universidad148.206.53.84/tesiuami/uam20557.pdf · sonidos complejos ..... 23 ... audhome$rlas de...

~

U N I V E R S I D A D

A U T O N O M A

M E T R O P O L I T A N A

I Z T A P A L A P A

TXkbAJO DE MEDICINA IV

A I S DE OIW MEDIO Y TECN CAS AUDl NETBICAS

I "... ,

RE~LIZACION:

MAESTRO: Alfonso martinez

fodol fo /i G z u f i o j iménez Genaro

-.

c

c

c

r.l

-1

- PROLWO - Este trabajo fue real izado con l a intención de dar a conocer pr inc i

palmente, l a s técnicas audiometricas mas senc i l l as que evaluan de - una forma precisa l a s patoiogias de oido medio que t ienen remedio - con con e l implante de alguna protes is s enc i l l a , como es e l caso de pro - t e s i s de l a cadena osc icular y e l timpano.

Por o t ro lado se pretende con este trabajo, dar a conocer algunas - n

patologias que aparentemente no son de gravedad alguna más simembar - go a i a larga i i e g an a causar grandes ie%iones que mas tarde solo Ip

se pueden remediar con pró tes i s que s i bién tratan de cubrir l a fun - cidn pero nunca en su t o t d i d a d ya que nunca se lograra este propo

s i to , mas sir@mbargo desde e l punto de v i s t a ing i ener i l se e s t M - realizando estudios cada d i a más certeros y grasias a l a tan a l t a - tecnologia a i& d i a se lograra este proposito.

-

- -

c

INDICE

CAPITULO I I4Anatomia y f i s i o l o g í a del oldo medios4

Reseña h i d t ó r i c a . . . . . . . . . . . . . . . 1 Anatomfa . . . . . . . . . . . . . . . . . . . 3

Enfermedades del o ído medio . . . . . . . . . 9

CAPITULO 11 14Fisica Acústica4t

Propagacidn de los sonidos . . . . . . . . . . 1 0

Intensidad sonora . . . . . . . . . . . . . . 20

Sonido pura, . . . . . . . . . . . . . . . . . 22

Sonidos complejos . . . . . . . . . . . . . . 23

Refracción y refBbxi6n . . . . . . . . . . . . 25

CAPITULO IIl 44Audiologla4t

Audhome$rlas de tonos puros . . . . . . . . . 36

Nledición del u m b r a l . . . . . . . . . . . . . 39

Enmascaramientos . . . . . . . . . . . . . . . . 4 1

I n t e r p r e t a c i 6 n clásica . . . . . . . . . . . . 44

Audiometria de Bekesy . . . . . . . . . . . . 52

Timpanometrla . . . . . . . . . . . . . . . . 34

d a a i o largo de la vida, req3eaoido o rene$bnbo de ammi foc ra e l noviialonto del a ire externo, con l o aru3. se cro8ba ur)* irpro dbn del maldo.

LOS grandes anatoaisks del ai&lo XVI fasroa3 los que descrl - bieron l a e pordones internas wlvlles del oido. Pemllo f t~e W6n- desorittid y did nombre a1 martillo y a l yunque. Zngnwf8, 1546 aeambt.16 el temer huerre6íllo, e l e a t r i b y l a s dos v m W s del- crrrool. aS 1561 mlopio desoribid agidadomcmte los hue sea l l a c y ems artitmlacionee, fasf com l a s dos blvisfones prfnolpales del- sido interno a los que bid l o s nbmbss actuales de oorrool ( d o l m ) y laberinto.

Bas-quio en 1564 eeorlbid ma ifnuoiornr d.dioirdo U Paw, en- e l que describe e l arbeoulo del * r t l l lo y e l coaducta que CQIU~C#-

1. ruw#aringe con e l ofdo modlo: e s b ooaducto o tr~mp8 11- rho - ra waoimre.

Entre l o s nvmexvms crientfflcos que 0ontrí-n a1 oonoai - mimfo de l a s ftmolonee del oido, hay des -0 rbrooan rm ree&wol- mionto eqecia l t Durexnej, dddiao, anatomista J Bbrajgci de Eiuñrf y eOtu@no, un autor Skliano. Dertpuis de l a &nqmeida y perfoedo- namiauto &el mícroaaopio Qoapaesto, bao18 1830, l o s o.pOt!,baE d s - fioas Oel oido fueron eetudiadoe por mnohos cic#itfiicos que c s i - namn con l a grirn contribtteión de ciortí , ~ll~fOmHsta italimw -0 - trabajo en WOrsburg, A l m a . Oorti reconooid y desaribtd todes - l o s elementos esenciales del dr- seruiitlm tamizmi del ofdo, e l oual redbe deede entonoes ar nosbre (1851).

hrchos CSantfiIcos, daiasiadoe para ser mmoion&ioa, han con - fribuido conoclmionfo actual del ofdo. Debems Moor sin embargo una msnaidn errpecial a Qmrg von Bdr6.y. qui& Ai. ualar40niao en - 1961 con el praiio ilorel de medioína y fisiolabg.br per OW deseribri- aienfos &e l o s meaonlsaoa ffsliass de estfaiillsreS4n doat- del b.n - col. Por medio de l a obsenaoidn direoto y con e1 UIIO iia iip~ tele - t6aaiea avansada y l a estrobomímeoopia dsloefrf6 odro los deUw - 80s mevimientas de l a base &el estribo ponen en marcha una 0ada - compleja que deaplrsi l a oabruu baoiiar, que QO aMpD (112 l a base con l o s tonos agudos y en e l vértice eon los toibos p v e u . Yon B6 - k h y es taniblh autor &e o t r a s de8aubriiicmtas Cue tratan wbre lb frmci6n coolear.

2

L

c

- c

_i

c

L

r L-

r L

c

L

c

L_

c

. e e1 pasito de vista fllogen6fioo e l 6r- vemt&bula~ es

l a porddn d a mtfgipr dcsnh.o de l a CripatCLa 6tiw. d e l e a o o+ ram1 em ads recimte y sa e~o l u~16n ea parrlela con 18 dol o$&- medio y l a d o n a de huesillos. %I ogB0 es un bz%an~ semaorial - complejo qua t ime un W p l e oslgen. Desde e l pude de *st8 de - so desarrollo e l odd0 medio es Unr mwld8U abrea y ae otf&t8 00-

nw una eva&naslbn de la faringe. La oadena de huesillom l o hac* a partir Be l a poroidn temixml del primen, y segunde car t i l rgo& de los areea farfngwos íaadibular y hioides). Bi ofdo exte.ni0 es una rodliloreidn del eatodermo wper f la i r l , por l a mal l a p i e l - atra en relaoibn rpscloarl con loa huesillos a nivel de l tiipou> y 91 coPdPato Yxtsrrro 0% modifloo 0.R cnnplir funcione8 de pro - t(MMldll, reorpaCn y oana.ntracibn del sonido.

. ZPL of6@, órgano de l a uudiai4n y el equilibrio, @e divide en

tree4 pa&.s; extema, redia e interns. ñl ofact mxtem es# *ora d e l edtmo, mIarb8s que la8 a s t m ~ s del of& ii-0 e Interno sstáa ubiartlas en e l haem@ taaposiil, dentno Us1 d.0.

x,a pared de1 OS& medio e a M forardr por Iia m r b F r r i . del t+ pa*a que oierra e l fondo del cmml auUiti.iB erihtw y eeO. 20- da por trem capam: una extenir rpidiriiae, 'I~LI media f l b r o a y x b na intern8 m~9oa.

oupndo se a b a e m e1 t- tleZY1 QBB e l OfosoSpb, e l 010 - mento que i d s destaca ea una projeceibn en fom de ombligo tri. - oorrespgnde a lo apdflais lateral eaart.) de1 . . r t l l l e . Ln

l a poraidn ten-; l a parte mgerlor se denomini, porcldn fllooidr. Iñi direooi6n l i ge raen fe oblicua hacia abajo y en línea recta con l a opdfts lrtercrl, se amaentra e l mango del J r t i l l o que esOI in cltaido en i o oapa media ¿el tfmp-0.

del thp&ElO que ü6fiL por debajo &e1 UiVe1 de 8s- O p b f i i C o r k 0s

3

r L

r c r

L..

L

r

Otros puntoe de refeniola del tfmpno son los l$episnfos que ionman loa pliegues interior y posterior, y que se dk8parra por - delante y por dotfils a l a altura de l a apbflisie laboral. A nivel- Bel extrema Inferior del martillo my un d m 8 r a & m i m li&e= - menate preiilnmte 11- oral2go. Deae 6mt0, parte 011 00- le- noso que se abre brcSaa abajo y hecici adelents, y Que reoibe e l - nombre de cono limin6so.

=TE3 Y 0011TrnIM).

La cavidad del oido medio es como una míia o primma tri"crpr lar de seis oaras que mide aprorriauQarmte 15 m en el sentido - vertical y anteroposterior y en aentMo tftSli+drm@X em muy entre - cl)r en l a poreíón meddia y es iIs amplia hada rrslbe y d a profunda per

FIGr

4

1id.B ganeral; algun8s 11.- pPO?mblaionte 01 oentido del gwt0. &a etaemla del raoibo un8 ram8 cfmnmie8nto (La1 grn%lIo at& co antm da &rea a1 narvlo lln@ml.

-* Le p8r.d anterior en 18 que Be abre l a trempa de aUtaqnio -

tima un oondriofa 4mso por 01 cp. dorm l a cadtida y un a u u l - qua mntlona U rd.oirlo da1 r r t i l l o . La abartaR da 18 trarp. de

tud total do la fi.eipa eo do 37 m 8proliiodriab%a. Sa abre on 01

oiao madi0 an 18 mi- emperlor de l a p 8 d 8nterior y 01 amplío- oOaa.tpct0 do1 mdauolo da1 mwti l lo me encuentra por arria. de 61.

1RlSCJUWS. Bl dsctl lo de1 martillo ea de t.mo8o contziaarrbla. So o r i e l -

iui an I8 cmparficie mparior de l a para ib cart l lag iaer de Ir - trompa de lhxetaquio, de l a poroldn adyacwte del ala myor del ea - fonoidas y üe las parados del aonducto. T o r ~ i n 8 por nadie de OII - $@6n redonda qua s o dobla C8i1 en ángulo reato sobro un8 eupina deea, l a apQflsis csalaarifoxme, y C N E ~ laterr3~nente l a caribaa- timpániaa. Está cubierto por l a mucosa y se Inserta sobro e l man- @Y del martillo, cerco 801 c u e l l o . &te mdeculo iraooiona e1 i.n- go del martillo haoia adentro por 10 que pone a l tfmp8no an tan - sidn. Eat6 insrvado por una ram6 de I8 rafa motora del trigómino.

El otro mdsculo del osdo medio es e l m#eaulo &el eetrlbo. Sa encuentra (111 10 parte posterior del oído medio por debajo del ad& tu8 ad antrum, que es l a abertura de l a mastoides en e l 8nto y co - necte a 6ste con e l oZdc medio. El mdswlo del estribo ee origtins on una pequeña prominencia y se inserta on La c8bab0 Bel ea t r i b . Su oontnccidn l leva l a cabes8 del estribo hacia afhera y abajo.- Fstl inemado por una del facial. La atmi& conjunta do es - t e e dos mdeculos no proauoa oopblos apreciables en l a posicldn - del tfmpano,

k 6 t 8 W O 80-5- 1i carfdad t w i W 8On 18 hfiw, 18 lo--

qq(0YPA DE WIS'PIQUIO. La porcidn de l a tr@m de ñustoquío me se cenecfr con a1 - '

afdo medio es 68e8. Desde l a cavidad tiarplnica se dirige hacia o-

bajo y iiporomente hacia adelante. (fig. 2). W k porei(n 6 6 r QO - rrampondo aproxiroauonte a UD taroto be l a longihrd toorl. Los - doa tarcios intornos fonien l a porciQn crutila@ma.; fr parelb&

6

eat4 s&ampre abierta, p.ro 1. airrtil8ginoSa est4 e*L.Trda en eu exfrom faringme uwpto &irante l a degluoibn o e l boatow, w- m e a b o s ea los que 1t2 lais se abre en f e r n triporal o se on- - cha por osatraacidn de l o a míeuuloa periea0irfllino extern o - teneior del velo de l paípdnr, y perieakfll.ino i n t w , o eleva- dmr de1 velo de1 paladar. B elwndor eat6 in-0 por 1. fu. hriryu del ncnno@alatiriao (plexo); e1 tenwr rectbo .u in- ai46 de l a yri. m8ntiiWar del M g & i n o a t r w 4 s de umrr m - bel -io dtioo. La mooma &e l a tmry emlid trpíuda por un- epitelio ciliado que ae continúa con l a maco.* de 18 nmcioMn-

8e y de1 osti0 medio.

.. . . .- _L . ...-

0

LOS 3 huealllos *.tan nupOdibOs b.nfro de 10 c8ridad ni-. mrman entre e9 una o a d m clo. trmumíte lu ritmeienee bo8 - de e1 tiap.no a l a vonfrna oval.

P i.rtilio reoibe rate nombre per ma f o m (fkg.3). &!a for - dos aplfida, uru, lap@ y una c o r k o ~~~~~3. BLL el ofdo nom - e l manubrio ea r i s ib le a travis de l a capa -tema be1 tfE?ano. a1 JimTp. t e f f forairdo por am cuerpo y dos apófisir, una corta y w - larga. ArLrtioola bop e1 martilla an forma lateral y oomo totlas 1- articuiaoiones posee una adpenla. eaZribo as e1 ho.miiio d s - prdxiw a la linea media. ira a i más pequeño pero *billa e1 mls in - portante ya cia. aimrra l a vQlt.89 oval y mu@ aia*iií-tbs grsr00.n ondas en l a perilinfa, qua son esenciales pur l a audiaidn. Se or- tionla con l a apófisis larga del Jnnqye por medio be tma rrtiaala- ción dril. Los hueeiilo8 aon !afbnht~ldos en siz porici6n denfro de- l a cavidad tsimplnioa p o r medio de vario8 ligoiiento8.

oad- d@ ht1e8illoa @at&. aomo 1111 a lútg . . de p.lPn~88 mb aumentm l a mersa del iqnaleo tnnamitibo por e l emfribo a la p c r i i i n f a del iabarínto de1 d a 0 interno.

-do por 18 mbbm, e l OOello, a1 m U b r I 0 0 de1 r V t I l l 0 y-

U

a

los s iy ientes cambios f$siios. tcr membrana puede hacerse muy grue - WL debido a l a inflamacidn, pueden aprreoer sorna blanquecina8 y - engrosadas circunscriha o de hecho, estar completusaite bl8nca y- engrosada debido a l depbsito de oolágano hialino en mu *pa media, oom remuifado de uno inf l~aacidn previa. laibión e1 tfnpam puede ado1ms.ri3e por p6rdida de l a cap8 media (meibronír propia), l o que oasi si-e se debe a alteración en l a ihmoiQn +Qntilatoria de l a fronips de ibstquio. BBz talas CPBOB de dis fbci6n ventilatori8, ir trompa de Ewrtquio puede o bien ventilar inwficientem.nto o per- m8necer sie8pre abierta, periitiendo que e l a ire penetre y Salga &

del ofdo medio con 10 rePgirPciih, l o que origina ieqtwmia y necm - ais de l a capa fibroaa (media). to membrana tim$hica puedo estar- retraida s i hay mi vacio dentro del ofdo medio, o bien puede sobre - sa l i r en esta oavidad cuando hay liquido, InfecciQn o ma msa de- tejido, como por ejemplo un tumor. Ea muy cooldm que haya perfora - ciones en l a msaiibrmm tinpónica; éstas pudem ser e1 remalimio de- traumatimaos y aoompañareie o no de tma altescrclbn subyacente. La ot i t i s media adnica que arana hacia apbera ai.pppro denota poriora - eidn de l a m.iPbrcrn0 tiwnica. m e a perforaoaiones pueden ser o x t r ~ mad.aemte peweñaa y di f ic i les de visaalisar, o bien ser m e s y evidentes a l a exploracidn; puodon clasifioarse en 4 tipos, en be- ne al sitio afectado: tubaria, at- , marginal y tie l a poicióm - p14eid8. LPs doe primeras por l o general 110 ac.p1ean txmplioacio - ne6; psro l a s dos ú l t i r s muelen ser d e grcrtes.

Q f/rmino miringitis signiflca iaplicreióoi de l a meabr8n8 - timplnioa. como se menciord, l a infloPaciQn ael tirpuro paieüe 8.-

p&ar a l a i n f lmc ión del oido nuiio c a l a s ot it ie nternas, Sin embargq miringitiei s i e i f i a a espesifieaiaento iiuilaiocidn que afoc- 4r primariamente a l a membrana tiq54niar. Hh l a miringifis hemod- gica o f i i c twu la r , que se censidera aauMdo por e l virus de l a in - fiasnsi, e l h8llasgo más no.t;.ble es l a fomoidn de vo8iarilas M) - bre l a mabrana t iqdnica J l a pared adyacente del conducto auáitg vo externo. kstas -lies contienen iiqaido sero~o, w e o am - difsreneial oon l a o t i t i s externa y e1 herpes *oster auditive (dn - drone ds Ramay flunt). S i oxiste dolor pueden ]L.olñamrse l a s ood- l a s eon una amja fina o un airing6toio afilado.

boa 9 w n de S s p O C t o rOj0 O viOldCe0. Debo hrUW60 01 diagndratiao-

ALTPPU cxams DE LA W D ~ A DB BEla T&QUXQ. tr trompa de a;is+mqulo conecta l a actividad de1 c ~ & o m d l o con

l a naoofaringe y p o r l a tanto e s t l inti#uien9e asiaeiono8r c m lam- anferiedados de mb8s caridades. Ei tercio extemo, CQCI~~O a l oído medic es de es%mcfurr 6s.0, en trnto que loa d*s tercios OXtenWB-

Iian fibrocarfllaglnoSOs. Lo froiipo de IRi8kQuio en e1 ai50 de 00-

Madl dif iere corn macho de l a de1 adulto. ~i aqua BS corta, aoipiis y su trayeatorfa ea h0riir;Ontml; h t a

os una de i r e rasomes por l a s cuales l a inn.mosi¿n de l a trompa do Bmstaquia es t8n f’reouonte en los niños, aobre todo d-te l a lac- tancia. A meditia que e l niño mace, l a tromp se alarga, ae elatre - uhp y toma un curso hacia abajo y hacia adentro. ~i. trompa normal - renite ae halla c e r m d a y a610 a@ abre por l a contraoaidn del docu - i o perleofrfilino externo dorrnte l a deglucí6n y en etras slfn.oio- nee, como bostesrr o abrir l a boaa. Loa Auiciones de l a trompa de - mataquio son: 1) ventiiaci6n y 2) drenaje.

Por tanto l a ventilmcibn pesmife eqtlilfbrat l a presidn rb.gst( - rim a s e b e lodoe de l a mesbrano tiapdnica. La trompa se abre corn conesouemcia de l a actividad msoular cuando oe pooenia una d i f e - rancia Be presi6n de 20 a 40 torr.

loa e l consultorio puede valorarse l a ventizaeib tubaria por - meilio del otcetcopic, buscando e l deaplauipiento lateral de l a msm - bnna timpdnlea; o bien, cuando hay ugo perfonioidn, esaochuldo con rm tubo de auadtacibn mientras e1 paciente cierro l a s norinas o - cluy6nüolas entre doe Bodas a l m i s m o tiampo que daplute (maniobn - de Toynbee)i también cuando ocluye I@aimente l a nul la y sopla con- f u e r a contra l a s narlnas ocluitl8~ y con l a boaa cer rNa , proüución - dose presldn sobre loa cidoa (mniobrr de Valaalv8). ’pubibi pude- haserse que e l aire penetre en e l oido rnedlPnfe l a politr~ríaeoi6a,

oonsiate en fcrmr la entrada de a i re a travb de l a naris, - mientrae l a aascfpringe se halla cerrmla cuando e l p o l e a t e deglute. &l a i r e también mede introducirse con una per i l la de Politcer a - m v d s de una oliva.

‘habl6n es poulble l a oateferirracibn at l a trimpa de mhietaqeo; eute procediniento ee utl l isó hace años pero hoy din m udo no ea - fr.cuente.

Loa alteraciones de l a Cronipp de i!Ustp~ui~ comprenden l a from- Po daraslado abierta, o au obstrucción. l a mtoclorúa del p i l a u J- e l psluiar hen4ldo.

- .. .. ., ., . .~ . . .~ . . . ._114 I*--- . ,. * ,

r"

L

TBQBPA DE WJSTAQUIO D w m . Bh esta altencidn 18 trompa 80 enouontrr 8biert8 e i q r e , por

l o cm81 e1 aire penetro en e1 ai&@ medio eon CNdr rrspír8cíbn.hn - fieau.noi8 e l prrclmte premOnt8 e l 8nteaedente de 1iD. diminucibn- impQrij8tIte de pesa, la @e produce p id ida de1 tejido adiposo 8im-

dedor del or i f ic io de l a tromp8 Qe %staqmio. k pew.rbi l i - te en escuchar 18 pT~pi8 resplrrcidn) o l a sanaacién de t.nw 11- "0 tapado" e1 ofdo. La ~uísirrno del tímpaio do estoa lyot~iente6 os - rtrüflcr y delgada y se meve heoia rdentm y afuera con or(L. iitairi- miento respirator io; este es un signo que permite e1 dlrgnd~tioo. - Un aitodo ~ i m p l ~ y si icrs de ccrreecián et3 e l i n s e m r un tubo de - V0ntil8Cibn a travbs de l a membrana del timpano par8 dim8inuir las- moleatias.

MIocIAlRu Dmd PALADAR. La micolonfa del p8lOdrr e0 un8 aitenoidn m y f(Ly., 021 l a --

aual los músculos del prliau presontlrn ocntr8caion.e r i h i C 8 8 pe - riddicos. Ento o r l e i n r ohasciuiao en e1 ofao del paoiaite, que pude aer 8taswltado tamblón por e l mbdiao. A m p s 0e desconoce l a cata- eX8Qt8 de 18 iaioafcnfsi tiel pal.d8r, se l a ña aaocl8do con lealonea ossoalarea, esclerosis ndltipie, anouri[15198 de l a orberia vertebral, tumeres y lesiones del t r l l o cere-1 y 8.1 cerebelo. CaaiQmente 110

es necesario ni- tnfui irnto; en raras ooocionea puede recurrir- ae a i o incialdn Bel mdscalc de l martillo cm e1 oido i .á io .

dad de 18 tromp8 d. hlwUi@ pUOdb OX'%&lk8r aUtOfOd8 4 COndc

La obstrucción de l a tromp8 de mstrqmio pude tener varlaa - caua8s que Incluyen inflamalanea como l a nam€8rim@tis a l a d e - M i d i tis.

%ana0 un tmnor nassfpriageo obtitraf. la tmmpa de Ibmtaq&,- el primer b e l l o w clfnieo pude ser l a preaan$i!ir de limaido en el- ofdo medio. Por l o tanto, en au8lqui.r ptboiento 8dulto con o f i t f a r medl8 sems8 uuii8tsr81 y crdníoo, debe eoniilderarse 18 poilbilldad de ím 08rCinOtP na~of8rfsagoo. -1 6batruoel(ln -bib pude 8.F de- bid8 s un cuerpo extr8fí@, cow un tap& introdlicitis par8 tratu- 0111 epistaxis o debido a trormn~mm i eoh ioo , por eJ.mplo durante rmr, 8dsnQideCt0d8 8greaiva que pued8 producir eIoli%rirrci.dn y cierre - de1 aonducto.

12

PAWDAR HIBDXDO. Bi palaear hendido silele producis dirf'uncith de l a taw- de -

mirtaqulo, debido a l a falta de u11 pun% de fijcuoibn ael d c u l o pp. ristafílino externo, io que evito que l a aooidn del mdsoolo produs- ea tinoaidn en e l orfiiaio de I8 tramp8 abribdalo l o cmi$ciente rante la deglucidn.

La falta de abertura de l a from o r i g i n a ir18 vautilación del osde medio y puede ser causa de innmoidn.

&I eenenl no osM indiorda l a .denoideotoda en l o s pocieniies eon paladar hmaiao o oon hendidiirr sirbanieaira ya que producirfa diir - fmcidn del paladar, danao lugar a vos nasal y remrgttrcidn de l o r

IB?arisan*D iRlT&lATOBIA NO Sum TIVA DPt OIDQ WrOT O.

doe de1 ofdo medio; aerotitis o -%ram, ot i t i s media uoroaa J gL t i t i s media mucoide.

lfqadO6 -Cia l a ni;BQfodnge.

b y tree tipos principales de procesos iniloiartorioe no mppn

ABK)!F!Z TIg (BAltO!rRAUHA). t. aerotiti8 es causada por l a brasca dismtnuoión de altifua -

darante el vriolo y tambih en al buoeo, y se pradaoe porque na 11- g a abrirae l a trompa de Jketaqaio. ciw(Lo l a diferemi8 do pre - sidn es mayor ae go am ae % l a actividad murcdlpr nomal no poáb abrir l a trompa.

Li. diaainuci6n relativa de preeidn o r i g i n a rm - d o om e i ofdo mulio, debido a l a cual e1 tfnipuro #e r e m e huoia den- y r e asla - fin los capilanir de l a meoira aei oido i d l o , que treemm lfquile y finalmente se roiapdn oon l a oonsigniente p4rdiid* de sangre haaia- l a eavide del ofdo medio y l a s oelai l lar a s t d i d u u . Hay hiperm - sia en uno o oabos ofdos y por l o g a e r a l tambtón dolor.

?&a lZteraci6n es aún ireauente en l a frlpolacidn y pa0aj.roe ae l o s aviones. Lair acoiones preventims ineiuJlen e1 P.O de @ma de mmsoar sin addcar, qtte aumenta a l doble l a fre@aencia de l a dsglu - ci6n; l a realieí~cfdn ae maniobras as V a l u a l a , robre todo dannte - al dereenso, y en a l v a dos e1 uso do ai& descongestivo MM- his-nieo antes de1 vuelo.

43

L

c

L

P".

L_

F

.... P

TITI IS IEDXA.

Las ot i t i s mediae seroaa y mucoide timan etiolodas algo ami.- jantas. La mag da l a ot i t i s media serosa es prinoip.limto 1. kr- ~ ~ d a c i 6 n de plaiapa de los rasos 6angufa.o~ M~ir ir cavidad da1 oido

tanto que l a otitia, media muwide es raauitrdo de uua saorooidn acf& va de l a s glándulas y quistes de l a mucdr~). que ravisfe l a oavidrd - del oido aodio. C. disfunoiibn de l a Wmpa de lbst.qttl es tam de los principales ?actores mut384les. Otros eon las vegat8cionaa admeidea, l a adenoiditis crónica, e l prladar hondido, loa ttamr~ros euoirrfn - eaos, e1 ba ra t r am (rerefitis) , e i n f í ue ionea asociadas come sin% sit ie o r lnit is , l a terapdtior eon rudiacianoe, y -bib defi&mn- ciae mefabdlioas e 1naon016gioas. A mentado l a alergia os un factor - que aantribtayo a l a prodiícoidn 68 los emdadow da1 ofdo madio.

Eafas enfermodades caraetarisadas por l a pmeonoia de lísuido - an e l sfdo medio aon d e fiemantes on los niños; oualrn manifestar- se 00- hipoacusia de conüuccibn, y por l o general son l a oawa Jn- importante de hipoacusia en los nifloa de edad ascolax. A maauáo se - compraaba hipoacnsia de CanducciQn raniaanite mayor Be 35 decibeies,- mediante los audiogtuias practicados an asctielrs o ea eshtdies de d2 toccidn. Es raro qae e1 niña manlfieeta rspont4rtaesIaIta aiguna Sinto matologfa; 01 maestro Hede describir a l niño adma fal fo de atenci6n. Por l o general loa &iu$fos descr$bai sus sfntonJs en f e w Piis e.pog tacrilar, antre eiios se inoluym l a sensacidn de tener l o s of(L0s "%a - pabosm Y menor aguaeao auditivr. A veces notan 8ejsSIa en 10 audi - cidn con cambios de posici6n de l a cabesa'; puede haber aaofenos 6ebi - do a l a presencia de liquido em e l ofdo medio; srrcui.nto pueüe haber v6rtl&w*

tratuaiento de estae aiteracionos ea inloialments m/dico y-- cuando es neceuario, quirdrgico. El trotoaoiento mSditm inaiaya Mti- bidticoa si eriste infección lmoteriana ( lo que ocurre 6 s f i e m a f a - maate en l a ot i t i s media muooide), an%ihist8mfziicos, descongestivae, ejercicios da vrritilaoidn de l a trompa tis mstuqtaio y deeeneibilisc. c16n se rssbrva soluaente para los cams en que hay alergia defidtls y domostrada por pruebes cut inera. Muchas vaceq san dti las los anti-

aidto, sobre fado debido a d&fermcirs d. paaS6n hl&@ü*tiCRt en -

14

-I.- - . , I . , . ~ . . ~ , . _ . < , . . . . . . . . . ~ . I ....

hiOt8dniCQS. Ei tmtumíento quíribgico se utiiim p.n los pacia- tea en l o s que e1 problem ae ha heoho crdniao a p e a r de 18 telo - p6Utlm m4dioa ooneemdora. B6to siepiiior generamente 18 presa-

ruglta consiste en l a incisidn ll.PiaB8 miTíngc~toari8 a tnv60 de Pr - -1 se inserta un tub de o*ntilaoibn. -do esM indimdo por ra - mne8 clfnlors, puede acomprflors. de rd~oIdectomf8. P fob0 de v e tilrcidn weie dejarse an e1 tfmpano hasta que se eXptü.98 en f o r u - emponmea, lo (lpe ouurre generalmente VWios meses o un .et0 des - puli. &e IIP incitiiicicía. si de nuevo hay l fqrido en e l ofdo medio, - debe repetirse el pracedimianta. IQi tal oaw puede d e un tubo - de Venfíl8Cidn mayor si esto es nece(IOFIo (fig.4). Ente trataaiiento no eoluiente ea efioas p8ra recuperar I8 8udlcián sino que c.Sbi4n- aontríbuje a elinbar 18s secuelOs de cronicldaU sobre todo: 1)sordera, 2) atrofia o adelg8eamiento de l a membrana tfmpdnlc8, 3)ero - sidn de los huesillos y sorden periarnente de conducaidn, 4)dep¿sI- t o de col&g-o hi8linimdo e l Ofdo medlo y l a membrana del C-8 - no (timpanosalerosis) ocn scrder8 de wnduocibn pe-ente, 5 ) o t i - t i n media czbniacr y colestwfoiu y 6) m o n a de colestarol y for - ioci6n de gldndui.8~.

el8 de 1 feddo en 01 ofdo media durante 8-8 0 @= mOSe8. k C i -

O t IT I8 MICDIA AWhA SUPWIDI. ñípdcrotes sefíalb que e1 "dolor 8gua0 del ofdo y l a f labre con

tin- y elevoda son temibles y8 clue bay e1 peligro de que el hombre Lniffr delir io y finalmente mera". Iicr otitis mdia rgnd8 -8 y

18 eastoiditls eran enfemedades ay irportantea rnfos de Ir infro- duccidn de l a ter8p6atiea con antibi6ticran 8 saüiaüos de 18 d i d a - de loa treinta. ACtM'laenfe l a m8yoZ.Sa de 106 gaciaites oon otitím- media aon tratados exitornir~to por e1 pedir- y e1 módioa - generai .

Hormalmenóe e l oido medio es estliril, l o air1 es 8orpr.ndemt.r- si ne considera 18 pobl8Cidn de i i c r o o r ~ ~ o s que a i r f e en la na - sofrrlnge y resto de 10 farinRa. 8ccidn f i s i o l b g i ~ a0rl-k de- l os c i l ios y l a s ensimas presentes en e1 moo0 (por e j q l o 18 mra- mlilatm) y de l o s uitiouerpoa, acfdo como meooniao de Uefmer aup-

do e l ofdo medio se expone a 18 aontuPiaocibn gar agentes mícrobir- nee durante I8 dc~iucibn. Por l o general l a ot i t i s medl8 rgud8 se - produce oorndo ae altera ea%e mecrnirrio fisioldlglco. Atlads do es - tan defensas de superficie, existe una nutrid8 rod de aapilares auk

rpifeli8lOf~ que 8pert8 f8CbTeO h-mlee, 16UCOü%%8 pO l im OT fO~U - cieares y otras o6lulas fageeftieis. La obstrucaidn de l a Groipr a. mstaquio ea uno de l o s f8ctores cau#ales primarios da 18 ot i t i s mm - día 8ena8. De este modo, se p i u b e l a prinoipal brrr.rr c o n m l a - invasidn bacteri8na y aquellas oaprs que orUinrri8monta no aon pa%$ m a p e a m coionimr e1 ofao rdlio, i m i r sa t.$iao y crasrir in iecei6n. Aunque la mayor parte 4e 18s infeociones mspirrDorira son causadas por agmtas virales, 18 ubyor l r de las ot i t i s mdias agu - das los mn p o r &cterlrs pió&enao. Entre 18s bacterias que se aisr lrn d a frecnentraaenta 0 0 t h Eiemophilua infiu.risrre, neumoeocos J ea - tropfacocos beta hsnolfticror. Hempphilus inilnmb.e mele ser e l d - croorganisro p8t&~em que se aneuontn d e a mmmdo en l o s niplos a0 noma de cinco años de ed8a, en tanlo qua Diplococcus pneumonlae y- l os estreptooocos beto hemolftieos se hailan con myor fremaneia - en l os niños mayores de cinco años de eaad. Anbs appos de brete - r ia8 son muy sensibles 8 l a p.niaIlin8 y l a uapieilina, que aon loo rntibi6ticos de eleccibn. Ehi 0886 necesario loa auifas coma flzmaoo rdicionrl o bien i a eritroaicinr como substituto, pnuien ntili&rroa en niños maneres de cinaa 8ños. El medicuento deba 8binistrarae - durante 10 airs por l o monoei.

Los sfntomas de l a ot i t i s medio *@da mpuradfa incluyen dolor, que coimnuirnte no existe mando e1 lfquido presente en e l ofdo me - dio no es prrrulanfo; fiebre, 1Mlest8r y algunas veces cefalia &e - más de otalgia. Anorexia y ocacionaimente n4use.i~ y vómifos, a a e l m verse eobre todo en 108 nifloe. L8 fiebre puede ser amy alta en 108-

niños peque5as.

í6

A B

C D ’*.

--.- -

E BIG. 4. A, ineisidn em l o a r m - 8 . B, 8.pL.roci6n de li-de -

C , csleoacidn ¿e un tubo de t en t i l 8c i&~ D, taba ¿e vent%-

h i i n ce1ecaaa.E. fabo de ventilacíbd t ip I y 11. 13

,., .

I. -

... < _

_.

....

P-.

I_

...

-. 1

-.*

-.,

LOS oídos son órganos sensor ia les constru idOR especialmenSe para s e r excitados por u na energía v ib ra to r i a nacida en e l a ire . Per- tenecen a s í a l a c lase más general de mecanorreceptores donde se- encuentran 1 os órganos de l tacto , l o s órganos senaiblelt a l a -- presión y l o s órgano s de l equ i l i b r i o . La f i gura 22 representa- e l conjunto de l a funcibn auditiva: en (1) l a onda sonona.. Su e s - tudio independiente d e l hombre aoae$ituye. l a físim acústica. - US vibracionea transmitidas alileíOid&?), aon traducidas en iañLii- j o a nerviosos. Eetos influjos son conducldoa (3), por l o s nervio &. audi t ivos hac ia l i os centros au d i t i v o s (4), im f i s i o l o g í a y l a gteJ r a f i a i o l o g f a de l a audicibn es t udian l o s mecanlsms 2,,3 y, 4.. kas informacionea audit ivas eon transmitidas luego de los centros au- d i t i v o s a l o s centroa vecinos de reconocimiento. Llb psicoacústi- a a (S,), e8 e l estudio de l a s reacciones de l organismo a dichos %o- nidos.& d e c i r l a percepci6n, l a integración, l a ident i f icaci6n,- l a memoria, l a e v o cac ih , etc..

Algunos esquemas nos recordaran las nociones aprendidas en l o s iIi timos años de estudios secundarios: todo emisor sonoro es e l a s c i - ento de vibraciones; p o r ejemplo, l a s doaramas de u n diapasdn - que osc i lan emitiendo un sonido estas dos ramas ponen en movimien t o l a capa de a i r e que está en contacto con e l l as . Cada moleculá de gas d e l a i r e no efectúa de: que un minimo desplazamiento aun - ,

lado y a otro de suposici6n de reposo, pero e l l a transmite a las- '

moléculas vecinas e l movimiento v ib ra to r i o que anima y que produce e l movimiento de l a fuente sonora, ~ í , por e l s610 t r ans f e r i r l a energía s i n transportar l a materia, l a vibraci6n se comunica - de canirihuidad se comunica en todas l a s direcciones..

I.- ~ropagac ibn de l o s sonidos.

c.

...

f L

El sonido só lo se propaga en l o s medios materiales.- cuando hay - e l vac ío en sonido desaparece.

a).- Conducción por los gases. b).- U~nduccibn por l o s solidos.-La ca j a de resonancia refuerm-

Ba ce ler idad de l a s ondas sonoras e s d i s t i n t a se& e l medio: Es de 34.0 metros p o r segundo en e l a i r e a 1 6 grados, y de 3 O00 me- t r o s en e l h i e r ro (Figura No. 26)

e l sonido de l diapasón.

. . . . . r 18

- . ..

_- I

11.- Medios de estudio de l o s sonidos.

j$l análisis de l a s vibraciones sonoras por micrófono oaptor y OSCL l ográ fo . .- Es un medio u t i l i z ado frecuentemente; l a l l egada de on - das sonoras sobre l a membrana d e l micrdfono provoca con l a misma - feecuencia, var iaciones de l a d i f e renc ia de l potencial en sua bornes. Estas var iaciones son transmitidas a unas placas de deaviaci- 6n v e r t i c a l de un osc i lógra fo temporal así se puede pe rc ib i r estas variaciones, apbicándolas a l o s bornes de una bobina ante l a que - d e s f i l a una c in ta impregnada de un producto ferramagn6tico que, - sencible a l a s var iaciones de induccidn creadas por l a bobina, toma una imantaci6n que conserva (c inta magnetofónica ). Se cons--- truyen tambi4n l a s d i ferenc ias de potencial. proporcion,J.es a l a s - variaaiones de l a presi6n sonora permitiendo as1 hacer medidas acll

sticas objet ivas.

111.- lntensidad sonora.-

S i una fuente sonora i r r ad i a en idos Los sentidos, l a onda sonora t i ene como super f ic ie una esfera, cuyo centro es dicha fuente. Llb potencia de radiacidn para es ta fuente YV se encuentra repartida so bre todas l a s super f ic ies de e t a esfera. La potencia rec ib ida - por unidad de superEicie (1 cm )) de es ta es f e ra disminuye propor-- cionalmente con e l cuadrado de l a distancia l a fuente sonora. S i Z es l a intencidad a una c i e r t a distancia de l a fuente mnora, - a una distancia doble de l a intencidad será pues cuatro veces me-- nor.

9

B;L contrar io , en e l caso de una i r rad iac i6n l inea l , , que es l o que- produce p o r ejemplo en dna transmicidn atraves de un tubo, l a por tenc ia sonora no va r í a con l a distancia si ae desprecian los fenó- menos l igados a l a absorcidn.

La inteneidad de un sonido es, por convencibn, def inida en un pun- t o de su trayecto por los va lores de modificacibn de c i e r t as propi - edades d e l medio que l e transmite: - - Variaciones de presi6n - Variaciones de energía por unidad de supereicie En una unidsd de presión, l a intencidad de l sonido será pu es expa sada ifsicamente en larrias; l a presidn de l a s ondas sonoras es --

. . . . . 20

extremadamente débil,, una f r a cibn ~ ' a e 'baria, mientras que l a pres i - 6n admosferica e s cerca de 10 barias. - e l e r g i o por segundo por centimetro cuadrado en e l sistema ceges i - m a l ; - o l a unifad práct ica en va t i o s por centimetro cuadrado: un bat io -- igual a 10 erg ias por segundo.

fi . q

I,!&- Sonido puro

E l sonido puro no se encuentra en l a naturaleza, su estudio es park icularmente interesante. Aparece sobre l a pantal la de l osc i l oscop io como una curva sinusoidal per fecta ( f i g 26),.

E6tos sonidos son producidos, por ejemplo, pbr un diapasdn y est& - def inidos por su frecuencia y su intericidad.

La frecudncia de l sonido puro es e l número de vibraciones completas- por segundo, expresada en c i c l o s o en hertz , terminos que son sinóni - mo s.

E l período 9L es l a duraci6n de cada c i c l o , de donde:

S i y / es l a cere l ldad de l sonido,hsu longitud de onda esta definida-

\

T=-F por l a reiaci f in A , V T , L S

.J

Todo e l mundo sabe que l a velocidad de l Sonido va r í a muchisimo con - e l medio por e l cuai se transmiten. Es mucho más rápido, por ejem - plo , en e l agud e en e l acero que en e l a ire . La longitud de onda depende, pues, no solamente de l a frecuencia s in - o también de l a velocidad, e s dec i r de l medio de propagaci6n.

~ o c i f i n de face.- La f i gu ra 29 nos recuerda l a noción de faee. Vemos en e l l a dos sonidos puros de l a misma frecuencia y de l a misma amplg tud en A. esto 8 dos sonidos t ienen sus máximos de l mismo sentido en- e l mismo instante, se dice que están en fase, En U están en oposici- 6ri de fase y en C están en cuadratura, es dec i r que sub fases d i f i e - ren en YO grados.

L- ~ n t e r a c c i ó n de l a ondas sonoras

Cuando dos ondas se encuentran en un c i e r t o punto de l espacio se produce un movimiento resultante que se obtiene hacienao l a suma geonb t r i c a de los mpvimientos componentes.

. . . . .

7 2

Sonido en fase.- S i l a s ondas t ienen e l mismo periodo y pasan con-- juntamente por l o s va lores maxamos de l mismo signo!, los movimientoa- v ibra tor ios se suman ( f ig . 30).

Sonidos ep oposición de fase .- si e l máximo de una de l a s ondas en- un sentido corresponde al máximo de l a o t ra en& centido opaesto, se restan; pueden incluso anularse s i sus amplitudes son iguaies. Es e l fenómeno de in ts r f e renc la : un medio de astudio muy ut i l i zado en e l - ñabon t o r i o de acústica.

Vd. - Bat idos .. Supongamos ahora que l a s dos ondas sonoras d i f l e r en ligeramente en - frecuencia; en un c i e r t o momento e l l a s se reforzaran, en o t ro se su- priminin parc ia l o completamente y e l oído o i r a un sonido nuevo d i f e - rente desde sus componentes, una se r i e & puosaciones o batidos que-- se vulven más fuer tes o mas débi les en una especie dElento uiu lar , . La u t i l i z a c i ón de l bptido e s un medio c ens i l l o para a f inar los ins-- trumentos de musica y generadores de frecuencia e lec tr icós .

vil.-^ Sonidos complejos.

Son l o s que se encuentran en l a naturaleea. 1.- Yonidos complejos peri6dicos.- 61 movimiento v ibrator io que l e s -

da or igen se produce con interva los de tiempo iguales. ejemplo,- instrumentos de nifasic@ ( f i g . 32). Teorema de Pourier:, Toda onda periódica compleja de frecuenciaF puede s e r desconpuesta en una suma indefinida de ondas sinosoi- dales de frecuencias respectivamente P, Pi?,, ERB, etc.... La onda sinusoidal, de l a misma frecuencia que l a onda periddica com p l e j a se llama vibracidn fundamental. La u t i i i z ac idn de anali; zadores armdnicos permite proceder a este ans;lisis de sonidos - complejos cuyo resultado está presentado por un espectro (fig.- 34). En abscisas están anotadas l a s frecuencias de los armonice s presentes de l a vibraci6n compleja, y en las ordenadas para - cada uno de l o s armonicás, un segmento r e c t i l í n e o de una long& tud proporcional a su amplitud.

$1 a

2.- La mayoria de nuedjxas sensaciones sonoras est& producidas por- l a s vibraciones que no pueden es tar represeñkadae por una CUNEL-

regular; No poseen ninguna periodicidad, entran eh l a categoria- de los ruidos .

. . . . .

.-.

.*

Frecuentemente son def inidos como sonidos indeciables, aveces tam--- bien como sonidos e r rá t i cos intermitentes. W c h o , no ex i s te ninguno de f in i c ión exacta d e l rui do.

J X l ruido blanco.- es tá producido por l a mezcla de todos los sonidos- puros de lanisma intencidad de l espectro audit ivo, s i n f e i a c i ón co n su fase , de manera exactamente d o g a a l a producción de una luz -- blanca por l a mezcla de todos l o s co lores de l espectro (,fig. 38). Un ruido blanco produce un sonido que puede s e r couiporado a un chorw o de vap i r que se escapa de un radiador. Por e l hecho de que un rui do blanco contiene en s í numerosos componentes t iene l a tendencia de i n f e r i r prácticamente todos l o s otros sonidos. Una t a i interferen-- c i a se llama enmascaratgiento..

IL- LE voz . Las vocales constituyen i&almente en l a palabra sonidos compuestos- de una frecuencia fundamental y de parcia les en relaciones armónicas S i l a ~mpos ic ibn frecuencial , l a amplitud y l a s relaciones de fase- de l o s parc ia les de un sonido son constantes durante un c i e t t o tiempo, e l sonido se llama sostenido. Es e l caso, cuando se emite una - v o c d de manera prolongada. Pero un caracter n importante a e l a palabra es de que se t r a ta raramente de un estado i m v i l ; l a palabra es t6 en constante cambio; es dec i r , que esta formada de ullti se r i e ck -- soniaos transitor ios. Estas modificaciones brusc,s de un estado a - ot ro se llssian transitorios.. E s t w t rans i tor ios estan furmado s y -- provocados, por ejemplo, cuando 61 vocal termina bruscamente. S i se - examlna en e l osci loscopio l a palabra "speech" se constata que e l es pectro d e l fonema S e s aperdddico cuando se l e compar, con l a curva - sinusoidal de l a í.

L- Beflexión. Refracción.

Cuando una onda sonora encuen t r a un D stAculo, es decir, un medio-- d i ferente d e l cual e l l a se propaga, una primera parte de su energia- se u t i l i z a en la re f lex ión. La onda re f l e jada se propaga en sentido- inverso de l a onda inf luyente y a l a misma velocidtid. Una segunda.. parte de l a energha se pierde por absorción o rozamiento ..

. . . . . L

F'

L

. .1 C ' . : , I

I

r..

C.

c

Fig 36

OSCiLO6RAMA PEL SOUIDO 3 7

26

En f i n , una tercepara parte de l a energía es u t i l i z ada en l a refr&- cci6n, La onda refractada es l a que penetra e l obstáculo y se propaga en e l nuevo medio con l a velocidad espec i f i ca de l cuerpo que l a - constituye.

Cuando un& onda sonora para de un medio a o tro que -tiene e l mismo- estsdo f i s i c o , l a cas i totalildad de energía es re f le jada. Así, e l - 99.9 5 de eiergía es r e f l e j ada en e l paso de l a i r e d agua . Esto &- prueba l a gran importancia de l a cadena tlmpano-oscicular, que cons- t i tuye sensación auait iva, sino una sensacidn de vibración, una vi-- braci6n t á c t i l ) De hecho como 105 sonidos son esepcionalmente puros e l oido percibe sus armonicos superiores. La cadena tímpano-oscicu- l a r , que constituye un adaptador de impedancias que permite a l sonido pasa r@ l a i r e a l medio Liquido d e l oído interno. E l campo audit ivo de l hombre se extiende de l 20 a 20 O00 hertz. Por encima de los 20 O00 hertz. se habla de sonidos ultrasonicos . - De hecho este l i m i t e superior de 20 O00 hertz. es propias de los individuos jobes. Desciende rápidaentecon l a edad. Puede reconocer- se unc. d i f e renc ia de frecuencias de menos de 1% . E l campo de intencidad sonora percept ible es conciderable. 6L l l m i t e i n f e r i o r , e l umbral , es muy próxiuo al umbra l de l ruPdo de l fondo' f i s i c o debido a l a energla térmica ., E l l i m i t e superior audit ivo está señaiado ? o r l a sustitucidn de l a - sensacidn sonora por una sensación dolorosa . La re lac ida de poten- c i a eysre e l umbral l iminar y e l umbra l luminoso es aproximadamente- de 10 ; es dec i r que s i se presenta gráficamente l a potencia en e l umbral por un milimetro, l a potencia dolorosa será representada por- una longitud de um mil lon de kilometros. Una t a l v a r i ación no se - puede representar m8s que en una escala logaritmica. La u t i l i z a c i h de una escala logar i tmica por o t ra parte, es sugerida p o r l a l e y aproximada de Weber-Fechner: La sensación c fece mom e l - log@*itmo de l a estimulación. Las medidab absolntad de intencidad s a nora son bastantes d i f i c i l e s Y t ienen poco h t e r é s en c l í n i c a , E l bel: es e l logaptmo de l a reiacibri de l a fuente sonora estudiada expresada en W/cm como l a fuente de referencia.. En l a prftctica e l bel es unii unidád demaciado grande, y se escoje un submfiitiplo, l a décima parge, que e s e l decibel , de dorue:

Siendo W la potencia de li- fuente estudiada y Wo l a de l a fuente de- re ferencia. En l a c l i n i c a se toma como fuente dereferencia la potencia de l a f u a t e sonora minima percept ible por un Sujeto de ofdo normal a l a fre-- cuencia considerada. Se habla entonces de dec ibe l ios de pérdida .

. . . . .

27

r.

...

Fi.9 37 "3 40

28

Los b e l s no nos pues a d i t i v o s . S i un v i o l i n emite a una potenc ia - W', 2 v i o l i n e s emiten una potene ia de 2 W, pero e l sonido no e s mas- que 3 db (10 l o g 2)) por encima d e l presedente. Así pues, e l umbral d i f e r e n c i a l de c e n s i b i l i d a d e s d e l orden de mag n i t u d d e l db. 10 v i o l i n e s no e s t a e s de 10 db, y c i e n v i o l i n e s - a 20 db , por encima d e l primero. En los e s t u d i o s de f i s i o l o g í a acdstica l a intencidad sonora s e mide Coil 1 a ayuda de un micrófono ca l ibrado . Se expresa frecuentemente- en expresiones a c ú s t i c a s . E s t a s e s t a n r e 1 ?<.cionadas a los decibe-- i i o s por l a r e l a c i ó n :

P1 n db=20 l o g - PO

2 p b p r e s í o n de l a fuente donora estudiada en dinas/cm . Po= p r e s i ó n de l a fuente s> nora de r e f e r e n c i a . L a d i f e r e n c i a de percepcidn de un sonido de 10 mseg e n t r e los oídos en p e r c e p t i b l e y s i d e para la determinación de l a l o c a l i e a c i b n de l a fuente sonora. En resumer,, una de l as c a r a c t e r í s t i c a s muy p a r - titulares d e l oido e s l a de a s o c i a 3 a l a vez: - Una e x t r a o r d i n a r i a sen s i b i l i d a d - Un campo de i n t e n c i a a d extraordinariamente extendido.

&

c m9 44 i

, . . , , . ., . . .~.. ~ . I . . ,-“a- l 1

A U R. I O L O G 1 A

OBJETlVOS DE LA AUDIOLOGIA

La audiologla, que es l a c i enc ia de l a audición, comprende l a evalua c ión de este sentido y l a rehabi l i tac ión de sujetos que sufren proble mas de comunicación debidos a de f i c i enc ias auditivas. Existen dos - motivos importantísirnos que hacen necesaria l a evaluación: 1) estable c e r el diagnóstico médico sobre el s i t i o y t i po de enfermedad y 2) . vz - i o r a r e i e f e c t o de i a de f i c i enc i á audit iva sacre a i aprendieaje, i a - interacción soc i@ , e l cambio de ocupación y o tras consideraciones que requieren que se o i ga o escuche en forma normal.

Las mediciones de l a audición pueden contr ibuir en forma importante al conunto más amplio de datos, esto es, e l interrogator io y e l examen - otorr ino lar ingo ldg ico , para a s i l o g r a r l a d x ima precis ión en e l diag- ndstico sobre e l s i t i o d e l trastorno y sobre l a s enfemedades e spec í f i cas en cuestidn. guarda,n re lac idn espec i f i ca cor1 e l s i t i o ,de 1, enfermedad, loqque permite d i ferenc ias estos trastornos. U s individuos que sufren enfermedades d i f e rentes en l a misma zona espec i f i ca (v. gr.: l a sordera por ruido y e l síndrome de iiíeniere, que afectan ambos e1 caracol ) r e f i e r en experienci- audit ivas muy d i ferentes , y los datos audiométricos permiten d i ferenc ias estas anomalias. Asimismo, l a audición de f i c i en te - afectará l a atención, l a adquisicidn de l lenguaje, l a precis ión de l habla y l a e f i c a c i a general de l a comunici6n por mecanismos que guar- dan re lac ión espec í f i ca con e l grad0 y t i po de trastorno. Los planes de edifacidn espec ia l y rehabi l i tac ión deber& va r i a r y orientarse con base en datos de l a medición ept iva , adem6,s de otras var iab les impor- tantes, como l a in te l i genc ia , ~ o t i v a c i ó n y apoyo fami l iar . médico se ve obligado a ’ v a l o r a r l a integr idad de l ofdo medio en forma a lgo ind i rec ta y e s totalmente incapa de examinar e i caracoi y sistema ne: - v ioso audit ivo excepto en su part ic ipación en los mecanismos de rea, puesta a i sonido.

TIPOS DE EVALUACLON AUDITIVA

De muchas formas pueden conocerse l a capacid,d de l paciente de o í r - formas que var ían desde procedimientos informales a mediciormxmuy es- tandari70das y preciass que requieren equipo especial . La habil idad y exact i tud de l examiniador gu ran estrecha re lac ión con e l equipo a su disposición, aunque más b i e h o n su educación, expecz- imagina- c ión .y talento.

El. supuesto básico es que las anomalías Cela audicizn

A medida clue l o s e s tuhos de l a audic i n son más f r e - cuentes y s i s t e d i i i c o s en 81 consultoCio, e s m6 probable qui ra h a b u d a d en su uso y se apliquen en l a pdc t i$a . p f tu l o nos ocuparemos de los t ipos s iguientes de pruebas

que se ad-

audit i v as : En este ca-

estudios con diapasón, audiometria por tonos puros, audiometria verba l , Es laportante que se consi- ppruebas especia les y audiometxfa en niños.

deren como un onjunto cuyos ob je t ivos suplementarios y complementarios formas un estu 3. io en ser ie .

TIPOS DE DEPICIENOIAS AUDITIVAS

por mddio de l o s estudios de .udicbn se ident i f i can t r e s t ipos de pérdi da de l a agude-8 audit iva, a saber: trastornos ae l a conducción, senso- r inervioeos, y combinados o mixtos. Los problemas audit ivos de l a con- ducción se aeben a amomalias de l oído eirterno o medio, esto es, e l mece nismo conductor. Los problgas audit ivos neurosensoriales son consecuen- c i a s de trastorno d e l cara%l, d e l octavo par craneaL o de l a s v i a s & d i t i v a s centrales. Ya no se usan términos m6 antiguos, como pérdida -- @*nerviosa** o **perceptiva*, pues l a mayor pa&e ae las enfermedades lla- madas * lnerv izw*@ ex is t ían en e l caracol o se nabian originado de esta - estructura; ade.iAa, e l tbrmino "perceptive" hace p e n p r en psicoi6gico. trastornos ncocleares** y "retroco&earean, cuando elconjunto de l o s estu- d ios t iene complejidad suf ic iente. La pérdilda a t i v a miata o co mbina- da se debe a i a vez a trastornos de mecanismos de conducci6n y neurose- nsor ia le s.

contexto Algunos consider? que es ú t i l l a di ferenciaci6n entre -

%.I

MODELOS DE PRUEBA

L,s pruebas de l a udición tienden w corresponder a al iguno de diversos m modelos conocidos. En casa modelo, es necesaria l a compraci6n por referencia. Con mayor f recue ia i a audicidn de l paciante seponpara con e l promedio de persor,as de audici6n nor&. Cuarido no se f ispone de un - estaar de LOS que es normal, e i examinador suele nacer , i as veces de nor ma, o sea de **ofdo prototipo" con e l que se compara la 'audic ión de l paz c iente. Obviamente, e l e e t m a r puede derivarse de l propio paciarite, como cuando se comprarari l o s resuitauos antes y desfiués de l tratamiento o ambos 01- dos se estudian a frecuentas o intensidades di ferentes. En esta catego- r i a se encuentra l a comparación de o importandia fundamental entre l a - conducción aérea y - l a conducción ósea. Estos thrminos describen l a su- puesta v í a que siguen l o s estímulos acústivos. En La conducción aérea (CA) part ic ipan e l o ído externo y medio en l a trnasmisibn de sonido na- c i a e l caracol y más alla'se pienda que Sta e s l a v i a normalsde ka tran smisión de l sonido. En l a conducción ósea (CO), se hace v ibrar e l c r c neo por cont,cto d i rec to con un cuerpo que osc i l a en forma perihdica, - como diapasón. Se supone que e l estimulo que se conduce por v í a ósea, mueve l o s l í qu idos cocleares (si bien esto es una expiicacibn limitad, de l a cuesti6n) 'y, a s í pues, no penetra a l o í d o externo y meaio. Békésy ha deinostredo que l a v ibración de l caracol e8 Ba misma, s i n &aportar s i e l sonido penetra p o r conducción aérea o ósea.

-

r L

I. . . .

*..

.... *-

-.

.-

I La prueda de conducción ósea se ha csnsiderado como fndece de l a inte- gridad de l caracol y 1,s estrcutr,s m4 pronimales. La audicidn normal por conducción dsea hace pen$ar que es muy probáble que e l funcionamien - t o coc l ear y re t roco lcear es normal. {CO) es normal pero ex i s t e pérdida de l a agudeza de l sisteuia t o t a l - - (CO CA), se l l e g a a l a conclusión que e l trasto& es consecuencia de l e s i ó n de l a porción restante de l sisteina, esto es, e l oído medio, e l oído externo, o ambas estructuras, porciones que no se pueden evaiuar s i e l estudio de l a conducción ósea proporciona resultados normales - Por ot ra parte, s i l a conducción 6se, no es mAs sensible que l a conducc i6n aérea (CO CA), s e deduce que e l trastorno t o t a l se debe a l e s i ón o a Cambios en e l mecanismo coc lear o retrococle,r.

S,i e l componente neurosensorial

,. BSTUDlOS POR DlAF'ASON

E1 médico, que dispone de un juego de diapasones cuyas frecuencias se encuendxan dentro de los l i m i t e s audit ivos, de l a s regiones más agudas a l a s m& graves, puede estudiar l a sensibi l idad auditiva. Los di.apasories c de l o s juegos ordinarios producen algunas de l a s notas Do de la escala musical, esto es, pxoducen l a s frecuennias de 120, 256, 512, 1024, 2 O46 4 O96 y 6 192 He. de l habla", esto es, 512, 1 0 2 4 y 2 040.

Swele bastar usar sdlo l a s llamaa,.s "frecuencias

UlllBRAL

Se sostiene e l diapasón por e l t a l l o y una de l a s . puntas se golpea - contra una super f i c i e f irme pero e l ás t i ca , como l a parte i n f e r i o r de l a mano o e l codo, Debe tenerse cuidado de no golpear e l diapasón -- contra e l borde de una mesa o de d g Ú n o t ro ob je to duro pues se prod2 cen sonidos armónicos, algunos de l o s cuales se escuchan a alguna d i s tancia de l diapasón, y que pueden insluso causar a ~ t e r a c i b n permane; - t e de l a s carac te r í s t i cas v ibra tor ias de l instrumento. Se sostiene e l di,pasÓn cerca de l ofdo, y se l e pide al paciente que señale e l - momento en que de ja de escucha,r e l sonido. En seguida, e l examinador se coloca e l diapasón cerca de l a o r e j a y mide e l interva lo entre e l momento que r e f i e r e e l paciente, y e l instante en que e l exa- Hz es e l abreviatura de herz io , que es e l simbolo contemporáneo

de t ac i c l o s por segundoq8. o sea. l a unidad de frecuencia. La frecpencia es e l número de vibr8ciones por segundo, de l sanido que se escucha; e;u:irda estrecha re lac ión con e l tono. El tono e s una experiencia - ps i co l ó gioa que produce l a frecuencia y que se c l a s i f i c a pr su si- t i o en l a escala musical. Cuanto mayor rá e l tono.

frecuencia, más agudo se -

PRUEBA DE WEBER

La prutba de Weber amplía l a experiencia bien cinocida de escuchar la propia voz con mayorintensidad cuando la persona se tapa l a oreja.

c

.... 32

tamo dtico

naufwd# d.*crLi &kM.ph

wanul Normal Ninguno holaieh R.ducido

L.Mo d. ia eudicidn rnio -

PLrdid. conductive Oído externo. madlo o ambos

Coclur. retrococlur, o ambos Pbrdida nwrosuiioiiei iitios

sitios

- 1 -

- CUAüRO 3-1. Resultados de ie prwba de R i m . tipo da doiiciencie

8uditiva y shlo del tremwno &ico

Poaitiva CA CO Norm1 o deficimi.

Nwative W < Co w naurounioiisl

Deficiml3 conductiva

Ninguno o eaiur-retroeociur

Old0 externo o medio

33

. . , . . .~. . ...~. .. 1. yc <

. .

E l t a l l o d e l diapasón en v ibración se coloca en l a l i n e @ media de 1 , f r ente y se l e pide paciente que senale s i escucna e l sonido en - e l oído izquierdo, o drekho, o en ambos. Por l o regular, e l paciente perc ibe e l sonido d e l diapasdn en e l oído en quela ~onduccidn ósea es mejor o en e l que e l componente conductivo e s mayor. S i e l sonido se escucha en e l oído que según 61 es m8s de f i c i en te , debe sospecharse - pérdida audi t iva por trastorno de i a conduccidn en e l ofdo en cuestón. S i e l sonido se eachucha en e l oído menos de f ic iente , se sospechará pérdida sensorinerviosq en e l oído de agudeza audit& a menor.

que el'"sonid0 se eschuciie en e l ofdo gue sufre trastorno de l a con- ducción y no en e l o ído mejor. Em l a prueba de Weber y otras que des cribiremos, es te fenómeno, que se observa en los problemas de l a con1 ducción y que consiste en mayor intencidad de l a conducción ósea, se

explicado por l a atenuación d e l ruido de fondo que se conduce po r e l a i r e y l a prevención de l a p6rdbda de energfa acústica en e l meg,- t o externo, f ac tores que se elimina- al cubMr u o c l u i r l a s orejas. La prueba de Weber es de mayor u t i l i d ad en los casos de trastorno uni - l a t e r a l , pero ta l ve, ocurra ambiguedaddes cuando un ofdo sufre l a - vez trastorno de l a conducción y neurosensorial (trastorno combinado), cuando se dispone de un diapasón de una sosip frecuensia, o cuando se usa sólo l a prueba de Weber. zsólo combinada con oCros estudios y no in te rpre tar la en form, aisla- da.

Tal. v e pare,ca i l ó g i x o a lgaciente, y a veces j tambi 'g ' e l examinauor,

E l c l í n i c o deber4 usar l a prueba de Weber

PRUEBA. DE RINNE

La prueba de Binne permite comparar l a audicidn de l paciente por con- ducción ósea y conducción aérea. E l tallo de l diapasón en v ibrp i ibn se sostiene contra l a apó f i s i s mastoides de l paciente (conducción 6- sea) hasta que ya no se escuche; en seguida l a s punta 8 se colocan - f r ente a l mismo oído (conducción aérea). El ofdo no&al eacuhcar6 de nuevo por co~~ducc ión a f ea e l diapasón, resatada, que se llama Rinne pos i t i v o (CA CO). Se pueden exp l i car l a impedancia en l a re18cíon- aiapasóncráneo . En e l pacienteque sufre pbraida aud i t i ve neurueensorial también se - producirá un resultado Rinne pos i t i vo , 8 1 es que e l diapasón es pre- fectamente audible cuando e l trastorno neu&osensorial a f ec ta p or i- gual l a conducción aérea y ósea (CArCO). Se usa e l thrmino de Binne negativo cuandoel paciente no puede escuchar de nuevo por conducción aérea el di,p&ón despues que deja de - oírlo p o r oonducción ósea (CA CO). La interpretación de los resultados de l a prueba de Binne se resume dn e l cuadro 3-l-.

?.

34

PRUEBA DE SCHWABACH

La prueba de Schwabach permite comparar l a conducción ósea d e l pacien- t e con l a d e l examinador. Con e l t a l l o d e l diapasón en v ibración con- t r a l a apó f i s i s mastoidea, e l paciente señ,la e l momento en que de ja - he escuchar e l sonido. En ese instante, e l exp inahor apkmca e l ta- l l a de l diapásón contra su propia apd f i s i s mastoidera y mhde e l t i m p o (en segundos) durwte 1 os cuales aún puede pe rc ib i r e l sonido. Existe un Scwabach normal cuando 1, conducción ósea de l paciente y exa minador son aproximauamente iguales. Se dice que ex i s te Schwabach p ro - kongado 6 mayor cuando l a conducción ósea de l paciente y examinador - san aproximadamente i gudea . Se d ice que ex i s te Schwabach prolongado o mayor cuando l a conducción d e duradera que l a d e l examiruldor, como en 1 os casos de périda audi- t i v a por trastornos de 1 9 cronducción. Cuanda e l examinador puede escuchar e l diapasón mucho despuee que e l pacifmte, l o que hace pensar en pérdida audit iva neurosensorial, se ap l ica e l término de Schwabach reducido. En e l cuadro 3 2 se muestra l a interpretacidn de l o s r ead - tados de l estudio de Schwabach.

&sea d e l paciente es apreciablemente -

PHUEñL DE BING

La p rueba de Bing es una apl icación d e l llamado *'efecto de oclusión**, en e i cuai se escucna e l diap,són más inensamnnte a l "tapar** e i o í d o norm@.. S i se abre y c i e r r a en forilia elternada e l meato audit ivo y - externo a l a vez Rue se sostierle e l diapasón en vibración cor i t ra . la p ó f i s i s mnstoides, e l o ído normalip e r i cb i rá un aumento y disminución de l a irniensidad (Bing pos i t ivo ) . Se logrará un resultado s imi lar s i ex i s te pérdida neuresensorial _udit iva. Sin embargo, e l paciente cuyo mecanismo conductor se ha modificado como en l o s casos de o t i t i s media u otosc leros is , no notará cambio alguno en l a intensidpa (Bing negativo).

CONFIABILIDAD Y VALIDEZ,

A l empredner repetidas veces 106 estudios por diapasón e l e m i n i d o r adquirirá habil idad en su uso. Los problemas de confiabil id,d (de repet~hubón idént ica) se deben a los errores da1 paciente y uel examinac dor en l o que se r e f i e r e a l "momento de inauaibilidad" desaparecer l a experiencia tonal. Las pruebas son másdi f i c i l es de l l e v a r a cabo en niiíos o pacientes cuyo margen de atenci6n es limiyada. Los c l in i cos debe& e v i t a r e l uso de diap,sones de frecuenci, baja - (12U y 256 Hz), pues e s necesario el iminar e l ruiao de fondo, como en cuartos a is laaos especia les, que por l o regular l e es imposible a l m& dico comh. los de 1 O00 y 2 O00 Hz, para l oga r resdtadoz ú t i l e s en l a prueba de Bing.

Por motivos f ss i cos , es mejor e l diapasón de 500 HZ que

AUUIOMETRIA POH TONOS PUXUS

La invernc i6n d e l tubo de v a c i ó h izo p o s i b l e s l o s tonos de producción e l e c t r ó n i c a . Los instrumentos i i d a d o s audibmetros, que s e c rearon - en l o s primeros anos de Lka década de 1Y20, producían a h las oc tavas de l a La intes indad tond. PO - d f a conservarse a un n i v e l f i j o , por l o que ya e l sonido no comenzaba de inmediato a desaparecer , como en los diapasones. El tono podia i n - t e r n i p i r s e a volunta o r e d u r c i r e e l a intensidadd a i n t e r v a l o s f i j o s - por medio de d i s p o s i t i v o s en s e r i e , de impedancia e l é c t r i c a , por 10 - que, as€ pues, s e tuvo opnrtunidad de c u a n t i f i c a r l a intensidad d e l - sonido. Fue n e c e s a r i o sdlo aplicar una graduaci6n en d e c i b e l e s &L c o d t inuo de l a i n t e s i d a d para que n a c i e r a l a e r a de l a audiometria moder - na por tonos puros.

AUDLOiúETROS D E TONOS P U R O S .

5 .

n o t a Do como l o h a c i h los diapasones.

El audiometro de tonos puros e s un aparato e l e c t r d n i c o que produce - sonidos re lat ivamente desprovis tos devruido o de energfa sonora en - forma de armónicos, e s t o e s , tonos llpurosvt.ExiSten aparatos que producen uns s e r i e de tonos que aproximadamente preservan las r e l a c i o n e s de oc tavas de tono Do, e s t o es: 125,250,500,l 000,2 000,4 000,y u O00 Hz.TambiBn producen tonos a i n t e r v a l o s de media o c t a v a ( 7 5 0 , l 500, j O0 y 6 O00 Hz). E l audiómetro s e compone de t r e e p a r t e s e s e n c i a l e s : un - o s c i l a d o r de f r e c u e n c i a v a r i a b l e para producir los sonidos , un aten= nuador para v a r i a r l a in tens idad, en forma c a r a a t e r f s t i c a por grados- de c i n c o d e c i b e l e g , y un tranductor(audifono o v ibrador 6 s e 0 , o a v e - c e s un d t o p a r i a n t e ) para transformar l a energfa e i 4 c t r i c á en energfa a c ú s t i c a .

CONDUCCION AEXEA Y CONDUCCION OSEA

El aparato s u e l e c o n t a r con dos f u e n t e s sonoras. Una e s t á c o n s t i t u i d a p o r los audifonos , que s e s o s t i e n e n f imemente c o n t r a l o a oídos por - medio de una banda sobre l a cabeza. Cada oido s e e s t u d i a por separado y los r e s u l t a d o s s e anotan en forma gráfica como e l audiogrma de i a - conduccidn adrea.LA segunda fuente sonora e s un o s c i l a d o r o v ibrador- de conducci6n 6aea que s e s o s t i e n e firmemente c o n t r a l a a p d f i s i s mas- to ides (o l a f r e n t e ) tambidn p o r una banda sobre l a cabeza. J%l vibrado hace o a c i l a r e l c ráneo , l o que produce wmbios e n los l i q u i d o s d e l - c a r a c o l .

. . . . . . . . . . . .

...,

...,.

Suele aceptarse que los resultados, que se unotan en forma grá f i ca - como e l audiograma de conducción 6sea, son producto de e l t@esquivami- ento** d e l oido medio, por e i estimulo, constituyen un indice de i a 11s *@reserva cocieur'* y son r e f l e j o d e l estado de l sistema de l nervio au- d i t i v o . Más adelante veremos como est& interpretacibn no es completamente exacta, pero si por i o reguiar ú t i l .

UMBRAL

El ob je t i vo de l a medicidn es determinar e l n i v e l de intensidad d a - bajo en e l que se escucha cada frecuencia; y , en consecuencia, e l um- b r a l de l sonido en cuestibn.

CERO AUDIOMETRICO Y LIMITES DE INTENSIDAD

n i v e l de l umbral de l paciente se compara con e l v%erot* audiometrico, que es e l promedio de los umbrales de un gran número de adultos $ jóvenes que no sufren transtornos audit ivos no presentan antecedente

da frecuencia t iene su propio cero d i ferente , y e l audiómetro puede p producir intensidades calibradas se& e l cero. Puesto que e l cero es e l v a l o r promedio de unbrales, deben también e x i s t i r intensidades menores en e l aparato que se use para medir los umbrales de pacientes & de mayor ugudeza auditiva. Las intensidades de l audi6metro pueden va r i a r de o0 decibeles debajo- de l umbral cero, osea -10 dB, a 110 decibeles por arr iba de cero. S i en un sujeto es necesaria una intensidad de 45 dEl por arr iba de l - n i v e l normal para que se perciba un sonido en part icular , e l n i v e l dd umbral de audici6n es de 45dB; s i su sencibi l idad audit iva es m4s c e r cana a l umbral y requiere s610 de 20dB POR ARRIBA DE LA CIFRA CERO? e l umbral ser4 de 20dB. S i e l sujeto es lOdB d s sencible que e l pro medio, su umbral de audici6n se simboliza con una c i f r a negativa, es* es -lOdB.

de enfermedades dt icas y no han padecido re 6. lentemente resfr iados. Ca

-

SIMEiOLOS EN EL AUDIOGRAMA RESPECTO A LA CONDUCCION AEREA Y OSEA

El. audiograma es una grá f ica de l a sensibi l idad de oído a var ias f r e e cuencias. Se reg is t ran l a s c i f r a s para cada oido por separado; l a s fie frecuencias se señalan en l a abcisa y l a intensidad en l a ordenada. - Los simbolos estándar para l a conduccibn aérea y 6sea se muestra en a c lave de l audiogram en l a f i gura 3-1.

._

. - . . . . . . . . . . . . . .

r

.- c

Se&n sec observa en e l audiograrna, los símbolos de conducción aérea se unen normalmente con una l í n e a continua. Los símbolos de conduccidn ósea se unen con una l í n e a interrunpida - cuando ex i s t e una d i f e renc ia entre l a conducción aérea y bsea; s i no- ex i s t e esta d i ferenc ia , l o s sfmbolos nunca se unen. E n este sistema de símbolos noes necesario un código de colores para ind icar de qué oído se trata. Sin embargo, s i se emplean colores, deber& u t i l i z a r s e e l r o j o para los simbolos de l oído derecho y sus l i - neas de conexión, y e l azul para e l oído izquierdo.

PROCEDIMIENTO BRA CONOCER 6IL UMBKAL

Carhart y Jerger (1959) conminaron a los c l in i cos a estandari,ar sus- métodos de conocer l o s umbrales de tonos puros a i adoptar l a s caracte t e r í s t i c a s de l a técnica de Hughson-Westlake que había sido aceptada- en 1944 por e l Comité para l a ~onservac ión de l a Audici6n de l a Acade mia Estadounidense de aftalmología y Otorrinolaringología. E l procz. - dimieato de Hughson-Westkake a veces se Uama e l *método asceMenWP,, pues se presentan los tonos al paciente por grados discontinuoe, desde e l n i v e l de inaudibilid,d hasta e l primer n i v e l en decibles a l --- c u d e l paciente indica que escucha e l estímulo . Cuando e l paciente reacciona al tono, e l estímulo se reduce de 10 a 1 5 aB.y se i n i c i a un nuevo ascenso. Se def ine e l umbral como e l n i v e l mínimo ai. cual se - produce l a percepación en a6 de l a mitad de los as-ceneos. El procedimiento siguientd es una adaptación de l a técnica de Hughson Westlake que pude usarse e h k a mayor parte de l o s pacientes. A cauna de l a importancia de preparar en foxcna adecuada l a paciente para l a prueba de audición, esta aspecto se t ratará antes de descr ib i r e l estudio de l umbral.

PFtEPARaCION DEL PACIENTE

1. 61 paciente deberá estar seritado de tal forma que no pueda v e r e l tablero de control n i e l examinador. Por lo regular bastará darle. - vue l ta a l pciente de modo que Quede de t r e s cuartos hacia l a deyecha o l a iXquierday en re lac ión con e l tablero de control. 2. Deber& quitarse los artúckos que pyedan impedir l a adaptación - adecuada de los audífonos o que de otras formas afectan l a medicibn. Algunos ejemplos son aretes, anteojos, sombreros, algunas pelucos, - $oma de mascar y algodón en e l conducto audit ivo externo. En este - momento e l examinidado puede v e r i f i c a r s i e l conducto audi£ivo ex te r no se colapsa, al observar e l movimiento de l a s paredes de l conducto ai e j e r ce r presión sobre e i pabellbn y e l trgo.*

Se han re f e r ido d i f e r e n c i , ~ entre conduccibn aérea y ósea de 15 a 30 decibeles o consecuencias de colapso de l cunducto audit ivo externo. Este problema puede resolverse aL sostener los aufiiorlos laxarnente cont1.a e l pabellón, presentar e l estímulo de prueba en un c a p o sonon

' 'i

. CENTRO DE DlACNSTICO Y TRATAMlwrO DE TRA- M: AUDICION. LENGUAJE

Y APRENDIZklE. A. C.

0

Expediente N6m.

FdU

a).

P

L . c

I..

a

CAREART: INTEHPRETACiON

ADAPTACION DE AUXILIAR AUDITIVO:

3. e l de

Las instrucciones deben se r c laras y concisas. Es importante qge paciente sepa i o que escucha& y qué t ipos de respuesta se esperan él. Debe alentársele a que reaccione al sonido m4 déb i l que pueda

apreciar.

4. Colóquele los audifonos paciente de t a l forma que e l o r i f i c i o de l o s coj inentes se encuentre directamwnte sobre e l o r i f i c i o d e l con - dueto p d i t i v o externo. Es necesario que l a reacción d e l paciente sea invar iable , l a prueba reconocer con f ac i l i dad e l examinador y no estorbe l a pmeva aud i t i ve Algunas reacciones deseables son sencillaiiiente levantar l a mano o un dedo, u oprimir un botón que encienda una señal luminosa., Son menos deseables los movimientos ce f&l icos vigorosos o l a s respuestas verba- l e s , pues posiblemente inf luyan en l a duración de los intervaloes en- t r e l o s estímulos, enmascaren e l estfmulo siguiente de prueba. Debe ordenarse al inaividuo que prosiga con l a respuesta en tanto escuche e l estímulo de prueba. Esto l e permite al examinaaor controlar en forma mayor i a conducta de respuesta a i va r i a r no s610 e l interna - l o entre l o s estímulos sino t a m b i h l a duración d e l estfmulomismo. Esto es especialmente importante cuando en e l paciente se observa un gran número de respues Liis falsamente posit ivas.

MliDICION DEL UMBRAL

1. Estudie e l oído mejor al usar la secuenQa aiguiente de frecuen- c ias: 1 O00 Hz, 2 O00 He, 4 O00 Hz, 8 O00 Hz, 1 O00 HZ (se rep i t e ) ? 500 HE, 250 He. A excepción de la repet ic ión a los 1 O00 Hz, puede usarse l a misma secuencia en e i otro oído. S i ex i s te una d i ferenc ia - de l umbral de 1 5 üB o d s en al& i n terva lo de octava estudiar4 l a frecuencia a ia media octava,tistudiará l a frecuencia a ia media o c t a va. 2. A: p a r t i r de una hntensidad unic ia l de l n i v e l de audición (NA) de O decibeles, se emit irá e l tono en incrementos ascendentes de 10 dB a.i usgr una uuracién tonai ae uno a dos segundos hasta que e l pacien - t e responda. 3. Deberá aumentarse l a intensidad de l tono en 5 dñ y , s i e l pacien- t e reacciona, será necesari9 disminuir e l sonido por grados ae 10 aB hasta que sea inaudible. 4. Se proseguir& los ascensos sucesivos por incrementos de 5 aB - hasta que se l o g r e una respuesta modal. Para e l l o , por l o regular no son necesarios mS de t r e s ascensos (véase e i cuadro 3-3). 5. Después de conocer e l umbral para l a frecuencia i n i c i a l , r e g i s t r e e l sfmbolo apropiado en e l audiograma. 6. E1 operador e s t u d i a d l a frecuencia siguiente del l a secuencia. f,se i n i c i a l , hará l a emisi6n de l sonido 1 5 a 20 dB ;por debajo de l - umbral ae lz frecuencia que se estudió anterirormente.

- .

c -

STATIC PRESSURE ! -200 *-

CONDUCTANCE (G) mmhoa .-

COMPLIANCE (E) rnmhoa

G MAX-G+200 - II

O +200 -200 MAX O +200 MAX

h I

r-

I

I

c

E YAX-B+200 U I

....

- REFLEX SL

r.

I I I I I I I

COMMENTS:

-. r.

c

.... o

L

CUADRO 3-3. umbral ~ w i n la respuesta modal

N I 4 d8 N M d8 -d. -. rap*.9. nrptr<i. -. Ninl d.

p1nnim.o qundo- *rm- -- - 30 dB 30 dB - 30 dB

30 dñ - 30 dü

- 35 dñ 30 dñ 25 dB 30 dñ 30 dB

25 dü 30 dñ 35 dñ 35 d8 35 dB

El~rrplo 4 35 dñ

El-vh 1 El- 2 Ekmcti~ 3 -

- - .-

-10

O

10

20

s i " s ' 0 e m

B r n Z m f BO

loo

110

FIG. 3-1. Audioarmu Y clave we muuba b dmbolao. d n d a r .

I

40

Por ejemplom s i e l umbral a l o s 1 O00 Hz e s de 50 dB, comenzará a e s t u d i a r la f r e c u e n c i a de 2 OW Hz a 30 6 35 dB. 7. Pueoe usarse e s t a t é c n i c a p a r a concer l o s umbrales de conducc- i 6 n 6seag1 adems de l o s de conducci6n aérea . los umbrales de conducción ó s e a p o r l o r e g u l a r no e x i s t e n en e l a- para to f r e c u e n c i a s ue 6 O00 y 8 O00 Hz.

Bn l a audiometria de -

VALIDEZ La compraci6n de los umbrales de conduci6n a é r e a y ó s e a e s a h un - f a c t o r importante en e l proceso de tom,r d e c i s i o n e s sobre e l tratamiento médico y quin í rg ivo de las enfermedades óticas. En muchos - c a s o s , l a c o r r e c c i 6 n de l a d i f e r e n c i a aire-hueso e s a h e l o b j e t i v o i d e a l d e l t ratamiento. Es de v i t a l importancia para tomar decksio- ne8 adecuzdas que se t e n t a n datos v á l i d o s ; por e s a racibn, e l exami- nados debe astar consc iente de t o f o s los f a c t o r e s que pueaen inva3.i - dar los r e s u l t a d o s de dicha medición.

ATBNUACION INTERAURAL Y A U D I C I O N CRUZADA

L a atenuaci6n i n t e r a r a r a l e s l a reducción de l a interisidad d e l estimulo cu,-ndo s e t ransmite de un oido ai o t r o . Por ejemplo, e l so- niUo de l O00 Hz que s e p r e s e n t a a un oido de 65 dB (en r e i c i ó n a l c e r o audiométr ico) , tal ve s u f r a una atenuación i n t e r a u r a l de 55 dñ an t e s de l l e g a r al a o t r o Oído en forma de un estimulo de 10 dB que s e perc ib i r , ' s J lo s i e l c a r a c o l e s s e n s i b l e a los sonidos de 10 dB Suelen usarse los términes t*audición cruzadal* o *I curva de sombre@l cuando e l p a c i e n t e responde a l est ímulo al escucharlo en e l o í d o q ue no s e e s t á estudianto. ocurre a t r a v ; d e l cráneo por coriducción ó s e a , aun s i e l est imulo - s e presente p o r medio de l o s audlfonos de conducci6n aérea. Se a c e p t a que 45 dñ e s una est im,c ibn razonable de l a atenuación - i n t e r a u r a i minima que ocurre a n t e s que s e produzca audic ión c%a- da en los l ímites de f r e c u e n c i a de 250 a O O00 HE. Aspfpues, cuan; do e x i s t a n d i f e r e n c i a s de 45 üB o más, los umbrales de l a conducción a é r e a e n t r e un oíUo y o t r o s e pondrá en t e l a de j u c i o l a v a l i d e , de los r e s u l t a d o s d e l oido menos s e n s i b l e . La a t e n ~ ~ c i ó n i n t e r a u r a l d e l est imulo que s e presenta p o r conducci6n _---- ó s e a e s casi nula. Al c o l o c a r e l v i b r a b a r ó s e a en l a a p ó f i s i s mastoides o e l hueso f r o n t a l s e producirán v i b r a c i o n e s p o r toúo e l c ráneo , i o que permite que s e produ ca una es t imulac ión esencialmente igual en ambos c a r a c o l e s . E s t a ine#is tendi@ de atenuacibn i n t e r a u r a l importante en e l caso de l a conduccidn 6 s e a a menudo c r e a problemas para i d e n t i f i c a r coiresctamente las r e l a c i o n e s a i r e hueso en e l Óído que s e estudia. P o r e jemplo, cuando e x i s t e una d i f e r e n c i a en l o s umbrales de conducción a é r e a e n t r e uno y o t r o o í d o s ,

L C ~ auuic ión cruzada con toda probabi l idad

41

I" u

desde e l punto de v i s t a t eór i co e l umbral de conducción o sea pu e& s e r tan sencible como e l UmttraL . de conducción aerea de l oido mas ~ h -

gudo. ¿ Ea . l a di ferencia aire-huesod e l o í d o que se estudia u xm-- separaci6n verdadera o a caso se escucha o estimula en forma c r u w da en e l oído que supuestamente no se estudia 'F

para. poder va l l da r l o s resultqfios de l a medición es necesario excl ir ir e l oído que no se estudia pos med i o de un estímulo de enmascsira miento e f ic iemte , de forma que l a s reacciónes del pasiente guraen - re lac ión con e l ofdo que se estudia. l a s caracter is t icas de l a ate nuación interaural pueden usarse para sentar reg las sobre l o s casos en que e s necewario e l enmascaramiento-.; en e l caso de l estudio I-

de l a conducta aerea, se debera usar enmascara.miento cundio e l nivek de presentación c e l estimulo sea 45 o mas des ive les ma5 intenso que e l umbral de condu ccmón o sea de l oido que no se estudia. Cunado se ha cen l a s pruebas de conduccih o sea debera enmascararse e l o- ído que no se estudiasiemBre que ex i s ta una d i ferenc ia aire-hueso- en e l o í d o somedido a prueba

ENMASCARAMIENTO2

E l enmascaramiento es e l ocultamiento de un sonido phr otro, o sea e l aumento d e l umbral de un estimulo que se produce por l a introducr. c i on de un segundo estimulo; s i b i e n s 1 sonido se produce enmascaa

miento mau e fec iente de un tono puso es otro tono de l a misma frecu

encie., ex i s te e l o t r o problema de d i f e renc iar e l estimulo de ermasca

ramiento d e l estimulo o r i g ina l . Para reso lver este problema se ba n usado var ios t ipos de ruido como eibthulos de enmascaramiento corn ruido blanco o guaussiano, rui do blanco conforiiiado (ruido rosa): y- ruido de bandu conformad a y r r ido de band& estrechm. De estos mi dos, ruido de banda estrecha e s e l estimulo que produce enmascL- ramiento más e f i c i en t e de los tonos puros.

De l o s var ios mdtoeos que se han adoptado para l a apl icación clfnica de l o s pr inc ip ios de l enmascaramiento-, l a técnica descr i ta o i r d (L950). es probablemente l a más senc i l l a ; s i n embargo, es l a +e-- t i ene menos probabilidad de producir err0re.s Burdos. Este método,- qu e suele llaqiarse "métodode l a meseta"'se basa efi datos que PO nd n de manifiesto que una vez que e l enmascaramiento l l e g a a su n i v e l mfnimo e f i c a z , l a re lac ión entre e l estimulo de enmascaramiento y a estímulo enmascarado es de t i po l i n e a l ; esto es, por cada incremen- t o a n e l n ibe l ed enmascaramitnto, l a señal enmascarada se e leva -- una cantidad igual. Por l o expuesto, enbnto que e l tono de prgeba-

se percibe (lipor audición cRi zada *I), en e.1 oído enmascarado, la k lación entre ei enmasoaramitinto y el tono permanecerá lineal. Cuan6 se llega ai. umbrai de enmalbcaramiento I n el oído que se estudia, el enmascaramiento adiciona en e l oído contrario no producirá despla zamiento adicional del umbral; se ha llegado a la *8mesetatv o al pun t o de “cambio I t . La meseta debería permanecer relativamente esta- bleal aumentar eo sonido de enmascaramiento de 15 a 20 decíbeles -- más a l l á del nivel en que comenzó la meseta.

Hood (WbO) describe las caracteristicas fundamentales de la prueba como a aontinuaciónse indica:

Los ruidos de enmascaramientotienen las mismas caracteristicas de 88

& enuación interaurai que los estímulos de conducción aérea que se presentan a través del mismo tipo de transductor, l o que puede ocu rir enmascaramiento excesivo cu ando el nivel del rui do de enmasca ramiento es mayor,en 45 dB o d s , qu el nivel del umbral de condu-- cció n del oido que me estudia. Al ocnrrir enmascar,damenrieto. excesivo se ponürfi de maniniesto la misma relación 1 ineal que se des cribio anteriormente. El métouo de la “meseta,” enibrma gráfica en- la figura 3-2.

Los vatímulos verbales siguen las mismas v5?eglasv* generales de ate- nuación interaural y audición cruzada que las que se describieron-- para los estímulos de tonos puros. Así pues, fn la audiometría verbal valldez losaismos cwiterios sobre la necesidad de enmascarami 8-1

to que se usaron ellos estudios de tonos puros; esto es, deberá u sarse enmascaramiento cuando elnivel de presentación del estima0 a sea mayor, en 45 descibeles, que el W r a l de ~onducción ósea del - o ido que no se estudia. El examinador debe prestar especial atee ción a esta- r elación al estudia la discriminaci6n verbal, pues l a palabras se presentan a un nivel de intensidad qu e es mayor que el ,

umbral.

Si bien se prefieren l o s ruidos de banda estrechaal enmascarar pur@ las frecuencias de estos miidos son demasiado limitadas como para - enmascarar la amplia gama de frecuencias del habla. Losestimul osde enmascarúmiento qu e se prefierenson el rujrdo blanco o ruido ver bal, esto es ruido blanco conformado pmr filtros para semejar el es pectro de frecuencias del habl,.

Sin emportar el tipo de ruido que se use, aeben establecerse nive- ies ef ica.ces de enmascarameinto para el audidmetro particular que-- se usa. Esto puede lograrse al calcular el promedio de los nibeles eficaces de 500, 1000 y 2 O00 Hz o a l hacer mediciones en un grupo

43

..._

..... de indibiduos con audición normal. E& ruido y los estímuios verba l e s pueden mezclarse en e l mismo audifono y conacerse los umbrRles d e l habla cuando se usan var ias intensidades de ruido. De esta f o r ma, e l examinador descubre e l n i v e l que es necesario para l o g r a r e l cambio a e l umbr,l que se desea en e l oído enmascarado.

COLOCACION DEL VIBRADOR m CONDUCCION o SEA

Las var iables relacionadas con l a c o i o c ~ c i ó n ciel vibrador dsea han desp etado considerable interés entre l o s investigadores que eatu-- dian l a medición de este t i p o de conducción. S i bien l o s estímuios- se transmitirán a l caracol independientemente de l s i t i o en quese co loque e l vibrador en e l cráneo, se ha prestado mayor atencióna la-- colocación en apó f i s i s mastoides hueso frontal.

Se tenurá mS cuidado a l poner e l vibrador sbbre l a ap6 f i s i s mastoides, que a l co locar lo sobre e l hueso frontal . E l t e j i d o que cubre- e l f ronta l es relativamente homogéneo sobre l a amplia zona en que - se puede colocar e l vibrador ósco, mientres que e l tejidoque cubre l a mastoides va r i a considerablemente @l desplazarse e l promotorio a direccidn i n f e r i o r , superior o posterior. La colocaci6n descuidada d e l vibrador; por ejemplo fuera de l promontorio y sobre e l t e j i d a - blando @yacente, f á c i l mente puede menguar e l estímulo que l l e g a -- al c a r a c o l a 1 5 dB. A kci inversa, s i e l vibrador se pone sobre e l pabelloh, aumentará e l estimulo en re lacidn alque be proauce por l a colocación correata sobre l a apó f i s i s mastoides.

La fuerza que e e r e e contra e l cráneo e l vibrador ósea puede produ- c i r var iab i l idad de l estímulo en c i e r t os casos. l a tensibri a e l re- sor te en l a mayor parte de l a s bondas de sostén d e l vibrado dsea, - por l o regular constituye una. fuerza. importante. N o deberá conf iar se ne l a s v i e j a s cintad de so tén que han perdido mucha ae su tens i on.

INTERPRETACION CLASICA

Los audiogramas pueden interpretarse según e l grado de pérdida, l a - f o rma o pauta de esta perdida, y l a s relaciones entre coriducción 6 sea y aerea.

En l a paradda conductiva "pura", e l grado de disminución de la a@& ea ser&, en l o que se r e f i e r e a i n i v e l de audicidn, de m8s de ceroa aproximadamente 70 dB. Las perdidas mayores de urs nuvel de audición de 70 dB deben i n c lu i r algún componente neurosensorial. Las ptrdi- dad neurosensoriales pueden ser de cualquier grado, aesde más l eves

44

2'0 5

1: I

NlVEl D i RUIDO 'N cñ

.

FIG. 3-2. MÉtoL'o ile " 1 : r i r w ~ ~ d i iiniento" de meSc!L ii. 11, ( a AB es la II 1

timulo y el ruido d e xiiarcaiainierito cuando el sonido : e I 5, "&.ha en el <ii lateral. B es e l piiiito de carnüio y BC es la linea da :,?e?ceta. Nótese que a nivel de enmaxar<imicnto no o m t r e n cambios en el uriitral sino hasta el punto C . A este i , i v r l , ,:I wi- do llega al oido que se estudia. y ambos oidoo se enccen:ran enmascarados I,rii i?as sivo). E" la ii,,<.a cn. , , x i - * ? Y O kina relación lineal c i:ie S I scii¡i:o y el ii?ii!o , ! e I I

.

hasta l a s m8s graves.

Muchos han estimado- que l a hipoacusia a l a s frecuencias a l t a s es de c a r a c t e r i s t i c a s neurosensoriales. Se ha considerado que l a s pérdi- das a l a s frecuencias bajas y de t i p o plano son conductivas; s i n em bargo , l a s excepciones a estas general izaciones son notables. Por- ejemplo, l o s audiogramas en d i f e r en t es fases de ' l a hidropesía endo- l i n f á t i c a pueden i n c l u i r péreidas a bajas frecuencias, a planas y a a l t a s frecuencias. Támbieh en l o s casos de tumores d e l ángulo cere- belotuberancial se obsevará cu l q u ¡era de es tos t i p o s de pérdida-.

Con mucho, l a contribucidn más importantea la in terpretac ión d e l au diograma l o cbnstituye l a r e l ac i ón entre l o s umbrales de laconducci on aérea y ósea, es to es, l a ex is tenc i , o no !Es una d i f e r enc ia a i r e hueso. Estas re1 aciones pueden descr ib i rse , eri t érminos genera ls en l a forma s iguiente :

Audiolo gf a.

1.- Cuando los umbrales de conducción ósea, ademas de normales, son 10 o más dB mejores (más sens ib les ) que l o s umbrales de conduc- c ión aérea, l a pérdida, e s Be t i p o conauctivo (figura3-3).

2.- Cuando l o s umbrales de conduccibn ósea son iguales a los umbra l e s a l o s umbrales de conduccidn aérea y ninguno de l o s dos umbra les e a normal, l a pérdida e s neurosensorial ( f i g 3-4).

3.- Cuando los umbrales de conducciSlr ósea son menores pero mejores que l o s de conagcción aérea en 10 o más dec ibe les , l a perd id ia e s mixta o combinada ( f i g . 3-5).

E l resultado de l a in te rpre tac ión c l á s i c a I% l a d i f e r enc ia aire-hue so en manifestación bastahte exacta d e l ttrastorno rea l . Cuando se- descubre una d i f e r enc ia aire-hueso, e s muy probable que e x i s t a al-- guna a l t e rac i ón de l mecanismo de mnducci6n y transformación en e l - oído externo o medio. Asimismo, l a inex is tenc ia de u ht: d i f e t e n c i a aire-hueso e s dato wa s i t-, n con f iab le de a f ecc ión neurosensoria

Nos ocuparemos elL forma dicioriai de l o s aydiogramas e s p ~ . f f i c o s y de l o s datos de l a audiometria verba l d e s p d s de L. beccifin. s iguiente sobre audiome t r í a verba l .

INTERPRETACIONW AUDIDaETRlCAS.

C i e r t os trastornos o enfermedades producen audioaramas c a r a c t e r í s t i co 3.

46

&1 a c o r r i r es tos padecimientos, se observan audiogrqas tbpicos. S in embargo, la inversa e s menos exacta. Todo audiograma puede corresponder a m s de un ,n&logo f i s i opa to i6g i co , Los e j e mpicos que se nuestran a contunuacnón sugieren es ta r t lac iones . ( Si hien muchos de estos trastornos son b i l a t t r a l e s , para esc la reces l a exposición por i o f r e gu l a r s emes t r a s610 un oído),.

OTITIS SEROSA

Iü audiograma que manifieCa t ras to rrios de l a conducción y que sB - observa con mayor frecuencia, es e l qur corresponde a l a o t i t i s --- media serosa. La curva&e l a conciuccibn &erea es relativamente plana, e x i s t e un grado de p fd ida l e v e a moderado, y be observa por l o regu lar una d f f e r enc ia aire-hueso en todas l a s frecuencias, se& - se muestra en l a f i gu ra 3-7. Los p r o g r m s de audifomekrfa m a s i í k que no toman en cuenta pérdi- das menores de 20 dB. probablemenhe no aebcubran es te t i p o de o- t i t i s serosa moderzjda.

O T I T I S MEDIA AGUDA, TIHPANOSCLEROSIS Y DESARTlCULACION DE LOS HUESILLOS

La impedancia mayor en l o s casos de o t i t i s media purulenta aguda, timpanosclerosis o d e sa r t i c dac i 6n de los hues i l l os produce una - d i f e r enc i a mayor aire-hueso y una disminución de la,conducci6n, se- gún se muestra en l a f i g u r a 3-8. M t e s e , s i n embargo, que l a puntug c i ón de discriminación es a l t a a pesar de l a pérdida mayor en e l um - bra1 ae l a conducci6n aérea.

PRESBIACUSIS

Es pos ib l e que e l proceso de envejecimienSo sea l im i t a a una s o l a parte a e l sistema audit ivo, por l o que e l audiograma de presbiacusia es poco aritivipabie. Se han sugerido cambios metabólicos, cascualres, a terosc lerOt ivos , cocie,res, mecánicos, - sens i t i v os (órgano de Corti ) , y también nerviosos degenerativos. ES razonable pensar que se prodece un audiograma d i f e r en t e prrra cada uno de e s t os c w b i o s , pero desde e l punto de v i s t a e s tad l s t i co - se observa un audiograma vvcarac-berístioovv ( f ig. 3-9), que pone de - manif iesto disminución de l a agudeza audi t iva , mayor& e l caso de l a s frecuencias a l t a s e inex is tenc ia de UTZ d i f e r e m i 5 ,ire-hueso.

y no

\

\ <

'SRT ~ UMBRAL Of RiCfKlMIViRB*L . FIG. 3-4. Umbrales por igual reducidos de conducci6n ahre. y 6sea. datos c@iac ler ls l i c~~s i!e 'a

deficiencia eudiliva semoringrv'nsa.

r1G. 35. Umbrales do conducción iere. y ósea de reduccibn desigual y persistancia de la diferriicia aire.huero, dalos caracterislicos d. la deficiencia rudiliva mixta o combinada.

I ,. *~ ~ ~~ ~. -. ~~ ~. , .- ~. - - __ - . .. .- I

48

reduce la discriminación verbal , que a menudo es menos de l o que podrfa preverse con base en e l mo e l o de audiograma que se obtiene- por f i l t r o puro. Esta insu f i c i enc ia en l a discriminación, mayor de l o que podrf, esperarse con base en e l pudiograma, se ha llamado - "regresión fondmica" y se cree que se debe a fac tores nerviosos, cz nt ra l es o de ambos t ipos. En l a f i g . 3-10 se muestran l o s auüiogramas de conducci6n d aérea - promedio en d i f e r en t es grupos de edad sucesivos que se estud,rion en l a f e r i a e s ta ta l de Wiscoinsin en 1954. E l audiograma promdio @ var io6 pacientes de O0 aiiosse muestra en l a f i gu ra 3-11 junto con - l o s resultados promedios d e l a audiometrpia verval . En l a f i gu ra 3-12 aparece un auaiograma neurosensorial plano que - hace pensar en afecta.ci6n de l a e s t r í a vascular.

SORDEitA POR RUIDO

En l a figura 3-13 se muestra l a mesca carac te r f s t i ca a 4000 Hz - ( 3 O00 a 6 O00 Hz) que se debe a exposición continua a l ruido. La muesca se hace 114s profunda y ,ncha al prolongarse l a exposición, desaparecer e l tiempo de recuperaci6n y aumentar l a intensidad, en especia l en e l caso de a l g u n o s pacientes. Nótese cómo a l a postre - es afectada l a discriminación vebal a madida que l a pérdida se en - cancha para trastornar primero e l ,udiograma a los 2000 Hz ( f i . 3-b y luego a l o s 1 O00 HB. ( f i g . 3-15).

OTOSCLEROS~S

A causa de l a naturaleza progresiva ae l a o tosc leros is , SOQ necesa r i o s por l o menflos dos a t r e s audiogramas"qtes que se p o n e de ma- n i f i e s t o l a figura guciiométria caracter fs t ica . Elprimer audiograma en l a f i gu ra 3-16 muestra una compresipon a l a s frecuencias bajas que se debe a l a f i rme -~a de l a c,dena de los hues i l los (anquilosas). La muesca de l a conducb5n ósea a los 2 O00 Hz se l l a m a "muesca de Carhart". El audiograma se aplana en l a f s e s igueinte y se h i f i e -

s t e una d i f e renc ia aire-hueso en l a s fases 1 y 2 (Fig. 3-17). Ea - l a t e rcera fase ( f i g . 3-16) ha empeorado l a conducción ósea y que& afectada l a audición de l a s Srecuercias más a l t a s l o que h,,ce pen- ear en o tosc l e ros i s cocle,r. Es f á c i l percatarse, puntuación ae discriminpcfon, por qué debe hactrse l a etapa3.

. HIUHOPESIA ENUOLINFATICA

Otra disminución f luctuante de l a agudeza audi t iva prác t i ca de audiometría en se r i e , es l a hidropesia En es ta enfermedad a menudo se ponen de manif iesto

con base en l a c i rug fa antes de

que j u s t i f i c a la endolinfática. E una pérdida ne-

urosensorial a las bajas frecuencias ( f i g 3-19) una pérdida plana & ( f i g 3-20) y una pérdida en l a s a l t as frecuencias( f ig 3-21), o biem se observan todas estas t r e s carac te r i s t i cae en cuestión de algunos días. E l grado de hipoacusia puede v a r i a r a veses de 30 a 40 dB. La

I-

C

c

I

I

ir

c

L

P

Y

c

L

r u

1 Li

1 v

U

in

5 7 0

$ 0

8 '30

- m - d<l E

* 2 60

7 10 ." > I

f 80

90

100

110

E;&--K.AGZf-l 1 'SR1 - WMl D( RfCCKlON V f U K

FIG. 3-8. Rasultadot audiornetricos csraclerí61icos de la desarticulaci6n da la CadeN de huesillos, 1 tlrnpanosclerosis u otilis media agudl grava.

N

*...

.I

I- - L

r-+

L I

L

+-"

c

I

SSTUDIOS,ESPECIALES: PRUEBAS DEL RECLUTAMIENTO ANOHMAL DE LA INTE / S I D A D SUBJETIVA, DE ADAPTACION Y DE BEKESSY

L a e x i s t e n c i a o falta de una d i f e r e n c i a a i r e hueso apor ta pistas iq por tantes en l a d i f e r i e n c i a c i ó n d e l t i p o conductivo o neuroaensori- al de hipoacusia . S i n embargo, e s t o s fenómenos cercanos a l umbral - t i e n e n muy poca r e l a c i ó n con las e x p e r i e n c i a s por arriba d e l u m b r a l que ocurren durante l a audic ión c o t i d i a n a . El umbral de recepc ión v? v e r b a l s e encuentra también en l a misma c a t e g o r i a de e x p e r i e n c i a a- penas audible . E l estudio de l a d iscr iminac ión v e r v a l , por su par te en forma s i s t e m a t i c a s e l l e v a a cabo a una in tens idad s u f i c i e n t e co mo para encontrarse en una c a t e g o r í a de intencidad s u b j e t i v a prome- d i o , y a v e c e s s e mide deliberadamente a l a " intensidad m&a cómoda" L a comparación e n t r e conducidn a é r e a y ósea puede producir sólo dab t o s genera les sobre e l s i t i o de l a l e s i ó n , s i b i e n posiblemente d e l l a forma audiométrica s e der ive informaci6n adic ional que permita - h a c e r r e f l e x i o n e s más e s p e c í f i c a s . E s t e estudio i n t e n t a a i f e r e n c i a r t rans tornos conauctivos de i o s de- t i p o s e n s o r i n e r v i o s o s , y l a r e l a c i ó n aire-hueso de l a pérdida por - l e s i ó n en e l octavo p a r , e s exactamente i g u d a l a r e l a c i ó n aire-hu eso que s e observa en un caso de a f e c c i ó n d e l c a r a c o l de los trans4 tornos d e l octavo p a r , son n e c e s a r i o s datos a d i c i o n a l e s . A demá de l a in tenc idad cercana al u m b r a l , l a in tens idad por arriba de # s t e puede producir información d i a g n ó s t i c a v a l i o s a . E n algunos p a c i e n t e s , un determinado aumento de l a in tens idad produce un ,peque n o aumento l i n e & de l a in tens idad s u b j e t i v a . Otros p a c i e n t e s , e s t i mulados con e l mismo incremeato de in tens idad, perc iben una experi- e n c i a mucho mayor en l o que s e r e f i e r e a intensidau s u b j e t i v a , como- s i e l incrementode de h i n t e n s i d a d o b j e t i v a f u e r a mayor. E l fenó- meno puede observarse en e l mismo p a c i e n t e al comparar un oído cone e l o t r o , de l o que s e deduce que e s t e fenómeno no guarda r e l a c i ó n - con causas neuro iógicas c e n t r a l e s .

PRUEBA DEL EQUILIBRIO BIAURAL ALTERNADO BE LA INTENSIDAD SUBJETIVA

En 1 J Z ü fowler hizo n o t a r a l o s médicos las v a r i a c i o n e s en e l i n - crenento de l a in tens idad s u b j e t i v a p o r arriba d e l umbral, y en los s i g u i e n t e s c r e ó una prueba clásica de rec lutamiento de l a i n t e n s i d a s u b j e t i v a que s e llamó prueba d e l e q u i l i b r i o b i a u r a l a l ternado de h in tenc idad sub je t iva(BAi ) . Se d i c e que e x i s t e t treclutamiento anormd. pos i t ivode l a in tens idad s u b j e t i v a e t o "rec lutamiento pos i t ivo i i c u d do s e o b s e r v a un aumento anormalmente rápido de l a intensmdad s u b j e t i v a .

. . . . . . . . . . . . .

..

,.,I.

...- I*

i _.

E s t a prueba, que s e hace con tono puro, compara l a i n t e s i d a d sub je4 t i v a que s e p e r c i v e en un ofdo, c o n l a que s e p e r c i b e con e l o t r o - a i conservar l a f r e c u e n c i a constante . Desde e l punto de vista i d e a l e s n e c e s a r i o que e x i s t a un oído normai o un oldo con umbral normala l a f r e c u e n c i a de l a prueba. En r e a l i d a d , l a a l a frecuencia de l a - prueba e s a h v a l e d e r a si e l oldo más s e n s i b l e e s normal, a c o n d i c i 6n que e l aumento de in tens idad s u b j e t i v a s e a más l e n t o en e l oído mejor que en e l o t r o .

AUDíOMETRíA DE BEKESY

Es p o s i b l e una representac idn gráfica ae l a d e c l i n a c i ó n t o n a l por - medio de l a audiometria de Békesy. Georg von Békesy (1947) c r e b una audiómetro que l l e v a s u nombre y que #$/&$d,$$p@h en forma automhti c a r e g i s t r a e l audiograma d e l p a c i e n t e cuando B t s busca e l umbral. S i e l tono e s a u d i b l e , e l p a c i e n t e l o senala a l oprimir un bokon qie en forma automática , reduce l a in tens idad d e l est ímulo. Al s o l t a r e e l boton cuando e l tono ya no e s audible Se produce una i n v e r s i ó n - de l a d i r e c c i ó n d e l cambio de l a in tens idad y l a magnitud a e l sonidi coinienza a aumentar una vez rn&. puede f i j a r s e l a f r e c u e n c i a por se parado como en l a audiometrla de tonos puros, estanaar . Em l a f i g u ra 3-38se muestra un ejemplo de audiograma de conducci6n a é r e a de Bdkésy que s e compra con un audiograma c l í n i c o e s t h d a r de tomos - pure s . Una de las muchas a p l i c a c i o n e s de l a audiometria de BeE6sy e s e l r e g i s t r o g r f i c o de las d i f e r e n c i a s d e l umbral cu,ndo e l ,. estímulo - e s p u l s á t i l y cuando e s continuo. En las figuras 3-39 y 3-42 se - muestran los r e s u l t a d o s de compraraci6n de e s t o s dos t i p o s de e s t i - mulos en cuantro p a c i e n t e s . En e l t i p o I, los r e g i s t r o s s e sobreponene, e s t o e s , no e x i s t e dife r e n c i a de i a r e a c c i ó n a l o s est ímulos pUsRti ls o continuos. e l - t i p o I1,los r e g i s t r s o s e s0bre:porien aproximadamente hasta l a reg ión de las frecuencias m dias, s i t i o donde e l r e g i s t r o continuo s e hace menos s e n s i b l e que e l d e l est imulo pulsatil y a menudo s e e s t r e c h a . En e l t i p o 111, e l r e g i s t r o d e l est ímulo empeora rhidamente y a pe sar de producir es t imulac i6n de in tens idad máxima e l tono cont inuo ya no es a u a i b l e . E x i s t e una separación notable de los r e g i s t r o s - en e l t i p o IV, y e l r e g i s t r o d e l est imulo continuo e s de 20 a 25 - d e c i b e l e s menos s e n s i b l e que e s est&mulo p u l s 6 t i l . E1 t i p o I de Békésy s e observa en l a audic ión norm@, la hipoacucia conduct iva y algunos tras tornos c o c l e a r e s l v e s . El t i p o I1 s e aso- c i a con h ipoacus ia c o c l e a r . Los t i p o s III y IV s e a s o c i a n con trastornos d e l octavo p a r , si biem e l t i p o I V tal bién en e l caso de l e s i o n e s c o c l e a r e s . Buchos c l í n i c o s coniiideran que l a amplituddel t r a z i e en audiograma de Békésy e s signo de l a e x i s t e n c i a o no de rec lutamiento. Concluya que e l tram e s t r e c h o de una amplitud d e 3 a 5 d e c i b e l e s s e acompa ñ a de r e c l u t a i e n t o . Debe t e n e r s e cuid-do g,l h a c e r e s e t i p o de dedu;

-

vez s e &serve tam- -

- 52

...

L

P

i-

c i

c

L

i

r

i

L

c

... c i o n e s pues, como en e l Caso d e l es tudio d e l umbral de diferer.cia, ia personal idad y o t r o s f a c t o r e s pueden prouucir v a r i a c i o n e s importan- t e s en la r e a c c i ó n O l a p rueba. Debrá usarse al pac iente como su propio c o n t r o l ai comprar un oído c o n j x a e l opuesto, o comparar una f r e c u e n c i a con l a o t r a en el miEimo oídom cuando los umbrales d i f i e - r e n con l a f recuenc ia .

i-

L

r L

....

s

i

F

AUDIOMETRIA DE LA IMPEDANCIA

L a a p l i c a c i ó n c l í n i c a de l a mt-dición acústica de l a impedancia no '-

general126 s i n o hasta l a in t roducc ión r e c i e n t e de aparatos que permiten mediciones r e c o n o c i b l e s y r e p e t i b l e s y que s e c a r a c t e r i z a n pcr s u faci l idad de manejo. La audiometria de l a impeaacia ha jug@do u1 papel cada ve , más importante en e l corqunto de evaluaciones audio; iógivas en s e r i e , y l a mayor par te de Ires c l í n i c o s e s t a r í a n de acue - rdo en que l a u t i l i d - d c l í n i c a de l a ,udiometria ae l a impedandia - r e s i d e eh, l a r e i a c i ó n particularísima e n t r e e l audiograma y las dos mediciones didmicas, e s t o e s , el. timpanograma y el. umbral d e l re-- f l e j o a c f i s t i c o . A cont inuac ión s e encontrará una breve d&scr ipc ión de ambas pruebas y de sus a p l i c a c i o n e s c l í n i c a s .

TIMPANOMETRIA A b

La timpanometria e s una medición i n d i r e c t a a e l a e l ast icidad (movi l i d a d ) d e l tímpano y s i s tema de h u e s i l l o s b a j o condic iones de per- s i ó n p o s i t i v a , normal y negat iva . Se introducel e n e r g í a a c ú s t i c a . ( n i v e l de p r e s i ó n de sonido de 95 a+) en e l o í d o p o r mtdio ae un tubo sonda; p a r t e de e s t s en rgía xse absorbe, mientras e l r e s t o s e re- f l e j a hacia a f u e r a a p a r t i d e l conducto audi t ivo y penetra en una - segunda p e r f o r a c i ó n d e l tubo sonda. En e l o í d o normál ,e l disposi . - t i v o de medición pone de manifiesto que l a energfa que s e r e f l e j a - e s menos que l a e n e r g í a que s e h,ce l l e g a r al condunito. Por o t r a - p a r t e , cuando e l o í d o e s t á l l e n o de l i q u i d o , e l tímpano e s m6s grue so o e l sistama osicualax . más r í g u a o , ai e n e r g i a que s e r e a e j a es- mayor que l a que s e observa en e l ofao normal. La Cantiñaad de ene rgia aue s e r e f l e j a e s m&s c e r c a n a a ia cifra de l a e n e r g i a or ig in$ de un n i v e l de p 4 e s i ó n de soniuo de 96 dB; a veces l a enarg ia que - s e r e f l e j a e s casi i g u a l a la o r i g i n a l . El timpanograma e s una representac idn grfica de l a e l R s t i c i d a d y de l a impedancia r e l a t i v a s en e l s i s tema t impanoosicular mientras dse producen cambios tn l a p r e s i ó n a é r e a (pos i t iva . , normal, negat iva) en e l meato e x t e r n o , p o r medio de un t e r c e r conducto d e l tubosonda. Las pres iones pos i tkva y n e g a t i v a reducirán l a e las t i c idad-de l sistema t impanoosicuiar y, así pues, aumentará l a cant idad de energ ía

-

~

que s e r e f l e j a (aumento de l a impedacia y disminuci6n de l a e l a s t i c idad) . -

LI

?-

-- FA

MILLIMHOS @220 HERTZ

-L

O O N P a> Q, o o o o o

MILLIMHOS @ 660 HERTZ

z P

I

.

.- *I

.- F

1

c

c

L

c

ii, -

!

- 200 O PRFSION en mm de H20

FIG. 3-43. Tirripanograma normal. La zona punteada es los límites de la variaci6n normal de la elasti.

cidad. . . . - ~ . . . -

..

c

CC

Se observará l a e l a s t i c i dad máxima (disminuiibn de l a impedancia) cuando l a presión d e l a i r e es normal (presión ambiente), y es te - $actor de l a e las t i c idadse reuuce a meaiua que l a presión aérea - se hace aumentar o diminuir en I r e l a c i ó n a las c i f r a s normales. En l a f i g u r a 3-43 se muestrael timpanograma que manif iesta estos cambios de l a e1asticidad.a meaia que se hace v a r i a r l a presión - d e l a i r e en e i meatu externo de un sistema timpanoosicualr norami Téngase pesente que en los suje tos de audición norm,l y en l a ma- y o r parte de l o s pacientes que sugren hipoacusia neurosensorial, se observar4 un smstema timpanoosicular n o r m a l . Jerger (1.370) ha creado una c a l s i f i c a c i ó n de l o s timp,nogramas - que toma en cuenta sus c e r a c t e r f s t i c a s normaLes ademAs de l a s que producen va r i o s t i P o s de trastornos oto lóg icos . S ib i en - , ex i s t en a l gunas divergenc ias sobre l ü : j d i f e r en t es t ipos , l a c l a s i f i c a c i o n se ha aceptado ampliamente. Los d iversos t i pos de l a c l a s i f i c a c i ó n que se muestran en lia f i g u r a 3-44, son l o s siguientes: T ipo A (timpanogrcma normal). Ocurre l a e l a s t i c i dad máxima a l a - pres ión ambiente o cercana aeLSta, - l o que supiere que l a p res ión en e l oído medio es normal. Tipo As. Ocurre l a e l a s t l c i dad máxima cuanuo l a presi6n aérea es - i gua i o cercaria a l a ambiente, pero l a c i f r a absoluta de l a elas- t i c i d g d es menor que en e l t i p o A. A menudo e x i s t e f i j a c i ó n o ri- gidez de l sistema de hues i l l o s cunndo se observa e l t i p o As. Tipo ad. Ocurre una e1,sticidad d x i m a considerable a l a presión ambiente, y l a e l a s t i c i a ad aumenta en forma extremadamente rápida a medida que l a s presiones se acercpn a l a c i f r a de l a presión - ambiente normal. E l t i p o Ad se asoc ia con discontinuidad ae l o s - hues i l l o s o con una membrana timpáica monom6rica. Tipo B. E l timpanograma es rel.ativamente '*plano** o en forma de - "domo'* y se observan pocos camtiios en l a r e f l e x i ó n de l a energfa a p a r t i r d e l sistema timpanoosicuiara medida quecainbian l a s presiones de l a i r e en e l conducto auditTvo externo. E l timpanograina de l t i p o B se re lac iona con l i qu i do en ofUo medio, engrosamitnto d e l tímpano o impacción de cerumen. La impedancia uei sistema t i m panoosicula,r se ca rac t e r i a sobre todo por l a imposiblidad de p r o auc i r compresión. Los camljios pequeños de l a presión aérea producen un e f e c t o muy l e v e . Tipo C. Ocurre l a esast ic idad máxima cuando se p roduce una pre- s i ón negat iva mayor de 100 mm de H/O en e l conducto externo . E l examen OtOSCópiCO por l o regu lar pone de Is c l a i s t l c a c i on a medida que se d i v e r s i g i gue e l equipo. OTRAS APLICACION.&S CLINICAS. Adends de permi t i r inrormkción sobre l a impemcia y elg+ticd,d bol sistema t impanoos i cmr , La audio- metría de l a impedacia pude s e r v i f para evaluar o trps funciones en r e l a c i ón a l a presión. Una de e l l a s es e l funcionamiento de l a trompa ae Eustaquio. Cuariao e l tfmpano es tá in tac to , l a presión - d e l o ído medio no menor de LOO mm de H - O. pone de manif iesto que l a trompa funciona de modo normal. Cuando e x i s t e perfoPaci6n d e l t i m - pano, e l examinador p w d e adapatar e l manómetro de modo que produz-

56

IMPEDANCIOMETRIA

TIMPANOORMA CURVASTIPO:

3) MJfomieci6n dd CAE.

REFLEJO DEL MUXULO ESTAPEDIAL btfmulo sworn an O. I. PM obtar nipnm an O. D.con275cpd.pug..

0.6Khz.I 1 Khz. I 2Khz. [ 3 K k . I 4Kh2. I I I I

1 I I I 1

Estimulo lonom ri O. D. prr obtener RgursrmO. i.am2iücp1.depnnbS.

0.6Ktu.l IKhz. I 2Khz. I 3Khz. I 4 K k I I I I

10

6 4

O

10 * c 4 a O

10 *

'6

4

2

O

mmH20

OD 0 o1 0

"A"

O D 0

o1 o I "B"

O0 o 01 o "c"

OD

o1 o "D"

OD 0 o1 o "E"

10 s 6 4 2 O

A I-

mmH20

DBSERVACIONES:

x 1 w - s ~ 4 a t i o 1 t 3 4 + - - -

-..

ca una pres ión de -200 mm ue H O, paraluego t r a t a r de ciescubrir cam b ios de l a presidn cuando e l paciente ueglute. Si p l a presión que - se produce se encuenrLra entre -1.00 mm de H O y l a presión a6rea am - bientd;despu$ ae deg l u t i r e l paciente var ias veces, se puede consi - derar que e l funcionamiento de l a trompa de Eustaquio es aaecuado. Este método puede usarse para v e r i f i c , r permeabilidad de l tubo de - vent i l ac ión . S i e l tubo de Be;it i lación e s t á bloqueado y no e x i s t e líquido en e l o í d o medio es más probable que ye obtenga e l timpanograma de l t i p o - A. S i e l $ubo de v en t i l a c i ón funciona en forma adecuad,, ocurr i rá - abertura esponatánea de l a ' trompa de Eustaquio cuando l a presión - d e l manómetro sea mayor de 200 nun üe H O. Puede conservarse l& presion negat iva s i e l paciente no deglute.

REFLEJO ACUSTICO

E l r e f l e j o acúst ico que se produce por tonos puros o ruidos, ha - sido un instrumento diagnóstico importante para l a evaiuación d e l umbral aud i t i vo y conocer e l s i t i o de l e s i ó n en l a v í a auQi t i va . El r e f l e j o ocurre en forma b i l a t e r a l aún s i e l estímulo se introduce en un s o l o oído. La contracción d e l m6sculo d e l es t r ibo , que se inse r ta en l a cabeza d e l hues i l l o homónimo, produce una mayor r e - &den en e l smstema timpanoosicular. En consecuencia, se incrernen- t a l a impedancia, que se manif iesta por aumento de l a . r e f l e x i ó n de l a energía d e l tono de ba ja frecuencia quese introduce en e l oído que se estudia. L a intens&.d d e l estimulo que basta para causar e l r e f l e j o en suje tos de audición normal, v a r í a de 70 a 100 dB - cuando se t r z t a de -tono puro o de ruido de tanda estrecha, y es un poco menor en e l caso de ruido blanco. La hipoacusia conductiva un i l a t e ra l l e v e a moderaaa (n i v e l de au- d i c i ón de 30 a 40 dB) hará aumentar, en una magnitud equivalente a l grado de trastorno c o ~ d u c t i v o , e l umbral de l r e f l e j o acústico cuando e l estimulo se introduce en e.1 oído trastornado. Tambipen se ha notado auaento d e l umbral d e l r e f e j o cuando las pérdidas coc leares son mayores de un n i v e l de aud&cjm&,nae 60 a 65 dñ en e l oído que se es timula. C i e r t os procesos patolópicos se han acompañado de aumento de l umrird d e l r e f l e j o . La inex is tenc ia d e l r e f l e j o acds.tico puede deberse a va r i o s facto- r es , entre o tors , hipoacusia neurosensorial impol'te en e l o$do que se estimula, l e s i ó n r e t r ococ i e a r en e l mismo o ído mencion,do, a- taque d e l nerv io f j c i a i d e l l ada de l estímulo, unexistencie congé n i t a de l e s t r i bo , ext i rpac ión qu in í rg i ca de l es t r ibo , y p fd ida COG - duct iva moderada o mayor en e l o ído que se estimula. E1 r e f l e j o acúst i ce puede aocurr i r pero t a l ve no sea mesuable. Por e jempio, e l aumento l i g e r o de l a tensmón qÜe cama l a contra- cci6n d e l músculo d e l e s t r i bo no modif ica en forma importante l a r i g idez de l a cadena de hues i l l o s en los casos de discontinuidad t i m = panoosicular. pues l a contracción de e s t e mfisculo no puede , f e c ta r l a r i g i d e z deL Listema.

PSICOACUSTICA CONTROLADA POR MICROCOMPUTADORA EN AUDIOLOGIA CLINICA M.E:Lutman MRC I n s t i t u t e o f Hearing Research, C l i n i c a l outstatión, Nottinggham Hearing s e r v i c e s centre , general hosp i ta l , Nottinggham NGI?GHA, UK Audi log ia c l i n i c a : s e pueden c l a s i f i c a r en sub je t i va y ob j e t i v a , l a s tecnicas de medie c i6n en audilogia. Las pruebas sub je t i vas conf ian en l a respuestas- desarrol ladas por e l paciente de acuerdo a c i e r t a s intrucciones, do nde s u respuesta contribuye para medir c i e r t a varmable. Las pruebas ob j e t i v a s solamente requieren cooperación de l paciente, en e l s en t i duo que s o l o debe e s ta r sentado y t o l e r a r l a s ataduras de l o s elec- trodos de medicidn . La mayoria de las pruebas son d e l t i p o sub j e t i v a . L a rut ina c l i n i c a inc l eye mediciones de senc ib i l idad como fun c i ón de l a frecuencma, mediciones de adaptación de ba ja intensidad de tonos continuos y mediciones en e l rango dinamico de intensidad. Se u t i l i z a n sonidos para condiciones aereas y vibraciones para con- d ic iones oseas. En U.K e l 1776 de l a pobla.ci6n t i ene notables perdidas de l a audicibn, y por o t r o lado en l a población cor r i ente , se ha estimado que - e l p r inc ipa l problema, es l a soedera sensor ia l , que generalmente no se resuelbe por interbención quirurgicg. En un suje to normal e l o í & es extremadamente s e l e c t i v o y puede considerarse quex t i ene un gran f i l t r o s e l e c t o r de frecuencias, E l e f ec to de l a perdida de l a . audi- c i ón sensor ia l reduce e s ta se l ec t i v idad . Esta puede s e i l a razón - por l a que pacientes no pueden entender palabras en l a presencia de sonidos competiitivos. Un metodo para apres iar es ta habi l idad e s l a detención d e l @(laguna) , donde e l GAP es disminuido a i maxim0 pa r a que e l su je to pueda detectax- . Un ejemplo de La forma de onda. estimuladora para detención a e l GAP esx mostrada en l a f i gura . En suje tos con perdida de l a audici6n sensor ia l , e l GAP debera s e r g:rande para s e r escuchado, comparado - con un oido normal . Fuera de las pruebas psicoacusticas, es necesa r i o para cont ro la r l a presentación d e l sonido, ; estimulos acumulabB y r epe t i b l e s en terminos de su intensidad y duraci6n. La funcidn de l a microcomputadora es l a de combinar l a s condiciones de l estimulo de acuerdo a cada su je to ; y esto por medio de un dgo r i tmo que es ei

que describe es te a r t i cu l o .

> ’ c - ’

psycwsoacoustrcs lltl Clinical audiology M. E. Ixtman MRC lnsrirute q/ Hewing Research, Clinical Outsrution, Norringhamshire Hearing Services Cenrre, Gerwral Ho~,,iial, Norringham NGl 6 H A , U K

This article describes the &sign und implemenration ofa microcomputer-controlled experime~ttol rig for p ~ ~ ~ l ~ o ~ c o ~ r s t i c ~ l resting in u clinirol settiny. F i rs t l j , the role oJpsychoacousricul m i s in clinical uudiology ir rrriei! rd briefly. Some i.omn1on test procedures are described. The requirements of the hardware und .ío/fiwre ure oiilliiie~l togelher II irh /h& r<ulizuiion. Finally, the performance of the systein und its usefufnrss we noted.

~ . - ~ .~ ~ ~- ~-~ ~~~~

Clinical audiology

in general terms, clinical audiological investigation involves measurement of many aspects of auditory and vestibular function, plus assessment of the presence and characteristics o f tinnitus. The various measurem‘ent techniques used may be broadly classed as ‘subjective’or ‘objective’. Subjective tests rely on the patient performing a task according to instruction& where his task performance contributes to the smeasured..~ . variable. Forexámple, he may be rquired to press a button when he hears a particular sound. Objective methods only require co-operation from the patient in the sense that he must sit or lie and tolerate the attachment ofmeasuring electrodes or probes.

The majority of tests of auditory function are of the subjective typc. Routine clinical assessment usually includes measurement ofhearing sensitivity as a function of frequency, measurement o f the dynamic range for intensity, measurement of adaptation to low-intensity continuous tones and assessment oispeech identification perlonnance in quiet as a function ofthe speech intensity. Hearing sensitivity measures (the ‘audiogpn) are obtained using air-borne sounds (air-conduction) from earphones and vibration (bone-mnduction) stimuli from a vibrator applied to the skull. The latter sounds can bypass the middle ear and are therefore not affected greatly by middle-ear defects.

The above set of tests is intended primarily to distinguish three major classes ofhearing impairment. Conducriue hearing losses generally involve mechanical defects in the middle-ear sound-conductingmechan¡m and aredetected by difieremes in sensitivity lor a iranduct ion and bone-conduction sounds. Sensory hearing losses involve defeas in the sensory apparatus of the cochlea whicb transduces mechanical vibrations into neural implses , and are characterized by a restricted dynamic range, normal adaptation and mod5rate-to-good identification of amplified speech in quiet. Neural hearing losses involve defects in the auditory nerve, or higher centres. processing auditor) inforinition and arech:ir:iclcrirrd by a wide dynamic r:rige, excz5xiie adaptation and poor spcech discrimination in +,iet. rcgardl<\s of amplification. 7hc t m n reii<+rinwUrul sub- C , : ~ L ‘ S thr tun latter classes and is appropriate in that it i5 oRen

by sensory damage, and to a certain extent, vice versa. In the UK, 17% ofthe population have a noticeable hearing

loss, and preliminary data írom a current population siirrey o f hearing [I] has indicated that by far the most common t y p e of hearing disorder is a sensory hearing loss. Thus, aithough the routine measures described above are uxeful in idcntiiyiiig conditions for which there are direct means ofintervention (for example middle-ear defects, eighth nerve tumours), they do not subdivide the major sensory category.

More detailed subjective measurements ofhearing functions have been used for many years in hindainental rcsearch into the auditory system, predominantly in normally-hvnring subjects. This type ofstudy is encompassed by the term psychoacoustics, and more recently interest has becn directed to psychoacoustical investigation o f subjects with impaired hearing. Such measures allow a fuller description ofhearing function than a set ofroutine clinical tests and may provide a better understanding of the problems of speech identification experienced by inidividuals.

One psychoacoustical measure which has received particular attention during the psst decade is that of frequency resolution, which is the ability ofthe ear to selectively detect a signal at one frequency in the presence of a competing sound at another frequency. In normal subjects the ear is extremely selective and can be considered to behave like a highly timed filter [z]. The eñect of a sensory hearing loss is to reduce this selectivity, or broaden the putative filler 131. This may be one reason that patients with such hearing losses find i t particularly difficult to understand speech in the presence of competing sounds, for example in a noisy environment. Also in quiet, the lower frequency formants (resonances)ofspeech may mask the higher frequency formants. Similarly. sensory hearing impairments may reduce the ear’s ability to resOlve the temporal structure of sounds [4], w&@ isalso impottant,in speech identification. One method of assessing this ability i s that ofpap detection, where a gap in a noise burst is adjusted to the minimum that the subject can detect. An example ola typical stimulus waveform for gap detection is shown in figure I . In subjects with sensorineural hearing losses the ?ap must be grater to he heard, coinpared with normal ears,

In ordertocarr) iiiitpr)cho;rcni~~ti<ral tests,jt i < n t y e < u r j to control the prrsint:rtion of bound stii:iiili accuri+telv :iv,d re-

, , . ~. , , . . * , “ , .. ..

1...

r

7

I__._ J O 500 1 ow

Time (ms)

Figure 1. Sfiniultis rncrlope and timing suiruble f o r meiiruring gap derecrion. The gup duration i s represented by ‘T‘.

ithm which depends on previous responses by the subject. Such characteristics are well suited to control by microcomputers and this artick describes the realization of a flexible facility for psychoacoustical testing under microcomputer control. The above examples of frequency and temporal resolution are used to illustrate the properties of the system and how it is used.

Psychoacoustical methods

Many psychoacoustical procedures involve the adjustment of parameters of the stimuli presented to the subject in order to obtain a target performance, rather than relying on differen- tiated responses by the subject to fixed stimuli. For example, the intensity o l a tone in a background o f n o i x may be adjusted to threshold; that is, the minimum iniensity at which the tone is correctly detected on 50% of trials. Alternatively, the duration of one toie may be adjusted until it is pcrceived as having a duration equal to that of.another bxedduration tone. Several classical psychometric methods exist which govern the way in which the stimulus parameters are altered. The method chosen for the test system described here is the so-called adaptive method. In the example of threshold iniensity o f a tone in background noise, the intensity ofthe tone would start at a level uhich could be detected on 100% of trials. It would then be decreased in fixed steps until it was detected on less than S q < of trials. Subsequently it would be respectively increased and decreased using smaller steps l o progressively ‘home in’ on the threshold.

Any psychoacoustical proccdure requires a means of establishing a correct response from the subject which makes allowance for guessing. A commonly used technique which is particularly amenable to computer-controlled experimenis is that o f three-alternative forced-choice (3AFC). illustrated in figure 2. Three presentation intervals occur which are often indicated by small signal lights: The critical stimulus which the subject has to identify is only presented in oneinterval, allocated at random for each trial. In the gap detection task, the no¡= burst would he presented in all three intervals but in only one interval uuuld it contain a gap. Tñe subject’s task is usually to press a button which correspo?ds to the critical interval, or to guess i f he does not know. 0,Mn feedback is giren iizing s i p d lights to inform the subject subsequently ofthe correct respons. One advantare of3AFC is that i t is not necessar) to describe the n:iturr of the critical inlenal to the subject, but simply that he must listen lor the intcnal which contains a stinlulus wliich appcdrs to he iditlcrciit froin the other two.

One mtawrc. oíficquznc) ie\ioIuiion i( the ps?chnacoustial t i i n i n x tunc iI’IC4. The technique uxcd to .>hiain a P-rc 84

~. ~-

L- L 1 - I - - - L - - L - - I J O 1 2 3 4 5 6 7

T m e (I)

Figure 2 . TjpicuI leniporul urrungeniriits of indiculor lights and stimulus preseiitution in u three-ulternutiae-

forced-choice (3AFC) paradigm used ro meusure gup derection.

iniolves presentation to the subject o í a probe tone (signal) at a fixed intensity, usually I O or 15dB above his threshold.. This signal can be detected on 100% of trials when there are no competing sounds. A masking signal (masker), often a narrow hand o1 noise, is introduicd the frequency of which is antred above or below the signal fr7,quency. The intensity ofthe masker is adjusted to the maximum level which allows the signal to bc detected on only WA of trials. Such masker levels are determined for various masker kequencies to plot out the PTC. PTCs are illustrated in figure 3, where examples are given for a normal ear and for an ear uith a sensorineural hearing loss. I1 c3n be seen that the I.itter demonstrates considerably reduced frequency resolution, akin to the broadening of a filter characteristic.

A i x j h i e n i e s - f i i x ü s é d io ostani PTCi ia ia &ÚW& to utilize the 3AFC paradigm described above, where the masker is present in all three intervals and thcsib.nalisndditionallypmcnt in one, allocated at random lor each trial. For each fixed masker frequency, the masker is initially presented at a low intensity and then adjusted according to an adaptive algorithm until the detection of the signal appfoachcs the 50% criterion.

Instrumentation for psychoacoustical testing

n e human ear is remarkable in its ability to resolve signals of different frequencies and to handle signals over a very large range of intensity. An audio-engineer would be extremely satisfied ifhecould produce hardware which could approach thc specification o f the ear. For this reayin, the hardware used for psychoacoiistical testing must be ofthe highest possible spicifi- cation. I t is only appropriate here to list the basic requirements

’

without detailing the means of achieving them. \

( I ) Very quiet acoustical environment (<2OdB [A]). (2) High signal-to-noise ratio (>WdB). (3) LOU, harmonic and intermodulation distortion (<O273 (4) Linear attenuation (within ZdBofnoininal over l W d B

(5) Sniooth shaping of gated signals lo minimize ‘spectral

(6) High degree of signal separation between signal lines

Thsrc rsquiremcnts impose~<eiereconstraints upon the [)pes of equipment that may b e us& psnicularly ihe siym-lo-ni.i* ratio. In ihs s),ien: drsrihed in thi~ariicle,considcrati~,ii ofthe a h \ e itquiremenis led t o two íundammtal Jecisiiins in it( dzsisn. Firiily. ih: -pcifiration uc1uld.k met mort c.i\,l> b? wine a rn ix~~<u~ .p : i ~ t r sirnpl? as 4 controller. n i h ~ r 1F i n ;*I*

range).

splatter’.

(>90dB) (\ec below).

O

250 500 1m m 4 m

Marker frequency IHzI

Figure 3. Psychoacoustical tuning curi,e (PTC) typical of ears with norrnal hearing ond sensoriiieirrol hearing loss respectioely. Signalfreyireiicy= 1000 Hz.

attempting to useit to synthesize stimulus waveforms. Secondly, it would be ~ s c s s a ~ y to design and build certain elements ofthe apparatus, rather than purchasing proprietary equipment.

Hardware Theconfiguration ofthe equipment is illustrated in figure 4. The

system achieves flexibility by allowing the attachment ofvarious signal sources, such as pure-tone oscillators, tape-recorders or noise generators to each ofthe three analogue lines. Each signal line may he switched 'on' or 'off independently by means of the three£hannel gate, which has selectable rise/fall times for each channel. Signal intensity is controlled by means o f manual

,siir,,llcd hy the diial digital attenuator in 0 5 d R \I ' j

I:+, j:t , , I 1 8 , C,lB.i1,itthrcesignalliiies:iicmincil and a > i ~ ~ ~ i , í i ~ lo t x , . ; ewitcd to the subject ria a .Teliyli<iriics l1)11 19

i.Ii<-'i,e. 17acilitirs also exist (not shoiin in thc figure) 1 0 route each Tipal line sclecti\ely to either or both of two carph<ines.

The ~ar inus requirements for the system spcciricatinn out.. lined ahore, necessitaied design and ciiristruction of the thice- channel gate, dual digital attenuator and mixer, whereas safety considerations made it desirable to build a special amplifier to isolate the subject electrically from the other equipment.

Control of the status of the t h r e gates and digital attenuators,was achieved, as shown in the figure, by means ufpi~rallel outpuls from the microcomputer system, which comprises a Cromemco Z2 with associated Roppydisc drives, VDU, timing and input/output facilities, connected via the IEEE-696 (S-100) bus and using the CP/M operating system. Communication bctueen the patient and the computer has entailed the use of a purpose-built response box containing indicator lights and push-buttons amenable to the 3AFC paradigm. These were connected to the computer via further parallel input and output lines.

Computer sofiware The development o í a versatile set of software required a major investment of time in the Setting up of the test facility: approximately four man months were devoted to this task. The principal requirement ofthe software was íornon-erpert users to be able to set up an experimental procedure within a short time and uithout any reconnection of leads and with a minimum oí restrictions on the possible range o f procedures. At an early stage, it was decided to limit the scope to tbe general 3AFC paradigm and the adaptive psychoacoustical method. However, within these limitations it was intended that there should be an extremely uside range of possible stimulus presentation arrangements and a wide range of rules for the adaptive algorithm.

h,

I- input/output ports

1, 1177 SiPafienfl response box Three-channel Two-channel gate r--- - -1

I I I __

1 A I Mixer Amplifier TDH-49

Signal I

Oscillator I I I I

t ' E I 2 Tape-recorder 1 1 - I I I : I1 I I

I sources I I I I I I I I I

I I I I I

I I

I L - - - - - I 1 Noise generator , I 1 1 ' I

3

C _ - - - - I

. . a .- , . :.., . I

I1:c :"<,"i ,<: 6 l . , , , :d , , , , ' ; i ' l ; i l . , . . i , l i . , a ! , : .>;,y non pr<igi;mnicrs s ~ ~ ~ + x t w l i ' h t a single inin tihe , ' ,.,grain \hould he divclupcd, which irniild Cibtain ihc i i ~ , ~ i t i ~ l t - 1, i r .

anieters , i ra spcLif,rd tcsl proccdurc rmii a w t dcon : i~ , l íilcs. 'nicse control files would bc productxi I-cfoich.iiid uring qxrial prc1imiii:iry programs. In fact, the systrin u5es one single run- time program and thrce coiitrol files. The control files govern (1) the stimulus tiniing. p t i n g m d the iandciin allocation olstimiili; (2) the adaptive algorithm; and (3) the specific text presented to the operator during the iunning ofthe main program. A set of three control files is required lor each different procedure.

Firs1 cunirol file Figure 5 illustrates the stimulus control information which is stored in the first control file, using the gap detection procedure asanexample.Itcan beseen thatthestructureofthefileisbased on the definition of 'events' which occur during the 'cycle duration', which itself is repeated three times in the 3AFC paradigm U p t o IOeventsmaybedesignated in termsof(1) the time at which they occur; (2) the gate or gates which are altered; (3) whether the gate(s) are opened or closed; and (4) whether the particular event occurs in ail olthe three presentation intervals, or is thecriticalevent randomlyallocated toone. In theexample, the variable 't' is implicated in the timing of event 3. l h e magnitude of the delay 'I' is governed by the adaptive algorithm and this delay is only applied to the interval allocated to contain the critical event. In non-critical events ?'is equal to zero and a gap nominally of 1 ms occurs which is well below the threshold for gap detrction, particularly with the riseifall times in the region of 5 m s often adopted for the procedure. I t will be apparent that the above arrangement allows a very wide choice of stimulus presentation characteristics.

I. . / , , .,~, _. . * ., .. . .. ~

( 2 ) ',t ~ ,

( 1 ) !f ; t i ,,'i:s4ion :?!o he :ilterrd.xhiih, r i ~ i t c r ~ ~ l ~ ~ t ~ , ~ ,,, I,

(4) I)eci?inn riilcs gowning 3 chai~zr in direr!:,," ( r ,

r 'his ~',~IJI11cti r r.lioi:ld i , .xrc ,8 . ,c . ,,, I , ~,,~,,,

uhtiiii ,, highcr percentage corri'ct rcymiix.

h > t h \Ii.>iild ct,,!>~gc ( ) re fiyiire 4).

in the adaptive proccdiire, ¡.e. niimher of , c - . I 1

rponses rrquircd for d zh3ni.e in one , I I ~ , , ~ I , , , , , ,?:' ~

iiuniber oíiiicorrect responser for :I rhangr. in 11. direction.

'main' parts of the adaptive procedure.

parts of the procedure.

(5) Numbers of reversals which cnnstituie the j , r ; : , . ~ ~ \ ~ . !..,,,,

(6) Step sizes (attenuation or delay) in the practice . (7) Number of reversals u,hich are scored in calculatin ~

average which approximates the final asymptotic ,fll,,c * '

Additionally, it is possible to modify the 3AFC prcx*.cilurc, required, by including a dummy stimulus presentation k ~ , . ~ the first of the three alternatives. This has been found i n ~~~

studies to reduce a bias, common in 3AFC paradigms thJi critical event is less often detected when it is in the first i n ~ e r , ; , l As for the control of stimulus characteristics, the aboxe allows an extremely wide range olexperimental proedudurir !,,

achieved.

Third control file The third control file simply contains two @character rtring\',f text which are presented to the experimenter each time them ,in

program is run. These are normally used to give the tiilc ofpje procedure and notes on setting up, plus an indication ,,r tk siimulusparameters, such as tone frequency. nhich mr) h.>..< :,, be szt for each condition within the general pmcediirc lhrr strings and another entered at run time are reproJuLL,l t.) , printer along with the numerical results oleach adaptnc run U)

document the data. .__ .L ~.

~~~ ~~~~~~

Second confrol file Thc parameter; used to control the adaptive procedure are as follows:

(I) Whether attenuation or delay i s the stimulus charac-

The philosophy of writing the abo\e pro:r:inis uar high-levellanguageasfaraspossihleand to ret:tin their.?,:! degreeoldeviceand computer independeiicc. In practice. it

I.rC

teristic to alter. necessary lo attach a small number of IOU-lcicl. Jrsrr-

On 1 -1

Interval indicator light I

4-- Cycle duration lo00 ms ______*

I I I

I --

Event number 1 2 3 4

Time (ms) 1 O0 480 481 + t 900

Gate number 1 1 1 1

Open (1 )/close (O) 1 o 1 O

Stimulus amplitude

!

i i ?

?

, , . : , : ~ : .,. ,'l,I/.llI.I , C i d . ' ! )

I y ~ ~ ~ ~ : ~ t i l c ~ ~ ~ ~ ~ ~ i i !~\rIi,~;t,;!~ .ii...iI': ' 1 ' i r

, i i t i , mi c\p~~i i~tcnt : i pr<.cfdiiics a i i h i n :I f i w rniiik:Ies. 11's ..!.!ciii I u s Iiciii rcli.ihle :ind tr:in~.port.ihie, in thc w i w that copiesofthc protot)p tun hate hzrn \ct up ~t ic~cs:. fu l ly with little eñort at soeral other centres. For iniiny psyycho;ic~,,ii.ti,.:iI procedures, the use ofthe microcomputer as a contri>ller i s inore appropriate than its use as a waveform qnthcqi/cr Nevertheless, for certain psychoacoustical tests involving curn- plex waveforms or phase dependence, the microcomputer iiiay usefully be developed further in the future as a synthesiier.

Acknowledgements

The purpose-built apparatus described in this article was designed and built by the stañofthe Electronics Section of the Institute o f Hearing Research, headed by Frank Brookes. Invaluable software support was provided by Tim Folkard.

References

I. Institute of Hearing Research (1981) A population study of hearing disorders in adulu. Proceedings o f r k Royal Smiury of Medicine. 14, pp. 819.821. PATERSON, R . D.(1976)Auditory filter shaperdcrivcd with noix stimuli. Journol of r k Acousricol Socicry al Americo. 59,

TYLER, R. S. FERN AND^ M. and WOOO, E. 1. (1980) Masking, temporal integration and speech intelligibility in indivaualr with noiseinduced hearingloss. In Disorders oJAuditory Function Ill. edited by Taylor. I. G. and Markidcr, A. (Academic Pres% London). pp. 211-236. TYLEK R. S. and SUM%ERFE~.D, A. Q. (1981) Psychoacoustical ilnd phonetic measures of temporal processing m normal and hearing-impaired listeners. In Ps)uho<icourrical. Physiological and Behorioural Srudies in Hearing. edited by van d a Brink. G. and Bilren, F. A. (Del& University Pres, Delft). pp. 45&465.

2.

pp. 640454. 3.

4.

Experience of using the system

The psychoacouttical test rig described in t h i s ;irticle has been u d extensively for rescarch work over a period of 18 months. Identical systems have been set up in three other clinical outstations ofthe Institute of Hearing Research and two htrther systems, with a slightlydiñereni arrangement ofgates and digital attenuators, have been set up at the headquarters psychoacous- tic section of the Institute of Hearing Research. The elec- troacoustic requirements have been met in all o f these and the arrangemcnts have proved to be useful and flexible.

Practical experience of writing control files for new experiments or procedures has indicated that new sets of files can be written and implemented by non-expert stañ within approximately 5minutes. given a short period o f explanation an" training beforehand. Thus, the time o f such stañ can be spent largely on productive experimental or clinical work, rather than on the non-productive overheads of reconfiguring equipment or rewriting programs. This benefit has entirely justified the initial eñort required lo write the original software.

Conclusions

__ The ~ . ~~ psychoacoustical test ria mak-u-y pf a microcomputer lo control ancillary hardware of high electroamustical specifi-

____ ~ ~ ~

Announcement The Third Medirerranean Conference on Medical and Biological Engineering Meeting in Portoroi on the Slovenian Adriatic Coast from 5 to 9 September 1983, the Third Mediterranean Conferenceisbeing organized by the Yugoslav Society of M e d i d and Biological Engineering and the J. Stefan Institute in Ljubljana. Main sessions include:

'

Clinical engineering Physiological measurement Biosignal processing Modelling and simulation Rehabilitation engineering Functional electrical stimulation Response to muscular training and exercise Patient monitoring Impact of new technology Artificial organs and biomaterials Pain treatment Biomechanics Ergonomics and:industrial physical therapy Applied biophysics Seniors, transducers and electrodes Education and training.

Workchops, alco, will k held. Driuils frum .ilrs Diirjlr L'de, J . S'refim I,islirure, Jomoio 39, 61 1XN I.juhlj<iiui. líryi,cl,ii-ia.

. - . ~~ ~~~~ ~.. ~ .

.-

Exhibition announcement MicroTrade 83

Organized by the group who ran the Personal Computer World Show, and sponsored by the journal MicroScope, MicroTrade 83 is exclusively a trade show. The exhibition is to be held at the Barbican Centre, London from 6- 8 July 1983, and will allow manufacturers and distributors to do business with the ever-growing number ofdeakrs (and potential dealers) without the distraction of crowds of end-users. Some o f the products expected to be demonstrated are: home computers, business micros, bar-code readers, interfaces, printers, plotters, VDUs, disc drives, soffware, books and other publications, and optical character systems.

InJonnaiion Jrmn M&ibuild Lid, I1 Manchester Square, London WlE 2QZ. Tel.; O1 486 1951.

* 87

UNA PROTESIS MULTlCANAL PARA PERDIDAS DE AUDICION PROPUNBA O TOTAL G.M Clark, Y .C. Tang Department o f Otolaryngology, Univers i ty o f Melbourne, The Royal V i c t o r i an Eye and Ear Hospttal , 32 Gisborne s t r e e t , East Melbourne V i c t o r i e 3002, Austral ia. Fue desarro l lada una p r o t e s i s multic:dml con l a s s iguientes carac - t e r i s t i c a s e l éc t ron icas ; provee de 22 canales separados de estimula ci6n, l a e l e c t r on i ca para e l implante d e l receptor-estimulador, i m - corpora un simple chip, usando c i r cu i t o s d i g i t a l e s y empleando tec- no log ia CMOS; e l chip es embuelto en una capsula de titanio-vanadio por medio de e lec trodos d e l t i p o ceramicos. Un tamaño compacto de- un procesador de palabras y un m i c r o f onad i r e c c i ond , estraeen l o s s iguientes parametros de 18.~ palabras procesadae: amplitud de l a s e ñal, frecuencia fundamental y l a frecuencia desarrollada. La frecue nc i a fundamental e s cod i f i cada como un rango de pulso e l e c t r i c o , y l a frecuencia desarrol lada por l a poc is ión de l electrodo. ~l procesador de palabras e s rea l i zado , usando c i r cu i t o s h ibr idos , y arregi? de compuertas CMOS. La p ro t ess i s multicanal va d i r i g ida a4 pruebas c l i n i c a s de PacientB con problemas de audición profunda o t o t a l . E l resultado de l a per- cepción de palabras, indican l a obtención de un juego de palabras - ab i e r t o para un balance fone t i co , de f r ases y oraciones. En todos b Los casos l a s pruebas mostrarón grandes l og ros s i g n i f i c a t i v o s que - cuando s o l o se u t i l i z a l a p ro t es i s s coclear.

EVALUACION DE AUDIOMETROS

Como parte del programa continuo, comparativo de evalución de equipo medico, patrocinado por l a U.K. Un reporte sobre audiometros fue publicado reelentemente " In formación de Equipo de Saludii (HEI ) NO 18 , conteniendo reportes de 6 semejantes eouipos: 1.-EL Kamplex AC3 y AC4 2.-El PEters AP33 y APb 3.-El Grason Stad le r 1715 4.-= Uamplex llPJ55B 5.-El Ampaid 207 y E l Ampiivox 2150 ó.-Stanmoro HA7iAT y HE1116 7.-Leaflets Vart DHSs o.-POBox21

L

* A multi-channel hearing prosthesis for -- - profound-to-total hearing loss

.. C. R1. Clark, Y. C. Tong Dcptrtt~w~t oJOtoIaryngology, Wnicersiry oJMelboirne, The R o y d Victori<i,i Eje arid Ear Hospital, 32 Cisborne Street, East Melhournc. Vi<rnri<i 3002. Australia

J. F. Patrick, P. M. Seligman, P. A. Crosby, J. A. Kuzrna and D. K. Money Nuduirr Lrd, Lane Coce. 2066, Atistruliu

A niirlri-choriitet cochlear imploiit hearing prosthesis prociding 22 separare channels ofstimulation has been deceloped. The elri.rroiiicsf»r the iniplaiitable receiver-stimulator hace been incorporuted on a single chip, using digital circuits and enipl»jing C.IíOS rech,iology. The chip is enclosed iii a riranium capsirle with platinum/ceramic electrode feed-throiigks. A pocket-sized speech processor and directional microphone exrract the following speech parameters: signnl amplitude, fundanierirul Jieqticiicy and forin~irit~eqi~eitcy. The/ltiidaainenral~eeque,ic). is coded as electric pulse rate, andJormantJiequency by elecrrode posirirw The speech processor hos been realized using hybrid circuirs and CMOS gate arrays.

The multi-cliciiinel prosrhPsis Iiris undergone a clinical triol oii four postlinguril!i. deof parienrs with profounll-total hearing losses. The sprecli perceptir~ii resiilrs iiidicute ihat they wrt' oble ro obtaiii opeit-set speech recognition scores for pkoiierically bdmiwd tiords, CID .soitpiices a i i d spoiiilees. In ull cases rhe tests slioweil .siynifjcant improceinents when using rhe cochlecir proflirsis conthiiied with l i p r e d i n g compored ro lipreadiiig uloiie.

. ~- . - . ~~

Keywords: MII~TI-CHAVVEL, HF.<RT\G PROSTHFSIS

Introduction

Patients with a profound-to-total hearing loss do not receive adequate helL@om hearing aids-d.2s-a consequence: have diñirult; in communicating. A significant number of these patients have residual auditory nerve fibres in spite ofthe loss of function of the end-organ of heariiig~ the cochkd. This means that th&e pdtients could receive additional help b) implanting electrodes close to the residual nerve fibrcs and stimulating the auditory nerve fibres electrically. With this goal in mind a reccdrch team in the Department of Otolaryngology and Electrical Engineering at the University of Melbourne de- vclciped a prototype implantable hearing prosthesis, which was surgically placed in three postlingually deaf patients with a profi>iind/total hearing loss in 1978 and 1979 [i].

A iiumber of psychophysical studies showed the patients could receive worthuhile help and perceive speech-like sounds wliichdifferedwitheachelectrodestiniulated [Z].Thisledto the develiqiinent oía speech-procci,ing strategy which was found to help thi. patients understand speech With this strategy the specLh was analysed. and the second formant frequency and aiiiplitiidc extracted. The second íormant hequency, which is imiit!itant for speech intelligibilit).. was used to select an elidi,ide producing a similar percept to thai perceived by i i /> i tntlly hearing subjects, and the fundaniental voicing frcci\t?ncywns usedtostimulateeachelectrodeat arate that rids

aly';q>rinte [3].Theaudiological tests showed that the patieiits wiii ahlc to attain useful speech discrimination. and this coiii~~lriiicnted their lipreading skills, and produced significant

A .L ic,~:ll of thc~~~eiicouraginp studies the device ha, beer, d e , . i;;cd t i , thr cliiiizd tri:il stage by the hioniedical firni. t i i~. .h~., I.ril. in <,wnpaatiori uith tlic Universit) of hlclhoiirne '1 1:s' c*>iti.;li: i i f i l i hL.ii~ing prorihc%is is illurtr:ited ii, fig~irr. I . I t

\rhiJi picks up 1 1 ~ spi.ccii ,ig,ialh, aiici

, . I , ,,IL, , , , . I ,,. . I ,

1111~~ , Wt', ,I> ['I]

o r Thi. t 1 ~ 1 , I '1. 1 1 ,

I . , '! , / , I , ' , , . \, ' , , , , , I

the implantable receiver-stimulator. This in turn excites the auditory nerve fibres by delivering electric current to the electrodes implanted in the mchlea.

As nerve fibres require electiical cuments to excite them, and as energy is required by the implanted electronics unit. power also needs to be transmitted using the above-mentioned induc- tive link. The electronics for the receiver-stimulator should be reliable and last for many years. The stimulator should also h. designed to deliver a variety of stimuli, as advances in speech- proassor design may require a number of different stimulus strategies, and it is a great deal easier to modify the speech processor than it is to surgically remove this receiver-stimulator and replace it with a more up-to-date model. Furthermore, ifthe electrode placed in the cochlea had to be takcii out and another one inserted this could cause damage to the nerve fibres. It is therefore desirable to design the device with a connector so that the receiver-stimulator can be removed and replaced without disturbing the implanted electrode array if an electronic failure occurs. l h e implanted receiver-stiiniilator and electwds array

~. ~~ ~

I I I

wiihin a hernicric container for protcction. Ilie coclilwr priir- thesis must he mechanically robust, otherwise it might he damaged by an impact such as a blow to the head. I t shoiild also be engineered so that repeated, small body movements do not lead 10 metal fatigue and fracture of the electrode Ic&,

p;irticul:jrly at the p i n t where they leave the stimulator unit. .The micr<sphone and slstem for transmitting power and signals to the impi:inted deviceshould beeasy to apply and reniove,and should alx, he cosmetically acceptable. Directional microphones are desirable so that the speech signals from a noisy environment can he cxtrdcled to enable the patient to obtain maximum u'e both at home and at work. The wearable speeih- processor needs l o he compact and light so that it leads to minimum discomfort. The power consumption should he kept toaminimumsothat thereisalongbatterylife.Thereshould be few controls to operate the processor and they need to he easy tn manipulate for the old and handicapped. The ~ar ious lead wii-es should be robust so that they will not break with repeated flexing and pulling.

Figure 2. trode iirraj iiiuiiufiictured bj Nucleus Ltd.

The multi-channel coclileur Nnpfiiiit ünd e l r c

These design principles have been implemented in thedevice shown in figures 2 and 3. Figure 2 is the implantable receiver- stimulator and electrode array. The electronics are enclosed in a titanium capsule, and power and data are picked up by the coil which lies circumferentially. Underneath the capsule there is a connector to link the 22 emerging wires with the 22 wires in the electrode array, ahich is inserted into the scala tympani ofthe cochlea through the round window. Figure 3 shows the speech- processor unit; also the headband which has the microphone and transmitter a i l attached.

Other multi-channel cochlear prostheses currently under development have different approaches to the method ofspeech processing.Atlasand hiscolleagues [SI havedewribedascheme nhich invol~es thcevtraction of the first and second formant frequencies a h o w values are then mapped onto pulx rates for separate electrodes such that stimuli on the two electrodes are those that vould, individually dicit percepts with pitches pr~port i~i ia l IO the formant k i n g represented. A thirdelectrode is stimulated vith a high-rate pulse-train whene\er the ratio n i hiyh t o l o w I r ~ ~ u c n c ? c n c r g ~ c r c r c d r a thrmhold. lkrzenich [6] .iiinuIat4 three or four dectrodcs. po-itioncd in the v.ala i!:npani in ths regions of tii:i\iin;d rcsponw fur the distinctiie i;-i>nant frsqiw.niic< ofqxcch. with handpa:s-filtrred analogue

J

signals. One channel coberr the region ofthe first formant, one or two the second formant, and the final one the highcr frequencies. And Chouard cr al. 171 haie 12 individual clec- trodes through holes drilled in the outer ndl ofthe cochlea. The speech signal isdivided into frequency bands according to a map derived horn unilaterally deafpatients. The encrgy in each band is encoded as the repetition rate o í a pulse train which is applied to the corresponding electrode. The pulses have fixed amplitudes but their width is varied from channel 10 channel lo equalize the loudness growih chardcleristics of stimulation at different electrodes.

Single-channel cochlear prostheses are also being developed to helppatientshearsomeelementsofthcspeech signal.Fourcin and hiscolleagues [8] use anextra-cochlear electrode to provide voicing information. They stimulate their electrode by a square- wave current source which is externally triggered by a voicing detector; this signal can retain precise information about the instants ofglottal closure, and hence fine detail ofthe temporal aspects of phonation if a time-domain detector is used. House and Urban [9], and Edgertnn ef of. [IO] use a system where a handpass-filtered (200 4000Flz) version of the microphone signal controls a non-linear modulator which allows spcech signals in the range 45 h5dB SPL to vary the amplitude of a IókHz carrier ware. This is used to drive a single electrode placed inside the cochlea. Hochrnair-Desoyer er <if. [ii] currently describe using gain compression followed by a frequency equalization netw,ork by which the microphone output from 1W Hz~~4000Hz is mapped onto an equal-loudness contour at a comfortable level. They are e\,aluating the relative merits of extra-cochlear versus intra-cochlear single-channel stimulation.

Engineering design

Speecli processor

The microphone converts acoustic speech sounds into electric signals to be processed. The amplified speech is thcn directed along three parallel paths with the extraction of the following threc speech parameters: amplitude envelope, fundamental frequency and formant frequenq.Thisis illustrated in figure 4- a block diagram showing the processing strategies. The output ofthe amplitude envelopedetector is converted to current level. The estimated fundamental dequency is converted to electric pulse rate. and formant frequency to electrode position. These three electrical parameters are;fed into the output section where the radio frequency signal isconfigured lor transmicrion to the implmtal receiver-stimulator. These components are now described i n further detail.

The automatic gain control (AC i ) amplifisr rcduccs the range of :iniplitude \slues or'the djnamic r:ingc of the ipczch sounds 10 k piucesed both h j the dc\ice :jnd h! the patient. For a wide rxnge ofinpyt risnal aniplitiidcs i t Lccps ihe p a k

,,,.,,, .. .,,'-a,. . y . <'ii ,.i . "U,

Figure 4 . A block rliogroin of the speech processor.

output amplitude at a level which is acceptable for subsequent processing and comfortable listening by the patient. The .AGC circuit has a last attack and a slow decay time. Thus amplitude peaks are used to set the gain of the amplifier, and generate stimuli ofconifortable loudness to the patient. Lower mplitude signals are used to generate sensations of a loudness which is then relative to the loudness ofthe signal peaks. The amount o f dynamic range reduction and the time delay over which this reduction i s achieved for speech are factors which must be carefully controlled to ensure a minimum amount of distortion of the speech signals.

Electrical current has been found in psychophysical experiments t o be related to acoustic amplitude by a power function, and the power coefficient can be psychophysically determined. I t should be noted that the power coefficient may vary from electrode to electrode depending on the size and location ofthe electrodes, and on the density o f residual auditory nerve fibres in the c o c h l e a a i r e c t ~ 4 J ~ ~ u ~ i ~ ~ ~ r ~ ~ ~ ~ ~ . b ~ acoustic amplitude is sarnpld by an analogue-to-digital converter. A look-up table, prepared according to the functional relationship, is stored in a block ofdjgital memory from uhich a current level can he read once the acoustic amplitude is sampled and an electrode selected.

Fundamental frequency is measured by passing the output o f theautomaticgaincontrol througharectifier, followed byalow- pass filter. After the D.C. component has been removed the signal is passed through a zero-crossing detector and a frequency-to-voltage converter. An,, analogue voltage proportional to the fundamental frequencyis produced at the output of the frequency-to-voltage converter.

The formant frequency is estimated from the output of a formant filter which is designed to cover the frequency range of the formant frequencies in question. The filter output is fed through a zero-crossing detector and frequency-to-voltage converter to produce a voltage proportional to the forniant frequency. Directed by the microprocessor, this voltage is sampled by an analogue-todigital converter, and the electrode number read from a look-up table in digital memorg. The correspondence between formant frequency and electrode number can he determined in psychophysical studies or deriied on the basis ofthe spacing between the electrodes in the cochlea and known physiological principles.

The three signal parameters corresponding to the three hunches o f the block diagram are continuously fed to the niicroprocessor and outp(11 block in real time. The microproce'~.\vir controls'ths inctances at Bhich the p;ir.iiiieter values are wnplcd and confiprcs the radio-freqticncy sipal I O he trans- niittcd to the implanted reccii.er-stiinulator.

The 'peech processor shoun in figure 3 has heen im- rlcmcnkd by hb brid circuit tcchnology using C\IOS intcgr.ited circuit chips. The special-purpose microproce&r has heen i .-plmrnlzd . , " , , , .,, : ! . , ~ < , l ~ d i dtf.i tr!l'/ 1'' '.>',T ( 1 ,'IC i::

;:Ioiiiul , , , , ~ . < ' r . ~ r i i i i i i /u~or

:iiid ,l:ti:b ílwii i l h i lxrch proce\>ot to the iiiiplmi~. , ' , .iii:i!r- stinidator hccause 1 1 is the most dTicicni and tclI;il,lc i i ioth<d A single radio-frcqiiency link ha* hcen i i \ d 1 2 Ii:,nw?it ILt ih poweranddata.Onepairufelectrodcsissel~c1r.d i i i r s i i i iiiliil' tn

at any time and a burst oí energy iixd both lo lraiisinit information about thestimulusand thecncrgy ncided for i t I'he receiver coil, which is outside the protective heinictic eiiclowie. is a single turn ofplatinum wire, while the extrrnd transinittcr coil haseight to I 5 turns.Thetransmission has bccii dcsigicd 90 that there is adequate power transferred over a distance of lOmm when the coils are coaxial, and some degree ofmis.ilign- meni is possible at shorter distances. Power and data :ire transmitted by amplitude modulation of a carrier wave of 2.5 MHz.

iriionhnchwniisci fi>r,i:iiisiiiittiii.~I

Imylriiitable receiver-stiiiiulator The recei\er-stimulator has been designed to provide the imost appropriate stimulus parameters for optimizing speech perception. When implants were first undertakenit wasconsidered that there should be almost complete flexibility with the stimuli so that the patient's percepts for different current levels, pulse widths, pulse rates and so on could be determined. For this reason there was an initial enifhasis on percutaneous plugs and receiver-stimulators which could best achieve this goal. Now that more is known about the range o f percepts possible, a receiver-stimulator can he designed to provide tlie appropriate $&IP.II wilhoutLhaving to be quite so flexible. The above principles and goals have been adopted in the design of the receiver-stimulator illusirated in figure 2.

The first important principle adhered lo is the requireinent that i t should provide constant current rather than C O I , , I ; , I ~ I

voltage stimulation. Constant current is required as tlie total charge flowing through a nerve membrane is responsible for neural excitation. Since the impedance of the electrode/tissue interface can vary, then a constant current source isneeded if the total charge delivered is lo he controlled. Theelectrical stimulus produced by the receiver-stiniulator is a charged balaiiccd and biphasic pula , which is used to minimize corrosion and reduce the production oftoxic productsat the electrode-tissue i n i e r k e .

This stimulus is presenf'ed between adjacent pairs of electrodes in what is termed the bipolar mode o f stimulation. Bipolar stimulation allows the stimulus current lo be Ioc.ilized cñecti\ely to a discrete group of auditor) nerve fibres. On the other hand, it may lead to raised thresholds unless the electrode is wellplaced. .The reccii.er-stiiniilator hasa modification where bipolar \timulation can be made not onlybetween neighbouring electrodes, but an electrode and the next but one or two. This spreads the electrical current further, and achieies a lower threshold.

The number o f sliinuIus channels to be used has bren a matter of wine debate for a number of years. Thedmice (figure Z)usn?l channclsofstim~laiionasthisislikelytoproiidc~iiorc information that can be used by the pdtient. It has hem shown recentlyin theclinical trialofthedevicethatthcsixp~ticnts !\ere in fact ahle to psychoph)sically scale the 21 different ch:rnnc!% of ztimulation indicating its uwfulncss. Furthermore, thcir .prech- p.rcr.pti<in rerults were appropriately good.

'nie'ii:iiiiiusciirrent pulcsc:in he : idjwid in .mpl i iu& L>\cr the range h u m 25 wi to 1.5 in1 4 in 3":. qicps. Tñi. aidc rang< ,>f currrnt>ha. hccn w l ~ - c t d t c ~ i c o i n d ~ i c .:ntiLipatc,i \

in paiicnt ibrc\hc! lJ< aiid di. ktr t Ic,.d\ dur. I,) ,i,?cring

j

* ,.,. i. .-: . . , I -7 , v

I : , ,,hit;,>,, /ivih i>i'tit.icii\ t i \ u c ;%nil huiie. I lx j",,

stcps in aiiiplitudc ,ire oí ihe w i ~ c w l e r :I\ t ! ~ rcp,r i id ,implittide iliffcrciicc limens.

.Ills prosthesis also has i t facility ubi, hy the piilic. u i k i i 1 1 <:.In he varied. ;ind this is a uwful addition a h !hi\ \tiinuliis p:ir:irneter can he used as well as current level to produce changes in loudncss.Ithas,in Fact, been shoun that i t ispossibletotradeoff current level for pulse width and maintain equal loudness proiiding the total charge is constant. With the device shown in figure 2 the pulse width can he Laricd from 20ps to 4OOps per phese in steps of 0 4 ~ s .

A s ihe perception ofpitch is important in speech perception, so also is the facility to vary the rate of stimulation. The upper limit on the perception o f variations in the rate o f stimulation has been shown to be about 400~800 pulsesfs in humans and 200400 pulses/s in animal experimental studies. The receiver- stiniulator has been designed to allow stimulus rates in excess o f io00 pulses/s.

?'he receiver-stimulator has been designed to make sure that the charge densities at the electrode are not levels that will cause tissuedamage.1thasbeen shown that with ourbandedelectrode continuous stimulation with charge densities of 1&32pCcm geolii/phase will not lead to damage ofspiral ganglion cells and this level has been employed in the current device.

The electrode array that i s connected to the receiver- stimulator packagehas22 bandsofplatinum which aremoulded into a silastic carrier with an outside diameter at the tip of 0 4 mm, and along the shaft 0 6 mm. This is a 'free fit'electrode sliich passes easily around the scala tympani ofthe basal turn of the cochlea for a distance of up to 25 mm, and this allows the elecirode to come close to the nerve fibres conveying speech inforniation. The advantage of having banded electrode rather than small discrete terminations is that rotation ofthe electrode as it i s inserted will not lead to misplacement of the electrode sites in relation. to the nerve fibres. and furthermore lower current densities can he u'ed and so not lead to damage ofrierve fibres or electrode corrosion.

Design rerilization In the realization ofthe receiver-stimulator design, analogue and digital circuits have been employed using C M O S technology. The circuit elements have been incorporated on a single integrated circuit chip. The data to the device is transmitted as a digitally encoded signal, which isdecoded bythisCMOScircuit. The circuits have been designed so that the coded instruciions from the speech processor are conierted into electrode currents for neural stimulation.

The electronics ha1.e been placed in a titanium capsule that has been hermetically sealed, with platinum ceramic feed throughs being used where thc receiver-coil enters, and the 22 electrodecontactsleave the titanium capsule.7itanium has heen used for the package as it is a proven material having been used in heart pacemakers for many years.

The receiver-stimulator has been designed with a connrctor so that i t may be replaced ifthere is an electronic Failure, without the remotal of the electrode afray, which could lead to damage ofthe aiiditory nerve fibre2 as u,ell as prolonging the surgery. Further, the reinccrtion of an electrode array could he quite dificul1:i'heconnector hasheendesigned tolast formany)ears, and thz platinum has heen uced to avoid corrosion. I t has heen dc\ign,!d for good elcrtrical i-ulation hetueen rlectrode connections. I t i s a150 simple to u-, and suitahle for use in ;I surgical theatre. . 6

-1 * , C , . ~ . -L< L.,~"-.* .. . 1 . ) ; , . . ) ]Iy 1<,1 ,,<l. I, ,'Kl,

l,l,pl:,llt?i,, i:l'l? .!,,,I I O cj

i i h c r c t l i i . , ~ l r . ~ i i ~ ~ ~ l r . ! ~ ; ~ d ),irc.~.il~, 111~ 1, ', 1 .r?c it'! ~.i~:ii.in~iitstr:in~n>iticd Ii-oin t ,Iihii u t ,1~1!11~!:1lg ilii. li:insmitter coil scrclikely iopr<i~iii<

coi~p<,i,iic$ :I <[>ir;dly wound electrode Icad. dL uithctand these mechanical stresqes over )ears of w e , I t i q ,

hiwiwr. irii-iiiniiirnded during surgical implaniatiori th.ii the proxiiiid clectrode he pl;icad in the groove under bmie to iminiiiiix clr,ctide iiiovcinent.

Thc t~.cei\er-~!iiiiitl~lor has been designed fur cake of surgical placement and cometic acceptability. Surgical experience as a result of implanting our first prototypc in 1Y78, which was rectangular in shape, and a series offour inip1:ints in 1982 using the present device, confirmed the view that a roiind implant is be pieferred. The bed can bemade very iicatly w i t h a milling burr. A round shape is also desirable for other reasons' it accommodates a circular receiier coil, and has no sli:irp projections.

Our anatomical and suígical studies showed that the maximum depth the bed could he drilled in the mastoid ;and occipital hones was 6mm, hut 3 ~ 4 m m was more usual. 'The maximum height superficial to the bone that is cosmeti~ally acceptahleisabout 5-6mm.Acceptability hasbeenimproved by rounding off the edges of the deyia . It was also our experience that the maximum diameter o f the unit in adults be about 35-40mm. In the packjige il1iistr;ited in figure 2, stability \!as helped by making the implant mushroom-shaped, with the titanium capsuleand connector hii\inga diameter of23 lnrn and making up the stalk ofthe mushroom, while the receiving coil helped to form the . cap. .~ ~~~

1 1 arca. Cimequeiiily, the presriii

.

Initial clinical trial The initial clinical trial was carried out on four postlingually deafpatients with profound hearing loss. They has 0% open-set speech discrimination scores. The? had rmived no significant benefit from a powerful hearing aid or \ ibrotactile device after a six-moiith trial period. There were no medical or radioiogicai contra-indications to surgery, and elsctrical stimulation of the promontory produced hearing setiutions. A series of speech- discrimination tests, includinga mmlified version ofthe minimal

es (MAC)test h t i e r y , were carried out after the six-month trial with a po\~eríul hsüring aid or vihro1;iciile device so that these preoperative rzsults c,,uld be compared with those obtained post-operatiwly 5, ith a cochlear prostheqis.

Patient 1 was a 37-year-old niale n h o developed a severe hearing loss at ihe age of fixe folloning the administration of streptomycin for burns. At the ti:iie he had coniplete loss of hearing in therightear,but wmeui.ful hearing in theleft. When he was 21 he suddenly lost hearins in the left ear, and was then totally deaf. Patient 2 wasa 62-ye.1r.old female who lost hearing gradually following recurrent otiti: media as a teenager. When she warpregnant inherearly 1 0 s t . ~ ~ ~ ~ ~ ~ I u r t h e r d e t e i i o r ~ t i o n in her hearing, and she wasihen no longer able to receive any benefit from a hearing aid. Patient 3 was a 22-year-old fcmalc who hecanie totall~dcaffolloainp :n :ittack ofmenin,' wtts ' :it the age of 18. Paticiit4 uas a 23.)ear*,id Tc1:ide who g~ :idually lost hearing follouins recurrent b u t s .-.fwLitls iiicdi:i as a child, and later required a number oft.iip~-. ,~r' .i .i> t~pcrati , '11s to i-cpiir perlomiations of the 1)~nipdnic .--:..?br~*nc and rotore the msicular ch:iin Shehasheinpror~ ...:.~1? J . , lbr nir~e>~ 'xs . ind ohtainrd no hmrfit f rom a t.carL:; ,/J 3: r ing that p-riod.

,, ,~.., , ~, . , . . ... I. I I,<. ,> ' ! l ,< : ,>tq . fii,r, This ,i:is uir<lcrt~koi in the 1:ittcr h:iV of I V Y ' . I'hc 1 .:i i ' l l lS

l c t , , , I ~ ~ d fL,r thcir first pht~c ,per : i t t t c It's1 \ ~ ' \ \ i g m : , io t i l l'?r<:e weeks i r t e r Jiwliarge from hi,<pii;il. .\I t'ie 51 4 k s t w4m ; i l l elect redes were tested and tht. : l h i ~ ~ ~ h o I i 1 s . coiiifort:i hlc IimiiiliieS leccls alid dynamic ranges for i..iih i.lcctrc>de uL'tc dctiiiiiined. In addition, the patient was asked to iarbk the \titniiii. uhich were balanced for loudness, at each clectiodc on the ha,;ic ofthe qiiolity o í sharpness perceived. All this iníoiniation, which is diflercnt for individual patients, was thcii put cinto J prog:im- inable read-only memory (PROM) in the piticnt's b p

processor. This was done using a diagnostic nnd prugr;uiiliiiiig unit. In this way, multi-channel stimulatioii could be optitnired for individual patients and allowance made for \,ariation i.n the electrode placement and individual nerve fibre populations.

The patients then had a course ofrehabilitation in the use of the prosthesis which lasted for two to three months. Following the course they were assessed with a number of spccch- perception tests lo see how well they could communicate. A series oftests was used as no single test is adequate in e\.;iliiating how the patient can be helped in their eteryday life.

Finally, the MAC battery was repeated using the cochlear prosthesis alone so the results would be compared with those obtained preoperati\ely when using a hearing aid or bone vibrator. The results, when analysed statistically, showed the cochlear prosthesis was significantly better than the hearing aid or bone vibrator on 1 3 ofthe 16 tests. The exceptions being the maleifcinale speaker, question/siatement, and environmental sound test, where both devices had similar results.

The audiological tests included open-sets of Arthur Boothroyd (AB) phonetically balanced words, the closed-set Monosyllable-Trochee-Spondee (MTS) test, open-sets of spondees, open-sets of CNC phonetically balanced monosyllabic words, open-sets of Central Institute for the Deaf (CID) ewryday sentences, open-sets of speech perception in noise (SPIN) sentences, and speech-tracking tests [12, 13 and 141 for cochlear prosthesis alone, lipreading alone, and cochlear prosthesis combined with lipreading. They were also adniinistered for most tests with pre-recorded material using an unfamiliar speaker.

The results o ímost interest are those obtained for open-set phonetically balanced (PB) words, open-set C1 D sentences, and open-set spondees. These are dificuli tests, especially as pre- recorded material and unfamiliar speakers were used. Furtherm'ore. they were open-sets, and the patients were not prei iously tested with the words. The results showed that, when u ~ i n g the cochlear prosthesis alone, the PB word scores varied from 2%--14%, and for the C I D sentence test from 2%.~38%. In all cases there were significant improvements when using the cochlear prosthesis combined with lipreading compared to liprcading alone. The spondee test, which was administered as an open-set, also gave good results for the cochlear prosthesis alone.

The results are interesting as opa-set \tord recogition has not k e n obtained iiith most *ingle-channel stiniulations, and al- thoush they show that communication with electrical stimulation aloneisdiifícult thisshould imprme as the full pilent ia l of multiihannel stimulation has not been explored. Furthermore, the results in patien! 4 susgest that she should be able to C<,nierw on timiliar tt>pics using the cochlear prosthesis alone \ ~ i t h ~ ~ u t : i n ) hclpfroni l ipreadin~.Thi~, in faci.isthecosz,asshe ib;ind but other pmple. She c m &) ih is h) h u r i n p speech. and nci t by ihc dctcciim ofpre- .:rr.in;::J +xils. Finally. the value of the deiice con alw he : : . - r . - 4 h! thc niinihcroíhour. a AA! ths p;iticnt finds i t ri.c.ful.

In the present trial al l four patients are using the prosthcsis all the time while they are at work or at home. A patient using the prosthesis to talk with her audiologist is shown in figure j.

Conclusion This clinical trial has shown that a multichannel cochlear prosthesis can provide significant help for postlingually deaf patients with a profound^ total hearing loss, and as its potcniial has not been fully explored i t ran be expected that furihsr improvements should be possible in the future.

Acknowledgements

We wish io acknowledge with gratitude financial support from a public interest grant from the Commonwealth Government of Australia. The research has been supported by the National Health and Medical Research Council ofAustralia, the Deafness Foundation of Victoria, the Channel IO Deainess Appeal, and Lions International. Special thanks to MI R. C. Dowell, Ms A. M. Brown and Ms S. M . Lusconibe lor the audiological evaluation and rehabilitation of patients; Dr B. C. Pym;in, Dr R. L. Webb, Dr Q. R. Bailey, DI B. Franr and Dr D. W. Marty for themedical and surgical management ofpatients. Our thanks to members ofthe Royal Victorian Eye and Ear Hospital for their help and co-operation. and Mrs H. Hodgens for the typing.

References I.

2.

3.

4.

5.

I

. .- I L

F

L

c

L

c

L

c

._ c

L

c

L

c

L

r-

- c

i

Measureinent methods

, A number ofpdramcters covcied in BS 5966 (1980) Specification for Audiometers, which were judged to be central to audioinetry, have been used as the basis ofthis technical evaluation. Other features which may be regarded as of secondary importance, for example those associated with less commonly used tests, have not been dealt with in detail. The full report in H E 1 118 does, however, note significant points which have arisen. Throughout the technical evaluation the objective measurements employed were those felt to be most appropriate to the verification of BS 5966 and for which these are agreed standard procedures.

In the case of earphone measurements, the acoustic charac- icristics of the measuring device will have a significant effect on the results. The Provisional Reference Coupler (1EC Publication 303,1970) has been used both for cheeking the conformity of air conduction calibration levels with BS 2497 and for recording the f r e q u e i r c p r e s p d ~ ~ channe!LthrZu&h-!& head- phone. Bone conduction .threshold values were checked in accordance with BS 2497 Part 4.

While meaningful comparisons can be made bet ween audiometers, because earphones ofthe 58me general type are fitted to each instrument, there are problcms with the interpretation of t h o a clauses in BS 5966 dealing with the measurement of frequency response. This is partly because the earphone and coupler characteristics u n combine to put the measured response outside the permissible tolerances, while in fact this artifactual condition may not be present under real ear libtening conditions. These problems are aggravated by the choice of artificial ear or acoustic coupler allowed by BS 5966. Different devices will lead to demonstrably diKerent results, although in thc Standard only a single set oftolerances are given. in view Of these uncertainties in the interpretation of the requirements of the Standard, some careful judgcinent is need in the evaluation of individual calibration curves.

The same measuring procedure was employed lor spectrum measurements on the various masking noises. A constant bandwidth filter was used whose frequency was adjusted to appropriate values. Basically ihe e&ct of the coupler would be significant only at frequencies above about 4 kHS and narrow handmaskingcurvesfor6and8kHzwerenotcunsideied in the cvaluation.

Levelsfor wideband masking are not provided in BS 5966, as each manufacturer is required IO state appropriate eñective masking levels lor the particular noise employed

Certain technical tests required by BS 5966 were undcriaken, although not reported upon individu:rlly. Each of the aiidio- mi'tcrs had an acceptably low warm-up time and none w& i~riJd) iiiliumced by inüins \«Itage v,~~i .~ l iun.

Evaluation reports

Kumplex AC3 This two-channel audiometer offers a compreliensi\e i.iiige of f:icilities selectable by conventional rotary switches. All w r ~ i r o l s areneatlylaidout on the front panelwith anilluminat,ddi\~'lay showing the functional state of the instrumenL

The tone interrupier controls for both channels :ire %if the touch type, and are therefore mechanically silent. Thcse are operated by bridging twoconcentnccontacts with the íiiigcr-tip.

Socketsare providedfortheconnectionofamicrophune and two tape-recorders, one of which is selectable by both ch.iiliiels. A free field output is available from each channel.

The Kamplex AC3 is well constructed with all wii t io ls conveniently and logically laid out. Internally the insttuI11,:nt is ,icatly wired and designed for ease of scrvice aiid c:4Iih xt i i in : all frequencies aiid calibration levels can be adjusted hy p,->eei potentiometers.

There are no major criticisms of the instrument as a hish- quality pure tone audiometer. The SISI circuit, in pariicular, works well with automatic display ofthe subject's scorc fibr the test, but improvements could he made in the electrical'inpiii .ind output facilities.

'A calibration error occurs when both channels are suiiched to the same carphone. If channel 1 is feeding a pure tone to the red earphone and channel 2, which is not carrying a signal, is also switched to the earphone, the-pure tone output uill drop by approximately 5dB. The situation can be made worse if the hearing level control on the second channel is iurncd lo maximum, giving a total calibration error of up to lOdB.

The iistruction manual could he written more c1e;irly xiid should contain Tar more information on the operation of ihe instrument l o enable the user to realile the full potrntial of the audiometer.

Kainplex .1C4 This versitile two-chsnnel instrument provides a nide unge of I;iciliiies dectable by means of push-buttons. Two biittniis >et the hearing lcvel,required ¡u each channel, on<: iiiw

level the other decreasing it in increments of 1 or 5dü. Touch operation eKects:a single-stepchange, while holding the button changes the intansity coniinuously unt i l the <ir'iicd Icbel is reached. Other ftont-panel controls are c m ~ ~ , ~ i c ~ t i I ) :iri,*nged in logical groups of input and output ~elcctws for lie two channels.

As an aid to, routine testing [he atidiwiictcr :iiit~~.,iaiii.illy ~rlaultstosensihle\alue,ror thcii.:ijor funciion,.~f~Cr.aiichon : . . , J ~~liencertaiii modech;inge<:srr inade. \ l t du le s fur SlSl and

thc Lwchei tcri. and for coniiection to a chart recordrr for ,e!f~ reading audiomctr). may bc inserted iato a bpeci.11 frttr~t p.incl Iri rhirlattcr mude. the output signal from t l w a ~ d i ~ m ~ t ~ ~ r can bt incrcixd or decreased by the subject a i a ra i l of 5 d R wing the patient response button. A version of thii aidim:dcr is aiail:ihlc with a computer interface.

The Kamplex AC4 shows evidence oí good design and performs accurately. All preset controls needed for calibration are grouped conveniently together beneath the t i p panel for case ofaccess. T o aid in servicing. circuits aregrouped on circuit boards by function, and the general standard of construction was considered good despite the compacinessand complexity of the instrument.

All functions and settings are made by push-button switches, and most switches incorporate indicators to wnfirm that selection is permissible. The clear, logical layout and display of selected settings make this audiometer relatively simple to ust:. The instrument is not matched by the instruction manual, howrver, which is too brief, badly written and does little more than list the functions of the audiometer.

Peters AP33 This is a well-made two-channel audiometer; it has facilities for pure tone and speech testing. with narrow or wide band masking, and inputs for both tape and microphone signals.

The hearing lcvcl controls are of the conventional rotary type, stepped in increments oí 5dR. There is also a vernier control giving an additional lOdR in steps oí 2dR.

The AP33 is a mechanically' robust, portable audiometer. The general standard 01 construction and wiring is good, and most internal components readily accessible for ease of service

.Y and dibrat ion. A ~ ~ d p t a s t i c i n ~ r u c t i u o n sheet gives a concise description oíthe controls and facilities. All controls are laid out conveniently and are positive in operation. The switch that reverses the function of the tone interrupter, however, is unnecessarily heavy to use. Thc path of test signals is quickly apparent from control settings.

While offering no advanced features, the instrument is straightforward to use and is capable of accurate pure-tone and speech measurements. At the time of publication this instrument is no longer manufactured.

Peters AP6 The AP6 is a two-channel audiometer which offers a wide range: of facilities, including continuously variable control of frequency and output level. I t is oí large and heavy construction and basically of elderly design.

A patent mechanical cam system is used for threshold equalization and to provide various other mechanical inter- locks. I t alro has two independent oscillators, speech audio mctry facilities and synchronous masking.

ManyofthefunctionsofferedbytheAP6arepruiidedbythe precision mechanical assembly around which the audioiiicter i h

built. This tried and tested design is outdated by today's technology, however, and it is surprising that such expensive nicclianical means should be used to provide facilitic, that modern elcclronics could produce at lower cost and with gre;itcr p r c i i h i . Thc coniplexity of the mechanical assembly h ; i s rehiittrd i n frcquciiry and hearing levcl controls that l a e l tli: s inr>~>th i iw

The intrrio, (oí ttw audioiiicter is neatly wirc.1, wi t l l ñil

C U I ~ ' ; ~ . U I : ~ , ~ I S acvi;sih!<: f a s i ..iiitip Aci.i .~; t c arecimtrolled h j Tihcd nir.ch:inic.il nicans. 7 h z manuíadurcr claim,. houeber, that u s r calibratiiiri of frc- qumcii.,irriot ncce,s ; í r~asthc~caresetduringmanii~a~tur< :and wil l ri.ni:iin con,t;mt for the lifetime of the instruniciit, excel'[ in thc ca,c of ni tjor mechanical damage. Calibration level, arc adjustable b) the user but should rarely need to be carried out.

Grason-Stadier 1715 The 1715 is a twin-channel audiometer offering a wide range of facilities for speech and pure tone audiometry. It is robustly constructed in an aluminium case with wooden end-panels.

There are inputs for microphone and tape-recorders and a range of electrical outputs for free field use. Microphones are provided for spcech tests and patient talk-back. A built-in loudspeaker enables the user tn hear the subject's voice when the instrument is used in conjunction with an acoustic booth.

This audiometer is designed and constructed to a high standard. internally it is neatly laid out using printed circuit boards and ribbon interconnecting cables. All components are accessible for servicing with calibration of individual levels and frequencies effected by preset potentiometers.

The frequency control and both hearing-level control switches are of a somewhat unusual design. A single printed circuit board is used to form the wafer ofall three switches, with the switch contacts etched on to the board. Although this design has many advantages, a fault developing on any of the switch contacts could necessitate the costly replacement of the complete board.

Test tones of 1500Hz and above were radiated by the audiometer, and could be beard by a subjeci not wearing earphones at a distance oí I m in a quiet room.

Kamplex TA155B The TA155 is a well-established mains-battery portable audiometer. It offers basic fac es for air and bone conduction measurements, speech tests and MITOW band synchronous masking. The instrument is housed in an attractive cabinet with all controls neatly and conventiently laid out. A hinged lid gives a c c c s to a uscful storage compartment which holds the transducer and accessories. The audiometer tested was supplied complete with a glass-fibre carrying case.

Generally, this is a well designed and constructed audiometer. It is neatly wired, with easy access to its interior lor repair and calibration. All printed circuit boards areofthe plug-in type, easily accessible for replacement if necessary.

The controls are well laid out,and clear in purpose, although the frequency, function and outp t switches are rather heavy to use. If the 20dB switch is used, t E nes at 125Hz and 250 Hz are completely disconnected from the output. The VU meter still deflects when the interrupter is plessed, however. and ii cmtmt thcrrforc be used as a reliable cqnfirmation of prescniatioii.

Sciious erroh i n output levGI can occur at the m.iiiiniiii: he.iriii:. level scriii:g ;!id al..:> whcn hin:iiiisl o~,t,,~.' i \, 1

Thi- is c . 1 ~ 4 h) thc p ~ > . , l dcsipn of t l i i switching a ~ ) , ui:i, t l i i ti<:.iriii: lcvcl C"lt lOl>. e

1 3 1

The internill construction allows reasonable access for servicing. Calibr:ition ofthe individual frequencies and levels is by means of prcsct potcntioinclcrs.

Headphones, bone vibrator and patient's signal button are all wired into a single plug, allowingtheleadsofthese three items to become easily tangled. Individual plugs and sockets would be preferred to this method of connection.

Ampliuox 21 50 The 2150 is a portable battery-mains audiometer oñering facilities for pure tone and speech tests, wide band masking and inputs for microphone and tape signals.

I t is a compact audiometer which fits neatly, with its accessories, into the rigid case provided. When the case lid is opened the audiometer i s fully accessible for use.

The audiometer itself is built into a sturdy plastic box, with all controls clearly marked and conveniently placed on a bell laid out front panel.

Generally, this audiometer is well designed and constructed, makingRxteasive use oíreadily available components which are easily accessible for calibration and service.

The well-made carrying case supplied with the instrument did not havea cableentry outlet holetoprevent thepossibilityof the mains lead becoming damaged by the sharp edge ofthe lip. lmprovements could be made to the input circuitry. The performance and calibration of this audiometer is generally good.

Clinical evaluation In order to obtain users' opinions the eight instruments were evaluated by audiological technicians in four clinics, and a further group assembled by the RNID. Each centre was provided with a questionnaire containing 22 items relating to the fcatures and fiicilities commonly found on audiometers. The individual scores Were averaged over all items and all testers in e¿ch centre and were then rank ordered. These individual rank ordering were further averaged overall items and all testers to give a single figure of merit for each audionleter. In this final ranking the Kainplex AC3 and AC4 were judged easiest to u x .

General conclusions I n use, an audiometer requires close interaciion uith the oper;iior and hence an) technical eualitation will reflect the rwrds of the mer. Some of the inhtiiimciits, \\hile pro\iding coniprshenGvc clinicdl Facili1ii.s di1 i i ~ t olTu logical prssm- tation. .md appmr to be h a d on dccign philo,i,phics w i n g

I O8

, , I , , . : ' , : ,<\<, , I

' , I h i > . I ,,. // I '., . 111 I

i l ~ d i , , t l l i \ ~ . \:> i ~ l i C ! i , i ' , # ' C I

~ n p h , , , c ~ ,A)..<. ~ ,y: iici.L' o n ilic i r l p t side. , iid dniplifiers 8nd speakers , .: ihc ~,iitput ..!de. Pi<,ii\ion .if xiich I.iciliiio tends to widen ti.; :r:iditimd rilk of tlic :iudiivnclcr to one of a signal switch;-ic unit in ;I \igul jirc>eniaiinn cystcm. This means that thc .tudiometer ~Iwiild i i i t r rLcc easily with other equipment typic.i! ofthat cmrniriily ti.r.d in cliiiic,il hituations. While iinpr0veni;i:is have been ni.iJe i n this dircctiim, wnie audiometers still ap'iir to include the ncccsi;iiy facilities as iifterthouplits rather i h n as fund:riiiental fcatures oí their design.

Diiringthesvaludtion it was apparent ib.it in somemodesof use thcre could he uncertainty over calibr.it:~~n levels. This uas particularly true when both channels ucre connected to a cnmnion earphone, and especially when mi .)ithe channels U a S

itself feeding both earphones. It may be i1 .vnsequence of the increasing Rc.yibility of audiometcn that modes can k established for which acoustic output levels .ire uncertain. and may be determined only by measurement. \\hile this may not indicate Faulty design, i t does represent a ,v.4hle source nferror and this aspect of audiometer design dew:\:s further thought.

An area of poor compliance with 6s 5966 concerns the characteristics of masking noise, both wide .id narrow band. In fact very few of the audiometers provided i::.isking sounds good enough to meet this Standard, and, althoi:r!I BS 5966 specifies more strinzent requirements than most i:.inufacturen have been working to, these requirements can k iulfilled, as denion- rtrated by the Kainplex AC4. N'hcn wide hi-.l mise isprovidcd, the manufacturer should supply informati,-:; <>n the effectiveness for masking. Generally this was not done .ind therefore it was impossihle to assess the significance of thc Luge variations in soundpressurelevelsoínoisefound on thei:?erent instruments.

A further area sadly remiss in the caw .?most audiometers concerns the provision of documentation The Grason-Stadler 1715 was particularly good but most ,,f the others gave information which was either inadequate ,Y not clearly presented, and in some cases poorly p r o d u i d It would be an advantage if the description and operati\-.n of the instrument were separate from a catalogue of audio!%'$Xl tests, if this is provided. The provision ofseparate bookl::> would help in this respeci

Another sinall but important point N.,> that several of the instruments did not habe markings on the c.i:phones io identify with which audiometer they wcre suppli.\! .ind for which the calibration levels ;ipplied.

The main conclusions from the clinic.+! :valuation are:

( I ) I t may be difficult to design a3 audiometer which successfully fulfills the requirements d a l l possible roles, f

such as the testing of both adults a d children. (2) Users are likely iobt strongly inflwlced h) the views of

their pcers. (3) Certain'house st).lri'oflayoutanc'-,ztiondoappear to

he preferred, the thrce audiomc:::s iiiost commonly rated as easy to use k i n g prC\ manufacturer.

X i sistroma ruditive humano peses muy rukrdoa cempen.ntes. Ea era senalble (I e l mevimiento rlO8ferie (BrewniaPo) de las melicu - las do1 a i re en e l cenduch 8UditfVO e a mevímiaWos &e 1. i..broUu boeilar tan pequeños cere un oenthime de e l Bilhetre de wn Ckms - de hidrdgene.

EL efde puede detockr freatiencias desde cerca de l e a 6 a H 5 - hasta l o s 20Hm en l e s jóvenes con l a mejer soaciibi1id.d; entre i y- 3 aH5 en intiividues nsrialea; e1 limite superior c8e aerw da l e s - l o ma a l a edad de 50 y 8 KHs a l a s 60 añee, con pórM&r d4 send- bilidad t u i b l h rprrente.

A d d s e1 efde puede discriminar manides e mides seleeaieruo- de une en prr t imlar (Biacte de l a fies- de ~ecktoti). Eete erti- l o sin embar@ Brat8 principalmente el ameate iisioló@ae del efde w rnrtemfa se i lustra en 18 figtln 2 y l a s fuuciiones de l a mayerlta de a i s subdivisienes sen mostradas am 18 f i é t . 3, r s i come l a trblr- 2 muestra l e s f M K e e y prepleilades de 81R\rnrs de man e i t r n c w a .

El efdo extreme proteja l a meabr8n8 +Jimflelea y pese9 ma pre- pira caractoristicas f i l trrates. Sebrotedo tiene una m n c i r de 10 a 15 dB. en e l mwp de frecuencia do 1.5 a 7 K€lz aen picos en las- fromencirs de resenancia da e l canal 8ndifiVe g -nab, 2.5 y 5 - KHs reepoctiv8monlo. Las vibracienes de l a ram- d m h % c u indu- cida per aarbies de presión en e l can81 ataditrive1 oen trrnsiiitidos - rl d d e redis. Bi patrón de vibracienes de l a &bran8 tir)(niaa - han side rec ientaenh expuestas aame una aerie rwimenle complojr.

E3 otde medie aaeplr 10 impedancia de l aira para l e a tejidos - del efdo interne. Les huesillas rctdrn cema un multiplicuiar de -- hersas ba jo un priraese hidradlitm con una voaWja i e d n i o l relrci? neda w n e l radio del aren de l a mmbrnna timpdriim a e1 Irea de l a vonWna ev81 (apraximadamenfe 22: l O 27 dB.). I>9 impeduiaia es lie rPriente uert i@ada p8ra crear un incremente en e l ancho de banda.- Sebretode l a reapuesta on frecuencia de e1 efde externa, y iebio ea- tá per arriba y cercano a l o s 4 KBE con un paso de purtiguapienb-

centrade en l a s 4 m5 y c6n un deciinaaiento pdgresive para f r o -- cumcias mayores. Hay var ies factere8 que pued.I/ afectar l a trans - misiin de1 oide medie y mu respuesta en frecuen4ia, per eiemple te peralmente en las infecciones del oi&e inedia y e s poniulentai.n.tie- en esteoescierosis. Sin embarge, l a desaprcrdibi8 re&icción de ir -

siwslbllldad nerriesa s peer aán 18 p&?dIü8 de 18 8udleión per cea - ducción pueden ser tratadas por tratamiente qulrúr@eo y mecanls - mes de soporte. Des funciones d a que re8lI.r e1 esde medio sen - primero Im a l r r I r a presiones por l a conecctón entre l a oavldnd de ride medie y 18 gRr$8nf8 (tromp8 de Eustrqoio) y segundo eld. de - phtecciin produoitla per reileje centractii &e des atismien i tvr l~ crades cen 18 snapencl6n de l e s hueelllea, e l $enser del thpu~. y l e s dscules OSt8pbdiCes. Esta pntecclón sebr.~898 les 10 dB. pa- n ma iatencia eumade 18 eetimlación es de r i imes 10 ase#. es d~ srgr8drble. Tablón es bien conocide qne les dBcirles son rotiva - des can niveles moderados de senide. %l ves l e s mbscules dan las- caracterfsticrs f l ltrrntes b8je un ublente saturade (se sabe que- aofñrrn cene un f i l t r o pasa altas).

Lo f i g . 4 i lustra 1. ulntamia de 18 cecler, l o f ig. 5 ilua- tra e l c o r t e tranavenal de un8 sección de 18 cwcles y 18 fig.6 - iuesCr0 en dletalie 61 órmno de -rei que es 18 perto sonserial - del efdo Interno. Mecanic8mente l a d c l r es ¿Irlalble lengiftídi - nrliente per 18 iubrana b s l l 8 r . L8 helicewomB l @ a 188 dife - rentes pronienes eatltlcas entre is. escala superier ves.blbular y - l a escala icmer tlaplnlca. La ventana redenda cons.biimye Ir cenec- cióa esencial entre e l (1116. medle y e l fluido lncempreslble de la- cóaiea.

Nuestro cenecimiente r c k a l üe l a Rmclón de l a incnbrrna ba - s i l a r es bost811te amplie g ~ a c l a e a l e s experiiemnks r.crlfs8dos por pen Beiceay, q u l h WQ un micreacopie lumlnese uen un8 fuente estro - b e s d p i ~ a &e ilti.inrolón pan ebselmr l a s +iblicrcienes de 18 m a -

basilar. Ibicen9ri que 18s reglenes de Poxima vibracaón &e 18 membrana est& 6Lceudadas p a n respenaer st deturminrifas iremen - C i 8 9 , een a t a s fiwcuencias en e1 ip lce de l a c6cler. Sin embrtm- 18 a lx i ia no es de tsne muy agudo, pues existen CsMidenrblOs se - brelopulentes entre 18s fmcuenclas que sebrepasan una ectava (u- na .ctaa es el doble de una frecuencir). Ven Bekesy usó cidlvores humanas y nus resaltados han side grandaente dlsimztldes, pues u - mnde pnia.les vivos por experlmenBI(Ieres usande l a t 6 a I a ~ de aIs - sbauer y con pruebas de ilumlnaclQn 18ser ne ob0.íenn.n IF.ndes aif2 renclrs en l e s reeultrdoe. Estas diferencias gtisdsa s e r deblde 8 - l a s diferrmcias de t6cnIcas y 8 18s dif'ere1¶Ci8# i i s r r s maire te j i - des vives y muertxw. La reaptaesta de 18 cialea be ilustra en les - Qirgrurs de l a fig. 7.

*

histoiy stordge h u ma,ie US shy off this on a nirmh.! of occasions. Microfiche equipment and msthod5 h:iie no1 gained much o f a foothold as a second tine of ke<piri: much information compactly and transportably accessible, though i t has proved its value as an insurance to computer sssteni failure in making medical history details available..

The technological advances which we see in many other countries, where a different system of general'practicr ir; current, have certainly bypassed us in the UK completely. On the spot, in the doctors' consulting premises. spectro- scopic examination o f specimens, special biochemical electronic equipment, x-ray equipment, the varied array o f physiotherapy machines - aU this we have left to the hon- pital services, having open access to most of it. This, needless to say, avoids capital expenditure for such equipment and its replacement for those in general practice. It also ensures that the level of technical competence for our patients is supervised in hospital, and instrumental and technical accurate functioning is reasonably guaranteed - far more than an average GP could achieve, even with the instrument makers

he!i> an.! srrii,ing This need t o seivicc and mJin!J, 3

elah,>r,its equipmen! in msny small units is of couni- one O f

the gsneril piohlcinr o f technological advances in grncnl practice

It is aitoni;hing that in the preventive health educ.:iti' 11 field w e still lñigely rely on pamphlets, posters and advutiw- m e n t i , w h i n we haie a nation that is alinoct cornplitrly hook: d on the N box.

i t is now over ten y e a s since'Amstrong was taking his first lunar steps, 290,000 miles away, whilst his blood prerwre was recorded on earth, The non impact o f technology is still remarkable. A general practitioner will possibly once in his lifetime have a patient on home renal dialysis, or with a bioengineered artificial linib, the major impact o f technology is yet t o come. It may be that the impact will continue to be slow, just as the impact of the introduction of general sanitation and piped watersupply was not without its problems and took its time. However, it is an exciting age t o see how much we can usefully assimilate.

Sound analysis by the ear L. n. ROME

Science Divirion, Polytechnic of Centmi h d o n . 11s. New &endish Street London WlU8JS.

THE human auditory system exhibits very remarkable acconr- plishments. It is almost sensitive to the random (Brownian) movement of air molecules on the eardrum or to movements of the basilar membrane as small as one hundredth of the diameter o f a hydrogen atom [ 11 yet it is able to withstand sound that can vibrate the body (Table I). The ear can detect frequencies from about 60 Hz to about 20 kHz in the Young (Figure I ) with a best sensitivity between 1 and 3 kHz (in normal individuals the upper limit falls t o about 10 kHz at age 50 and 8 kHz at age 60, with losses of sensitivity also apparent). If we consider the number of possible discnmin- ations o f frequency and intensity ("difference limina") that can be made, they amount to something more than 300,000! [2 ,3] . Add t o all this the complex discriminations that can be made and the ability t o extract weak signals from noise (the 'cocktail party effect') and the performance of the ear can be fully appreciated. A wide range of disciplines have been required t o reach our present level o f understanding. There have included anatomy, physiology, psychology, acoustics, engineering. medicine and others. Much of our knowledge has inevitably been drawn from work on animals. This article will concentrate mainly on auditory physiology. The anatomy of the ear is illustrated in Figure 2 and the functions of the major sub-divisions are shown in Figure 3. ?able 2 shows the sizes and properties of some of t h e struct-

The outer ear protects the tympanic membrane (eardrum) arid has its own filter characteristics. Overall there is a gain of 10 to 15 dB in the frequency range 1.5 to 7 kHz with peaks a t t ' tc resonances of the ear canal and concha, 2.5 and 5 kHí 1 .-.i'xtivzly 141. The vibration o f thc tynipar,ii nicnihi.inc i n i i u i c d by pres>urc climgr\ in the ear can,il is trsii\iiiitt:.J I,, !'IC ossicle C h i n o f lhe iriidillz car. The vibration pattern (it thc t> r u p : t n i ~ n ~ e m I , ~ i , c he.: recently l>.,cn sliown ti, br "<. j C U * I i i , I r [ i]

1.11.. i i ~ i . ~ ~ i ~ i i ni.i!ctis.. ttic i i , i p c , ~ . i i i . of :,ir Iii II,." < > r 111,: ¡ I , . , ', C;.' ti\\!,<, 'I 8 , <, I I : .li I ai, ,?', d I,ydr.,i,ii, pr;,s (1:

ures.

'

$ . ' / 1 '

force multiplier with a mechanical advantage related to the ratio of the area of the tympanic membrane to the area of the oval window and stapes footplate (approxiin&l>c qL-- or 27 dB). The impedance is actually slightly undermatched to provide increased bandwidth. The motion o f the Ossicle chain is complex, over the normal range the stapes footplate undergoing a rocking movement. Overall the frequency response of the outer and middle ear is flat t o 4 kH7 with a

Table 1. Sound intenaities. Sound intensity b measured in deciiels (IdB= one tenth of a Bel). Decibels m e w r e I ratio of intensity or power to a reference level, number of dB = IO loglo fl/Io) The reference levcl normally used hearing U W B SPL, the p u n d r e sun level close to the human absolute threshold (IO-' W/cm or 1 x IO-' Nlm' or 0.0002 dynlcm'). Note that intensity is proportional to the squareof pressure (amp1itude)and that number of dB = IO loglo 11& = I O log10 (AI/AI)' 20 loglo (AiIAz)

P

~ ~ ..

Sound

Nearby jet engine Loud rock mncert (close to thrcrh- hold of pain) Road drill and loudest animal noises (bats. whales, howler monkeys) Shouting Normil conversa- tion Good mirrophonc's best rc,p>n-e Hhi;p:i Re'.. , . . I ,. ,. 1 C , < . I

-~

~

intensity Amplitude Sound Level Ratio Illo Ratio AIAo dB SPL

10"

10'0 IO'

106

lo3 1

io5 100 3.2 io3 70

1 o3 60

31 6 30 I O

~.

I l l

1 1 2 10 20

FrequeKy KHz

F-

j igure 1. Typical auditory threshold c u n e for a young Wult (the minimum perceived intensity at each frequency is measured).

L Y

A

I C

Figure 3. Schematic section of the ear (cochlea uncoiled) 'o illustrate the functions o f the major divisions. Many ;tmcfures can be identified from Figure 2 and the fo l low 'ng key identifies others: A scala vrsti0uli. B scala media. n rrola rvmonni D oraan of Corti. E helicotrema

1 t t

Rode of Action Vibra t i on i n a i r

Gatherliiy Fund. p ro t ec t i on and l o c a l i s a t i o n

I

MIDDLE EAU INNER EAU

Mechanical " ibrat ion F lu id v ibrat ion and mechano-

e l e c t r i c a l transductioi

Transduction of rrechanical cmergy

equal isat ion and energy, frequency

to d i g i t a l s i gna l conversion

.~

NERVOUS SYSTEU

Electro-chemical

Transmi-sion and

d i g i t a l pulsed information

ana l ys i s O f

.

l u l l < , ir

iúghr, ;

t:-..ii 11,;

1% ,,-,;<x in <)<te, nerve d i operaiirf are perf;. is carrii:i the thro protectii involved tympani that the the laten against i musclts inuscles sound cc and they

.

~c F.

?gurure 4 ources, ? coch1# vindo w

G-

"-..

I-.-.

l . J.. i

! I

'Yairre 5 ii:ileO 'W"1a I,

<.a:, :, . , : I ;o,

... .. ~L . . ~ ~ . .

hallow heak centred at 4 kHz and a progressive decline t o 'nigher frequencies Various factors C M effect middle ear

transmission and the frequency response, for example ?mporarily in middle ear infections and more permanently

. n osteosclerosis. However. unlike reduced sensitivity in - n e n e deafness, conduction losses can often be treated by

operative procedures and deaf aids. Two other funclions -.re performed by the middle ear. Firstly, pressure equalisation

~ s carried out by the connection of the middle ear cavity to %he throat, the eustachian hibc. Secondly, some measure of

protection may be gained by reflex contraction of two muscles l n v o l v e d in the suspension of the ossicle chain, the tensor

:ympani and the stapedius muscles. However, it is unlikely +hat the protection amounts to much more than 10 dB and

the latency of at least 10 msec is toolong toafford protection --against impulsive sounds. Also it is well known that the

~ muscles are active at moderate sound levels 161. Perhaps the muscles alter the filter characteristics to suit certain ambient

sound conditions (they are known t o act as high pass filters) e e n d they may also reduce non-linearities.

-1 - L

c

..... c

I

c

i

c

I Figure 4. The inner ear (drawn by the author from various -I sourceJJ. Key: A semicircular canal. B vestibular nerve.

I 1 C,cochiear nerve, D cochlea. E round window. F oval P window.

B

C D

E

F

. . . . . . . . . .

The drawing in Figu;: 4 illurtrates the andtomy of thc cochlea, Figure 5 illii,tiitcs a crws section'of one coil of the co.:l'l :.I and Figure 6 sbo\ts in detail the organ of Corti which Is the sensory p:irt of the ear. Mechanically the cochlea is divisible longitudinally by the basilar membrane. The helicotrema equdises static prcssurr differences between the upper scala vestihuli and the lower scala tympani. The round window provides the essential pathway back to'the middle ear from the incomoressible fluid of tbe cochlea.

T h e hydromeccanical response of the cochlcd ir un. douhtcdly complex (71 Ths mo>t imporiani rc',ponw 15 thl t of the basilar membrane and the organ of Corti It must l>r

Table 2. Some o f the dimensions and properties of structures of the human ear. Mwnr or ranger arc $ven (information collated from a large number of uiurns).

Outer ear Pinna

Auditory canal

Mid& eu Tympanum

Middle ear avi

Malleus lncur Stapes

Inner ear Oval window

Round windou Cochlea

Helicotrema Basilar mcmbc

Organ of Corti

Hair cell.?

- Dimensions and Cnaracter ___ Concha volume 2.5 an' Concha resonance 5 KHi Crow section 0.3-0.5 cm' Length 2.3-3.0 cm Diameter 0.1 an Volume 1.0 em' Resonant fre-

quency 2.5 K H Z

Diameter 7.5-9 mm Arca 0.5-0.9 cm' ThiJiisU 0.1 mm

Breaking strain DUpkcemont foz a 250 Hz tone:- 1.25 nm at 75dB SBL 0.5 nm at 65dB SPL

Volume of owides 0.5 - 0.8 cm' Weight 23-11 mg Weight 25-32 mg

2-4.34 mgs Weight Area of foot plate 3.2 mm Displaaments for constant pressure at the ear drum of ldm/cm':-

-.

Weight 14 ny 1.6 x 10' dynlcm'

-v+-.-. 29 m'

125 HZ 75 x 10- an 200 Hz 28 x lo-' cm 400 Hz 20 x lo-' un

18 x IO-' cm 750 Hz 1500 €12 10 x lo-' cm 2000 Hz 6 x lo-' an 2500 Hz 2 x 1 0 - s a n Maximum displacement 0.1 OUR

Dimensions 1.2 x 3 mm to 2.0 x 3.1 mm

Dimensions 2.25 x 1.0 mrn Number of I U ~ N 2% Volume 98 mml Length 35 mm

0.08 ~ 0.04 mm' Area Length 34 mm Width, basal end 0.04-0.08 mm Width, opical end 0.42-0.65 mm Ciois sectional area at basat end 0.00053 mm* crosr SeLtional area at apical end 0.022 rnm' Number inner hair cetts 3.500

I

. . I

:Aa ... .

i

remembered that the basilar inemhi..ne is nari ’mv Jí t h c ! of the cochlea and progressively broadens towards the apex. This observat.ion led to theories that the basilar membrane was responsible for the separation of frequencies in thesound stimulus. in 1857 Helmholtz postulated his theory that auditory nerve fibres each signal the frequency of a disting- uishable tonebecause they are connected to specificresonators. However, the anatomical structures relating to the resonators could not be found and there is now much evidence available to discredit the theory. For example a tone pulse o f only two cycles is only 10% worse than a continuous tone for pitch discrimination yet it would not allow resonators to accumulate sufficient energy to ailow such a discrimination. Also observations of the basilar membrane show that it is not under the tension required by the resonance hypothesis1 11. Wever [SI has summarised the problems inherent in the resonance model.

Our modem understanding of basilar membrane function is largely due to the experimental work of yon Békésy l l l who used a light microscope and stroboscopic illumination to observe and measure basilar membrane vibration. He found that there were regions of maximum vibration of the membrane for each frequency, with high frequencies at the apex of the cochlea. However, the maxima are not very sharply tuned (Figure 7) there being considerable overlaps between frequencies of over an octave apart (one octave is double a frequency). The phase lag of the vibration progressively

1~.1\,;5 : : , . r a i d s [fie apex of the cochlea. Il ie h i ! : i rm, .m- braiie inotion is thus in the form of a trdvi.lliiig wave (l’igiire 7a) which increases in .irnplitude and then dies away more abruptly. With high frequencies the amplitude peaks occur towards the basal end of the cochlea and with low frequencies the peaks occur at the apical end. The peaks are more sharply tuned at the base. Thus each frequency has a maximum amplitude place on the basilar membrane which thus acts as a Fourier analyser (Figure 7d). incidentally a theoretical advantage of a travelling wave is that transients are largely avoided because storage of energy does not occur 191.

Von BekCsy used human cadaver ears and his results have been largely supported, often for living animals, by workers using the Mossbauer technique [ l O , l I , l 2 , 1 3 ~ , capacitance probe techniques [ 141 andlaserillumination techniques [ 151. The diffcrences in the results may be related to details of the techniques, to differences between Living and dead tissues and to the species used, although some aspects of cochlear non-lineanties remain to be resolved. The responses of the cochlea are illustrated dirgramatically in Figure 7. It must be remembered that such illustrations are much simplified, not least because of the lateral attachments of the basilar membrane and the other structures associated with it. Confumation of frequency being related to cochlear place comes from post mortem studies of cases of deafness caused by cochlear degeneration. For example, a patient who could

!’

igure 6. Cross srctiuu o l f h e cochlea to show in detail the orson ufCorti(dr0wi~ig by the ,~ut l ior from Iii.~tologiCol obsur. otions andfrom a w n e t y of sources). Key: A spiral limbus, B apirul sulcus, Crpiral lirnina, D spird guiiglion. E brunch of ochlear neve. F outc? hoir dls. G n m e fibres, H rods of Corti, I inner hoir cell. J border cells of fnnrier mlcur, K scab erfibuli. L Rcirincr’s iiirmbrilne. M stria rus(.uIaris, N s&la in~dia. O tectorial membrane, P Hrnsriii d l s . Q Dciteri rlk, R Cloudius’cri!,~. S rpirirl hgoment. T barilar inrrnbrane. U scala ljmpani

.~~ ~ - . -. ~ ~ ~~ __ ~~.~ ~ .. ~ ~ ..

not hcdr above 2048 Hi WAS found to have a deyenerJti bawl end of the cochlea. Such studies show that 2 k H z corresponds t o about 11 mm from the oval window, 4 kH7 to about 8 mm from the oval window and 8 kHz t o about Smm from the oval window, results which are perfectly compatible with the studies described above.

Transduction o f the mechanical vibralion of the basilar membrane into electrical signals to the brain is complex and does not occur in one stage. This process is also probably the least understood in auditory physiology. The sensory cells o f the organ o f Corti are the hair cells, one row o f inner hair cells, and three rows of outer hair cells (Figure 6 ) . The cilia (fine projections of the hair cells) touch, or are emhedded in, the tectorial membrane. The basilar membrane and tectorial membrane are not hinged in the same plane and their relative movement duringstimulation probably gives rise to a shearing force on the cilia of the outer hair cells and possibly on the cilia of the inner hair cells [161. It is certainly the mechanical forces, in whatever form they take, that cause electrical changes in thereceptorcells. Steady

- w,

I

I

I - ..., - ....

I , ,

state ( r i i t in; ) potentials a n d sticiuliii generated pot<i:tiil changes ociiu i n thc cochl-1. The scats media exhibit, .I rezting putentiil of + 80 ni\', thc highrst s f e ~ d } pohitiii potential in the body. l h i , potential is produced by ?ne%:.' dependent ion exchange and is maintained by the stri3 vascularis. I'hc hair cells have an intracellular potential of about ~ 70 mV and there is thus a potential ofahout 1 S O r;iV a c r o s the cilia bearing membrane o f a hair cell. Perhap, this potential gradient is needed for modulation by the stimulus to produce a receptor potential (see below). Potential changes induced by a stimulus úicluae the little understood summating potential, which is of uncertain origin and occurs asa baieliriz shift during the stimulus, and the cochlear microphonic (CM). The CM, first described in 1930 by Wever and Bray 1171 follows the form of the input waveform. Much of the avaüable evidence points to the CM being a receptor potential generated by distortion of the cilia of the hair cells. Von Békésy first indicated that this might be the case by demon- strating that the cochlear microphonic is proportional to the displacement of the basilar membraneI181. Then with

a

b

C d apen A

base apex

distance

frequency

frequency KHz

lguri 7. Diugrattrimiir reprerefilarion of the propcriies o f I ~ L . borifar membrane: (I shows rhree travelling waves in fhe irilar niemhrani, at a n inrlunt of rime ((i) high frequency. (if) medium frequency and (iii) low frequency), b show,s the rape o f a iypicul h i r i iw cun'c of the basilar membrunc ohiairir.d by niea.rirring the amplitude o f vihrarion ai a poinl a.y a I trcl ion o f frcquG::i<j. o f Sti!7ildUtion. c shotis fhevrrrical exitalion pa1lern.r along the hnrilor memhmn: reriilling from itnulaiioii at t w i frwurticies amf d shows pcnk bosilar mcnrbronr vihruiion for of1 frrquencics ai a function o f dislance ong lhe nii.>r;hram

.... .~ ~ . . ~ ~ ~~~ . ~ . .~ .~ ~

the introduction of differcntial rri oi,ling by 'Tawki arid Fernandez in 1952 1191 it was s h o m that the CM origiuates in or near the hair cells 1201 and that the amplitude and phase of the CM recorded at different distances along the cochlea mirror the form of the travelling wave 1211. .Thus, for example, the amplitude of the cochlear microphonic for a high frequency is greatest at the base. Because of thjsrelation- ship the CM has been used to study cochlear function and has been in human electrocochleography (auditory nerve recording and brain recording have now largely replaced the use of the CM in studies of human hearing defects). The CM apppean to be related t o the signals to the brain in the auditory nerve, the action potentials, as complex components in the CM can be found encoded in auditory nerve signals (221. However, there is unlikely to be a direct causal reationship between the CM and action potentials as inter- mediate steps of chemical and/or electrotonic signal generating mechanisms almost certainly occur between junctions (synapses) of the hair cells and afferent nerve fibres. Theresult of the whole transduction process i s to convert an analogue signal into a digital code o f all or nothing action potentials that conduct non-decrementally towards the brain ('afferent' direction). Before considering how frequency and intensity of the stimulus are conveyed in the code it is worth considering two further aspec!s of cochlear physiology, f i t l y the difference between inner and outer hair cells and secondly the role of efferent nerve fibres from the brain t o the cochlea.

It has been suggested by Tonndorf [23] that the two hair cell types respond differently to shearing forces on their cilia. Danos e f al ( 161 and Billone andRaynor 1241 havesuggested that the outer hair cell potentials are proportional to displacement of the basilar membrane and that inner hair cell potentials are proportionalto velocityof thebasilar membrane. It has been further suggested I241 that there may be diffei- ences in sensitivity and frequency selectively between the two hair cell types. The two types also exhibit structural differences and whereas several afferent nerves contact one inner hair cell, several outer hair cells may be contacted by only one afferent. Observations reported by Spoendlin in 1970 (251 that 90-9546 of the afferent nerves supply the smaller number of inner hair cells was initially greeted with caution. However, with Spocndlin's further work [26;271 and the support of other researchers 126,291 this finding has come to dominate the literature in this particular field. Continued investigations using histological and electrophyo- cological methods and methods utilising drugs that selectively damage outer hair cells have yet to explain the details of the functional differences between the two hair cell types. It may be that a very recent report 1301 of a new mutant strain of mouse that lacks inner hair cells and that has apparently normal outer hair cells, but that has been shown to be deaf in behavioural tests, may shed some light on this problem.

T h e inner and outer hair cells also differ in their efferent innervation. Whereas efferent fibres synapse directly on the outer hair cells they synapse with the afferent fibres close to the inner hair cells. The efferent fibres originate in the superior olive, a nucleus of the brain stem. The fibres of the inner hair cells have been regarded as coming from the same side of the brain (uncrossed olivocochlear bundle) and those to the outer hair cells as coming from the opposite side of the brain (crossed olivocochlear hand1e)although this may be an oversimplification 1311; Rasmussen 1321 f int described the efferents and subsequent studies have shown that they inhibit the responses of auditory nerve fibres (331. As might be expected the CM is largely unaffected. The inhibition may amount to the equivalent of a 25 dB reduction of the stimulus. Behavioural experiments suggest that animals with the crossed olivocochlear bundle cut have a poorer frequency ir<oliing power, especially in noise. *The efferent system ,,,;pe;irs to be iinportdnt for orgsnised rcsponces, perhaps .,cling as a gain control (altlioiigh absolute thrrshulds are not

~~ . ~- ~~ . .. ~ ~

?....A, iu.., r

.dtcird) (31 perhaps acting to improve fiequency wlictivity (the reviews by Eldredge and Miller (341 and Iiirato [351 give further information).

We now come to the question of how stimulus frequency and intensity are coded in the auditory nerve. Since Tasaki published the first report of activity in single auditory nerve fibres in 1954 1361 many studies have been made with the notable landmark of a monograph devoted to such studies by Kiang e f al in 1965 1371. Generally the auditory neurons show sponianeous activity in the absence of a stimulus with rates of action potential firing between 1 and 150 sec.'. The maximum firing rate of the all or none action potentials is about 1000 sec-'. When a stimulus reaches a threshold value the spontaneous rate increases and continues to do so fairly linearly with increasing stimulus intensity of the stimulus until the response saturates. The dynamic range over which the rate of firing increases is often little more than 40 to 50 dB and there may be little variation in the thresholds of different fibres 1361. This observation has implications for the coding of intensity over the dynamic range of the ear as a whole (seb below).

Each neuron has its own characteristic frequency at threshold (the CF). On either side of this frequency stronger intensity tones are required to elicit a response from the .

Fiequew KHz

Figure 8. Responses of indzpidual auditory nerve fibres (diagrammafic): a shows t6ree typical funing curvesfor indiviúual auditor). nerve fibres ( f h e intensify required IO increase the firin8 rote above the base-line level U

measured foreachfrequency rested). b. iminfenriry curves for a iingieaudifory nerve fibre feorh r u n e is ohruined by tnaintoining fhe stimulus iilitensify consrunt arid meomring the discharge rate as afuncvion offrequency). Six curves are shown for inrrnsifier in ÍOdB sfcps where OdB ir fhr iufenrif? justfai1ir:y roo l f e r the dirchorsr rafe o/rliefibrc.

~ ~~

r -7 A

r I 1 I. I I P T I T

r

n I

I I

J !,+tire 9. Diagrammatic representtrtion of the ascending

1 lirory paihwriys The pathways of informarion from riglit ear are shown as solid 1iric.s and from the left

' ' us dotted fincs. Szih~divisionr and h k r wirh brniri I L A ' C ' J > rurh a,p the cirfbcllum ui id rcIicul,:r formofion are 11, 1 rhow,,.

~. .. .. ~ .. ~. .. .. . ~~ ~ ~ ~~

c "

.c: 1 ?;,t. 1 I ly 19Yü

ahui,! 5 kt l t p'i.!.:-lo ni prohi!'l) b:e !!.s dr>,hn b ? C : I J X

the inazcurxie, ('nel!:d jitter') ap7ro:tch the pe:iorl of the ton:. Thus at 10% to medium frequznsisz the ph3s<-Iockir:$ mc.hariinrii COLI'$ ad2 to the place principle in coding frequency. l'he fait that the neiiil! reiponie c u r x s are shlrper t h r i the basilar membrane responie ma) indicate that neural sharpening is occurring by thc procesi o f lateral inhibition along the cochlea. The sharpening cou!d also be due to intervening non-linear mechanical events [131. The latter is suggested by the results o f Arthur et al I441 who showed that a second tone close in frequency t o the C F of a neuron can suppress thc firing rate o f that neuron but that the suppression appexs to be too rapid t o be neurally mediated. With two tones having overlapping excitation patterns a neuron normally is captured by one o f them. Interestingly if three harmonics are presented a neuron may phase-lock t o the missing fund3mental[45] , a mechanism that may provide the basis of the perception o f pitch in complex tones. Click stimuli have also been used to study auditory nerve function. Clicks contain a wide range of frequencies and stimulate the basilar membrane correspondingly. In single nerve fibres the response is normally a train o f action potentials at intervals that correspond to the C F of the neuron concerned.

However, i t cannot be assumed that interpretation o f frequency or intensity is simply a matter o f brain centres analysing the responses of individual pattern. This can be illustrated by the following examples. For two different frequencies with overlapping excitation patterns on the basilar membrane that have different intensities, the excitation at one place can be the same for both. At moderate intensities one auditory nerve can bestimulated b y a rangeof frequencies. Conversely, therefore, many of the 30,000 to 40.000 fibres will be stimulated by one frequency. F o r a moderateintensity of 40 dB SPL (sound pressure level) thin may be as many as 25% of the total 1461. Thus i f the frequencyis changed by 1% (an easily perceived change) and 10.000 fibres are active in each tone. then 200 fibres will be gained at one end and 200 fibres will be lost from the, otherend. Therefore 98% oi&e-.. fibres concerned are activated by both tones. Intensity coding is also complex. Auditory neurons have a dynamic range o f up t o about 50 d B compared t o the 150 dB range o f the ear. intensity could be coded by different neurons having widely spaced tiiresholds, but as previously stated this is not the case [381. It may be that thecritical factoris thespread of excitation and saturation along the basilar membrane as intensity increases.

Auditory nerve fibres terminate in groups o f neurons in brain stem nuclei. These neurons in turn contact higher brain centres (Figure 9 ) . At each stage information processing and analysis occurs. In the higher centres neurons tend to be switched on or off. This may help t o define the edges of frequency distributions and provide information on the occurr- ence of intensity bands. This form o f coding may be utilied because auditory nerve information, of for example phaselocking, could not be preserved through the inaccuracies o f successive processing stages. Lateral inhibition between neurons sharpens the divisions between frequencies and narrows tuning curves. Also interactions between neurons can produce more complex responses and detect features of the stimulus.~ The structures of the brain nuclei are ideally suited for this processing, normally exhibiting tonotopic organisation. The cells are arranged in terms of their frequency response in relation to their place on the basilar membrane (471 and for a particular frequency the cells maybe arranged progressively with intensity. One further important feature o f processing in the brain is the intemonnection and mixing o f information from the two ears (Figure 9) . This is especially irnportant for spatial perception where the mixing primarily occurs in the olivary complex of nuclei of the brain stem befocc. phaselocking and intciiqity informiition is d r w d e d (see hclvw). Soilii: neuriin.5 in thi. olivary ~ o i n p l i x specifirally r r~pond t o dif¡cii.iircc in aii::al time of the stiniiilus at e&Lh

ear and sonle respond t o intensity d i f f e i i r , r , : i . ( I f t h ~ . I l . . i , l i i s is lateral to the head the opposite ear will be in ocoilstic shadow.) T i ~ u s the responses of such cells change progrrssive- ly as a stimulus moves around the head. After such cells have analysed the information available in t h e incoming fii,re!i it can again be coded by on and off mechanisms through ihe higher centres.

In the space available i t is not t h e author’s intention lo examine further the processing of auditory information b y the brain nor to attempt t o introduce thearea of psychoacoustics, b o t h warrant a n article of their own. The interested reader is referred t o works by Webster and Aitken 1481 and Moore I491 respectively.

In summary, the analysis performed by t h e ear is iarg,eIy shaped by the mechanical responses of t h e outer , middle and inner ears and the form of t h e all or nothing coding signals in t h e nervous system. Whereas many of the mechanical processes are fairly well understood, our knowledge of t h e transduction process is incomplete. However, recent advances in t h e study of t h e hair cells may develop this area rapidly [ S o l . Our knowledge of t h e coding of information from t h e ear to the brain and within t h e brain has improved dramatically, but only small numbers of t h e huge total of neurons can b e sampled individually. The benefit to be gained from t h e rapid expansion of auditory physiology lies in our ability to bet ter understand auditory dysfunction, especially of t h e sensory apparatus and W N O U S system. It is perhaps,fortuitous that this expansion of knowledge has occured alongside developments in other areas, for example electronics. I t is now possible, for example, t o contemplate considerable ‘success and development of implant techniques for direct auditory neNe stimulation in cases of cochlear malfunction.

~ ~._____-.- -. ~

REFERENCES (11 BbkCsy, G. von (1960) Experiments in hearing. Mffiraw Hi4

New York. 121 Stevens, S. S. and Davis. H. (1938) Hearing, i t s pwcholog) and

physiology. Wüey, New York 131 Whitfield, I. C. (1967) Coding in the auditory nervous system.

Narurr., 213, pp 756760. (4) S h w , E. A. G. (1974) The external ear, in ‘Handbook of Sensory

Physiology’Eü. Kiedc1,W.D. and Neffs,W.D. Springer, New York. 151 Tonndorf, 1 and Khanna, S. M. (1972) TympaNc membrane

vibrations in human cadaver ears studied by time averaged holography. J o u d of rhe Acoustical SoOcry of Amcncn, 52 pp 1221-1233.

(61 Simmons, F. B. (1959) Middle car muscle activity at moderate sound levels. A n ~ k of Oto-Rhino-LulyngoloU, 68, pp 1126.

171 Tonndorf, 1. (1970) Nonlincarities in cochlear hydrcdynamia Jountol of the Acoustic~l Society of Amen.cn, 47, pp 579-591.

181 Wever, E. G. (1949) Theory of hearing. Wüey, New York [9] Bzkésy, G. Yon (1970) Travelling waves as frequency analyrn

in the cochlea.Narure, 225, pp 1207-1209. [IO] Johnstone, B. M. and Bayle, A. J. F. (1967) Basilar membrane

vibration examined u,ith the Mossbauer technique. Science. 158, pp 389-90.

[ll]Johnstone, B. M,Taylor, K. J. andBoyle,A. J. F.(1970)Mcch- rnics of guhea pig cochlea. Journal of rhe Acoustical Sm’ery of Amencn, 47, pp 504-509.

(121 Rhode, W. S. (1971) Observations of the vibrationof thebasilr membrane in squirrel monkeys using the Mksbauer technique. Joumdof theAcourticalSociefy ofAmen’cn, 49. pp 1218.1231.

(131Rhode. W. S. and Robles, L, (1974) Evidence from M6r:;baucr experiments far non-linear vibration in the cochlea. Journal of the A c o u r t i d Soriery ofAmenca, 55, pp 588-596.

(14]\Vilwn, I. P. and Johnstone, J. R. (1972) Capacitive probe measures of basilar membrane vibration, in “Hearing Theory”, IPO. Eindhaven.

1151 Rohlhffcl, L. U. E. (197?)Artudyofbadlarmcmbrinevibiations 11. ‘The vibratory amplitude and phaw patlein along the basilar membrane (port moilem). Acousrico. 27. pp 66-81.

116) Dsllor, P., Billone, M. C., Durant, I. O., \Ving, C. Y . nnd Raynor. S . 11972) Cochlear inner and outer hair cells: functional differ- rnier. Science, 177. pp 356-358.

[ lí ]\i‘crer, E. G . md Riay. C. (1930) Aiiion cur~ent\ in ihz ;¿ilitoiy

I . r i e in ri!pc.nse to acoustic rtirniilaiirm. froceedings of r:ie

húii.i~iolAcodrmy of Science. U.S.A. 16, pp 344.350. 1181 Iiékésy, G . von (1951) Minophonics produced by toiichiog ilie

ioclilear partition with a vibrating electrode. Jounlol of the Acuustinrl Sociery o f A m e n w , 23, PP 29-35

[19]Tasaki, 1. and Fcrnmdcz. C. (1952) Modifiration o f cochlear microphonics and action potentials by KCl solutions arid by direct currents. Journal of Neurophysiology, 15, pp 497-512.

(2O]Tmki, I , Davis, H. and Eldredge, D. H. (1954). Exploration of cuchlear potentials with a microelectrode. Journal of fhe Acoul- fiad Sociery ofAmedcQ, 26. pp 765-773.

[Z1]Tasaki, I., Davis, H. and Legouú, 1. P. (1952) The spsce-pattern of the cochlear microphonic (guinea pig) as recorded by differ- ential clectxodes. Journnl of rhe Acoustical Society ofAmerica. 24 pp 502-519.

[22] Pfeiffer, R. R. and Molnar, C. E. (1970) Cochlear nerve libre discharee Iiatterns: relationship l o cochlear minophonic. SoencP 167, pi1614-1616.

1231 Tonndorf, I. (1970) Cochleq mechanics and hydrodynamics, in ‘Foundations of modern addilory theory,’ Ed. Tobias, I. Y., Academic Press, New York.

1241 Billone, M. C. and Raynor, S. (1973) Transmission of radialshear force to cochlear hW cells Jouml of rhe Amusticol Sociery of America, 54,1143-1156.

1251 Spoendlin, H. (1970) Structural basis o f peripheral frequency analysis, in “Frequency and periodicity detection in hearing”, Ed. Plomph, R. and Smoorenburg. G. F., A. W. Sythoff, Leiden

1261 Spoendlin, H. (1973). The innervation of the mchlear receptor. in “Proceedings o f a symposium on basic mechanisms in hearing”. Academic Press, New York.

and models in hearing”, Fd. Zwicker, E. and Tcrhardt, E 1271 Spoendün, H. (1974) Neurpanatomy of the c6chlea. in “Facts

... Springer, New York:.

1281 Smith, C. A. (1972) Preliminary observations on the terminal ramifications of nerve fibres in the cochlca.Acra Orolaryngologicn. 75 pp 472-485.

Iz9)Wright, C. G. and Preston, R. E. (1975). Cochlear innervation in the guinea pig - II. Arta Ofol<rryngologim, 78,335-342.

1301 Deol, M. S. and Gluecksohn - Wielsch (1979) The role of h e r hair cells in hearing. h‘unire,; 278, pp 250-252.

1311 Fcx, I. (1968) Efferent inhibition in the cochlea by ihe olivocochlear bundle. in “Hearing mechanisms in vertebrates”, ClBA Foundation Symposium, Ed de Reuck. A. V. S. and Knight, J ... Churchill, London.

1321 Rasmussen, 1. (1943) “Outliner of neuroanatomy*’, William C. Brown, Dubuque, Iowa

[331Teas, D. C., Konishi, T. m d Niclmn. D. W. (1972) Electro- physiological studies on the spatial distribution of the nossed olivo-cochlea bundle along the guinea pig cochlea. Journal of rhe Acousticnl Society ofAmencn. 51, pp 1256-1264.

1341 Eldredge, D. H. and Miller, I. D. (1971) Physiology o f Hearing. Annud Review of Physiology, 33, pp 281-310.

135) Iurata, S. (1974) Efferent innervation of the cochlea, in “Hand- book of Sensory Physiology” Vol VU), ed KKdal, W. D. and Ncff. W. D., Springer, New,York.

[36]Tasaki, I (1954) Nerve impulses in individual auditory nerve fibres of guinea pig. lounid of Neurophysiology 17, pp 97-122.

[37]Kiang, N. Y-S., Watanabc, I., niamas, E. C. and Clard, L. F. (1965) “Discharge pattern) o f ringle libres in the cat’s auditory nerve:’ MIT Press, Cambri4ge, Mass.

(381 Kiang, N. Y-S., (1968) A ‘survey of recent developments in the study of auditory physiol Y Annals ofOrolo~ . Rhinology and Loryyngology, 77, pp 656-35 :

1391 Kiang, N. Y-S., and Moxon, E. C. (1972) Physiological consider ations in artificial stimulation of the innerear.Annalsof Orology. R I ~ 114-73n. --.rr . - ~-

14uJ Rule, I . E , Ilind. I . E, hnderson.D.1. and Brupse, I . F. (1911). Some d i m s of stimulus inlrrnl on the Iríponu of auditory ncrre fibicr in the rquirrcl monkey. J o u r n a l ~ f N ~ i ~ p h j s i ~ l o ~ .

I.

34, pp 687-699. 1411 Rose, I. E. , Burg% I. F., Anderson, D.1. and Hind:, J. E. (1967).

Phase locked response to loa-frequency tones in ringle auditory nerve fibres of the squirrel, rnonlr).. Jouninlofh‘euruph)fo~ .~ ,. 30, pp 769-793.

[42]Ro%, J.E.. Brua-e, I.F.. Anderun, D.J. rnd Hiid, J.E. , (1968) Paitrini oí activity in ringle auditois nrrre fibres of the rquirrrl munkey, in “lrcaring mcchanismi in icrrrürdrcs”, ClBA Fwnda- tion Symposium, Ed dc Surck. A.V.S. .nd Knight. I.. Churchill. Lundon.

MICRQPROCESIDOB W AüDIOiOQIA Y BEULLOFISIOLOGIA.

Los microprocasadores tienen mehas aplicacienes potencialoe - en les campos de l a audioiogía y neurofisielegfa, por ejomple en - l a s p raaks de diagnbstice, soperte auditivo, di8cplbstiCe, archive- de l o o expedientes de cada paciante, etc. Este a~ t f cu lo diecute la- utilidad ai l o a primeros des cases, recerdande ex u- croidn de lest- rIcrsprocosadores J tratara de exponer a l g u ~ s cmclusienea ceme - 108 beneficies que nos propereione e l UM de éstes.

LO prueba principal .s andielegfa es e l andie- do fane pu- TO, que aide e l umbral bo1 paciente a l e s aenides do b s ~ ~ puree. - El oqulpo aaecuade para esta p m e h ha estade eihtpenibla .II 188 elg nit388 per muchos años. La ventaja del aicreprecesader es e1 faci l i - tar l a colibracibn de e l equipo 7 en l as pruebas d a coaplejas pro-

porciona parbotros da ccnteo y crenemettg je . Las rW~Uit88QS pueden ser transferidos a una cenpuao8era para alarcenmdente de datas. - i f inme de h t e s cambios a alterado l a velocidad O exactitud de l a s pmebas clinicas. auís complejas pruebas psice-roúetíacls, per e- jomple detecci6n de perferacienes en l a roabrana timpbica deberfr- beneficiar l a apiicaciin de l micrepmcessd6r, pera ramaente se 112 va a eabo esta prueba en l a prlctioa. TQdaVfP e l micnpracoaador no ha tenide iapicte add .

&a reqmeata eiáctrica audionétrica es extmoameaite usada ahi- ra en audiolegfa come m a prueba ebjetiva de oído, partfculaniento- en infantes.

Otros baneficies es que m4tedos l e s cwies han sido des8ml l a - 40 en üiSteW8 brSrrd0S COmpUtadQra, pd- abr0 ser i io luides equipe eatandar.

aI pruebas de eídos ai infantes e1 microprcpceaodor ha side a - plicade para autoaaticer l a ala-, refiejo a l 8onido.El aicropnoe - sader es esencial en esta técnica, pero e l iac.bgr limitante &e l a - prueba, es quo es eo10 tmo medida de l a respuesta do senides 8l.b.- y ne ha side alterade, La p-eba del aisteaa vebtibúiar involucra - medicienes en l a fase de velccid8d baja de1 nystaepirs requeata a - l a estimuiaci4n cal6rico.

c

L

- c

LOS 3 principalae pruebas en naurufisiaiegi~ men EEC, ioLG y - petenCi8les evacadas. El registra de EEO y EER han tiido 8fec - tadas. Sin mbareo, came a t uiotd preoimente, a l mioropncauatlor fac i l i ta cansider8blunenta a l r eg ia tn de potenciales evocadas.

En suma, e l moJtor l q a c h % e l a ic2upnceWer en pnaabas da- diagn68tico en audlelegía y neurefisiolegía ha side inormmen*r - a l número de PlcLlIda&es dfiles, can paqueñes cambies en l a vele- c1d.d o exactitud con l a cual l a s pruebps san liavadas a c8be.

DO vuelta a l a aplicacibn da l e s miareprocesodores para am- da aud i t l a , ayuda canvenclogal, l a cual cempenba a ofdam dafílclas mpiifiaando a1 senido, tedavía ne se ba viste La aplicaoi4n dol - mlcroprecesador.

gar a severas rastriccionaa aún C Q ~ l a aicroeledtrinfoir de hay. E3 requerinilente de un tamefie coaae'biclriauitm acmptrble da 1u - Bh sum, a l iicnpncescrdar esta la jas Be S O ~ O rnaontr8r un-

mejor urn en la6 pruebo y seleccl6n de ayuda.

c.

Microprocessors in audiology and neurophysiology J. C. Stevens Shefield Uniuersity and Health Authoritj, Dep<irtiiirnt o/ Medical Physics ond Clinical Engineering, Royal Hollarnshire Hospitol, Glossop Rood, Shefield SI0 2JF, U K

Microprocessors have many potential applications in the fields of audiology and neurophysiology, for example diagnostic tests, hearing aids, patient diagnosis, patient records and training. This paper will discuss the fint two of these, reviewing current usage ofmicroprocessors, and will try to draw some conclusions as to the benefits they have brought.

The wain test in audiology is the pure tone audiogram, which measures the patient's threshold to pure tone sounds. Adequate equipment to perform this in the clinic has been available lor many years. The advent oftbe microprocessor has facilitated calibration oftheeqüipment and in the morecomplex tests provided counting and timing functions. Results can be transferred to a computer for storage, useful in industrial screening. None o f these changes has altered the speed or accuracy o f clinical tests. More complex psycho-acoustic tests, gap detection for example., should benefit from the application ofthe microprocessor, but there is rarely time lo carry out these tests in practice. lmpedancc audiometry comes second in terms of numbers of tests performed in an audiological clinic and recently many automaticmachines have been added to the range available. As yet the microprocessor has not had any impact here.

Electric response audiometry is widely used now in audiology as an objective test ofhearing, particularly in infants. The microprocessor has found wide application here, as witnessed by the recent explosion in equipment available. The averaging, display and storage of data can now all be handled by the microprocessor which should lead lo a significant reduction in the cost o f this equipment. Another benefit is that niethcds which have been developed on computer-based systems, for example machine swring, can now be provided on standard equipment. Howeier, only small changes in the speed or accuracy should be expected as the microprocessor has not changed the basic scnsitivity of the test. In infant hearing tesfs the niicroproceisor has been applied to automating the startle. reflex to sound. The microprouessorisessential to this technique but the limiting factor ofthe tesi,'that it is only a measure ofthe rcspoii'e to loud snunds, hachoi been altered. Thefestingofthe

vestibular system involves measuring the slow phase velocity o f the nystagmus response to caloric stimuli. The use ofmicroprocessors should lead lo a reduction in the cost o f equipment which calculates this velocity.

The three main tests in neurophysiology are EEG, E M G and evoked potentials. The recording of EEG and E M G bas been little affected. However, as noted pie\ iously, the microprocessor facilitates considerably the 'recording of eToked potentials. Normalized averaging is now a standud feature o f equipiiient and it should he possible lo p~oYideion.g~es~as!ished ideas o,n larger systems, such as phasÚ-distortion free filtering of the waveform. Analysis anddisplayoftheEEG hasbeen carried out on computer-ha\ed systems for many years. The microprocessor will enable sich facilities to be provided on standard equipment. in a case such as replaying and displaying of24 h EEG data the benefits are obvious. However, in the case ofanalysing the EEG data there are still too many paohlems to foresee the routine use of such analysis.

In summary, the main impact of the microprocessor in diagnostic tests in audiology and neurophysiology has been to increase the number of lac s available, with little change in the speed or accuracy with which the tests are carried out.

Turning lo the application of microprocessors to hearing aids. The conventional beariing aid, which compensates for hearing loss by amplifying the sound, has not yet seen the application of the microproressor. The requirement for a cosinetically acccptable s i x places severe restrictions even with today's microelectronics. A possible application, given the increasing ability 10 alterchar~icterisiicsofa hearingaid,is to use a microprocessor to decide qn the optimum settings at any instant depending on, for example, the level o f ambient noise. Special hearing aids iiiiolvingimore than amplification, such as frequency tr:inrp<isition aids,; cochlear implants and sensory substitution aids, requirc considerable processing ofthe incoming sound. .The inicroprocmor will have a part to play here, provided that theequipmcnt c#n bemadesmallenough.Finally, in hearing aid wlection and testing, the microcomputer has a natural nile in &ta retrieval bnd automation o f rests.

In summary. the inicroproqessor has so far onl j found major u r in the tr'tin- niid xlectiob of aids. The difficulty in other xcds is t h a t th~.rs i s inruflícient knoiiicdie : ihwt the best

~ ...

. :o7

Implications of microprocessors to equipment management Peter J. Howlett Deparfnienf of Medical Physics, Sourlionpron Genero1 Hospiral, Shirley, Southumprm SO9 4 X >: U K

This paper first examines the scope ofequipment management, from evaluation and specification o f equipment through operation and maintenanceto itseventua1replacement.A distinction i s drawn between the evaluation ofavailahle equipment and the specification ofthe requirement. Commissioningis seen to have a number ofaspects including documentation and inventory, as well as operation oí the equipment. Inventory describes the relationship . ~ oftheequipment to all otherequipments in a health unit ordistrict.Inventoryrelatestomaintenan~inthatitcanbe used to schedule maintenance and the results ofmaintenance are normally recorded in the inventory. One interesting aspect of operation is whether a change of function of the equipment is possible without modification oí the equipment hardware. A significant. role of the microprocessor is, incorporated into equipment, to enable such changes offunction to meet changing clinical needs without the need for significant equipment modifications.

This leads on to looking at the various applications of microprocessors in equipment and their implicatYons to equip ment management. ?his is examined under five headings:

(1) The microprocessor as a separate computer. (2) The microprocessor as an 'add-on' computer. (3) The microprocessor as an iniegml wmpohent o f

(4) The microprocessor as an essential component o f

(5) n i e microprocessor as a 'self-diagnostic' component o í

Under the heading of the 'Microprocessor as a separate computer'a number ofaspects ofthe role o la separate computer are indicated: to provide a record ofeach piece ofequipment for l,ospital,~district, etc.; lo provide management information for replacement programmes; to provide a maintenance history br each piece o f equipment;, lb provide planned preventative maintenance scheduliii~; to provide a rapid response lo hawrd notices or manufacturer's rnodificiition requircments.

The current slate of microcomputer de\elopment makes thcni an idcal tool for handling infixmatim :iiid inforiiiation- procilsiing deicribed aka\?, \ihich is not riadily pos5ihle uith manual. paper-bawd \>wms.

2118

equipment.

equipment.

equipment.

A microcomputer can also be connected to other equipment viaasuitableinteríace.ln thiscontext,as an 'add-on'computer, it can provide a data-logger io collect and store data from the other equipment and also present that data to the user in a way that the original equipment is not capable of. Examples ofsuch enhanced functions are: t~eiid~analysis ofphysiological rneasure- ments; enhanced display such as graphics; rapid calculation oí further, sometimes mathematically complex, data; storage of data; and communication with other syitems:

I t is of course possible to incorporate the microprocessor into equipment but the way in which it is incorporated may not leave it with the same flexibility as the fully progranunable microcomputer. Some oíthS functions o ía microprocessor as an integral part of equipment are: 'Super chip' compact electronic circuitry; sales gimmick; switchable change of equipment fun0 tion without hardware modification; modularity o í systems permitted by the microprocessor coping with the ditierent configurations; and providing and controlling a signal bus between various circuits. An example of the last-mentioned iunction is to be found in 'equipment marketed by Stewart Hughes Ltd, which permits UQ to 16 channels oíinput with up to seven functions such as filtering on each channel, to be reorganized under soltwarecontrol. This introduces the concept of'sottnare rewiring' which may be in more widespread use in the future.

The above indicates that in some circuinstaiices, such as sales gimmick, although the micaoprocessor is an integral part of equipmenfit isnot necessarilyanesxntialpari ofequipment.1n some Orcumstanas, however, it may be an essential part of equipment: electronic control of mechanical movement; a m - puter rewnsiruction of data to piovide an intelligible nian- machine interface;flexihle display facilities; storage ofdata; and communication with other sptcms. An obvious example oísuch. an essential pari ofequipmerit is the integral computer uced in a C A T scanner to control the acquisition of the image and to permit its subvquent reconctruction and display. Whilst in the p a t ihis function may often have involred the inclu,ion of a

miniwnipiiter, the distinnmn hetween minicriniputers and I i c roco~~puts rs i s~cominginr reas in~ly hlurrcd. FUC ewnple the puucrful IIEC VAX computer is now a\;i:lahle on a chip.

Fimlly, the microprocessor t , ~ l d s out hope h r the future in

~ -~

I

.

üI4 BECWSO DE MESPESIA 1QN"OFOBETICA PARA W YI-RAUA FMPlrñIOl

Q. eetratificade epltelio eucamoso de l a mambrEna tia)dnfee. coma en otra parte & e l cuerpo, es Impermeable o l a aplicooiim üe- agentes aneatéeicos típicos.

Hasta recientemente l a dnica alternativa da anestesia ha sido l a inyecclin de una soluciin anesthica dlroctoaente dentre de l a piel del canal auditivo externo. Sin embarga, en pacientes fre - cuentemente se ha .noeatratio que esto t6cnlce crCrisa d s meles+ias que una miringetomfa llevada a C 8 b . sobro l a a@brona t i q l n i c a - inanesteeiada .

Este problema ihe finalniente vt?nd.de cuande fue desaabierte- que ionen cargados pcsitiouonte de una s0laoi~m de llgnecaine e drian ser estimulados para tmlgrar a travls de l a mapuriicle del- epitelio de l a membrana timpdnica, per e l pase @e pesoefia co- tritnce ellctFica, una tdcnicr IlriaIbdO ientoferesis. Q. eptimfmm expresado en e l primor reperte de este m6teao (bmeau of al (21 ) h8 side amp~iamente justificado por muohea subsbeuennfes publica - cienes. La anestesia Iantoferétfea es ahera usa40 en proceatiilen- h e , t a l e a cem miringeforia o eiectroceclooara+!fa ( 3 ) , en l o mem - brann timplnlea.

Ei piupQsite de este artha lo es presentar en deltalle l a t ic - nica de via proyecto e l mi. f'ue diaefíodo on e l bepartiient o f O i l - nical Me8sarwient a t tho Meddlssex Heepital, y rep** l e s remal- tadss de nu u80 en plltctica clinica.

CI%CUITOS DEZUTADOS Y OPEBACIOñ

El aparato ilustrado en l a f iR .1 es UR simple mecanismo de- est8de si l ido con demanda de un mfnimo sistema opeFLtiv0 y p. de - un&* de energia externo.

Electroniamente e l equipo consiste de un trrnsfskr de - efectm de campo (Fm) que trabaja cen una fuemt+ do eerriente y- ármancie un d x l m e de o a 5 de una fuente de v 4 a r (Is 18 reltios. El ai.rnsi8t.r mantiene una corriente eonstanto csonslderande les- cubies en l a resistencia de carga para los efebtes de pelarica-

ción, par un perlo8e determinado, con un cronómetro eiecóPIPIoe, - e l oorl on e l crrw de l a anestesia de l a membrana tlmp&l.cir che -- qua 10 nin. Uno Be 180 m6s impartantea pz~pls&nies dei mecanleii. ea e l ascenso en e l petencial de l a corriente de O a O.%. En un - perieds de 40-60s. La crracterfstica de l a l ey ile l o s cuadrados de FET es me%lflcado de tal manem que l a taaa in ic ia l de incrsmente- en l a c o r r i e ~ ~ t e es pequeña, l a tasa se inoromnqa con e l t i m e - - p1s asf come e l anestlsico ea aonducido dentre. Efectes latersies- traasltories, tales come vdrtipo y hozripioo, causodes per una tasa in ic ia l l a r@ de increaente de cerriente a d men cvitrrdoa. M- cerrlente entro l e s electrodes del paciente auaionta a 0 , 5 d ea Bo- terminade per una resistencia y dibdes E8 y D3. EL paciente esf. a salvo &e l a cerriente de fue dado una resistenala de car@ I$.,, en serios con e1 circuito de eleetmdos a d que es imposible una cb - rriente mayor de imA.

&i activar e1 circuite, se ceneota sia y Slb sen cemaaio~~ Y-

nnt8zkeuemto; de este mea0 o1 eapacitor C2 es aargade a 9 V J q3 y Clb sen descargades. Velaije de coia)uorta de F (un FgT) es dnl- ma, perml*lendo que l a cerriente fins0 den%re Lle l a W e e de 01, de este mode cenmutisnda e l hraneleter se permite qwe C2 descarga* 0 - través de R2. Eatas badaa en e l volfiije de cempuert. F2 (un pm),- permiten l a corriente de n u j e entre l e s electmdes del paclento - en un mlxime de 0.5 mA, sieade limitada por R8 y D3 . UuaaBe 5. - y .Lb estas to0almente cargadba, Fl y de este modo 5 es ceniutrdo J C2 empieza a carparse. Este eaum que e1 voltaje de cca)uerfr on F2 mba T asf reduce el f ln je de cerriente del electroáo de nuevo- e 0 1 1 ~ . Los oemparentes aon mentades sebre un c i rcn ik imprese en- tan tablere y albePpI&o en al& contenedor apropiadaaente alelado.

La f i p r a 3 muestra l a r e l ac ih corriente eontm tie-0 n i h e

les electmdoa del paciente. Les intervalos TI y T3 sen deterilzuh des per l a cbmbinael6n C2B2; T2 es deteriliaado por CiIi.,..

E3 alsefio lonireferético del middlesex Heap&tOl es epurado de- l a mima fama que un equipe cemerclalmentib dlspenlble. Deapu6s de

examinar 01 proceder del paciente, e1 ofao es earainode para aso - m r que e l ama1 auditive externe no esta bloyasrde por de?mis o

cera y que 18 sembrsnr tiaadnica esta intaata. Ea anestesia ients - f4r6tica no debe aer administrada s i l a membmna tiq4nic8 Qat8 - retrafda o hay aialfeeraoción Y no puede mr per ei simple use del- aspectre sicele. W n a mezcla de soiuciin fresca I l a tesperrhm - del cuerpo consistiendo de 1 a1 de milésim de uarenaliaa y lml - del 4% de lign.#ina es adrinistnde dent- del canal externe auditive. El electrode activa, construida Be una Imneta ae acero - inoxidable de d ihetn, Feduoide y ceipletoiente ais18b en ou cara sxberns p ~ r una pasta de polielefina, en aniuaaeepraite itrser-

%ado tientre del can81 auditive. El electrodo (pmeba) es sujete - ra ma lugar crin cinta adhesiva con BU extrmi8ad hwedecld8 en 18 soluciin anest6sica dentn de l c8nal auditive, p e n no townüe l a superf%aie Be l a mubr8na t i M n i c a . Un electrob para es nsa - do cam^ e l electretie de referencia, sujete a l OTitebRso r e e m - do un buen CQnt8CtO aet8i-piei utilisondo ]posta pare r lecm8es.

Aieran.s pacientes pueaen experimenhar M pqqueño crier o c e ~ quill- en l a zena del electrode de referencia, p e n a1ffun.s -- otrea no sienten incomodidad 8ipxm8.

Iiitroduction The str;itified squamous epithelium ofthe tympanic membrane, as elsewhere in the body, is impermeable to anaesthetic agents ;ipplid topically. Some ofthe earliest attempts lo overconie this cliíTiiculty involved macerating the skin of the ear canal and tymp.oiic membrane with solutions of bicarbonate, glycerine or phenol before applying cocaine to the raw surface. These early techniques, together with their later development and the pharmacology of topical anaesthesia for the external auditory canal and tympanic membrane, have been compreherisively reviewed by Uhde [ i ] .

Lintil recently the only alternative to topical anaesthesia has been the injection of an anaesthetic solution directly into the skin of the external auditory canal. However, patients often found that this technique C d U S d m t W * disomkwt than e myringotomy carried out on the unanaesthetized tynipanic membrane.

n e s e problcms were finally o w c o m e whcn it was dis- covered that positively charged ions o l a solution oflignoaine could be cncourirged to migrate through the surface epithelium o f the tympanic membrane, by the passage o l a minute electric current, a technique named iontophoresis. The optimism ex- pressed in the first report ofthis method (Comeau et ul. [Z]) has been amply justified by many subsequent publications. Iontophoretic anaesthesia is now used in many centres as a preliminary to carrying out niinor invasive procedures, such as myringotomy or electrocochleography [3], on the tynipanic membrane.

The design and construction ofequipment lor iontophorcsis is not unduly complex aiid it Yhould be well within the scope o f the average hospital's physics department to build a device. The purpose ofthis paper is to present technical details o l a device which was designed and built in the Department of Clinical Measurement at the Middlesex Hospital, and lo report the results of its use in clinical practice.

Circuit details and operation

The apparatus illustrated in figure 1 i s a siniplc solid s1:itcdei ice demanding a minimum of operator control aiid rcquiring no external power-supply. .

Electronically, the equipment consists I w figure 2) o l a field dTcct tr:in~i.tor'(IET):icting on a currcnt SOLIICC :ind dr:iains a nia\iniuni d O 5 i n A from an ISVhatter! .upply. n e tran\i?tor mainlain, ihe cim\t:int current. rcgardlcw of t:Ii:in;c' in Imid

io.

i).m

.O"

Pat¡c.nt safe;! cr,-urd in thr <\<ni ofcoiiip cilrri'iit limi!;vLg rc\i*ti>r R,, in &e> tbith the clr.triirlc circuit. yi th:ii it i c i i~;po~~ihlr for a current grutcr that, I 111 \ t<i f l k w .

io actiiate the hrcuit, switches S , , and S , , arc. clowd monientarily: capacitor C, is thus charged tu 9 V and C , , and C, , are discharged. The gate voltage o f F , (;I FE-í) is at a miniiiiuni. allowing current to Ros into the base of T , , thus suitchingthe transistoron toallou C , io discharge through R,. This lowers the gate voltage on F, (a F ET), allowing current to Row between the patient ekctrdsl to a maximum of O5 rnA, this beinglimited by R,andD,.When C,,and C,,bscomefull) charged. F, and thus T, are switched olT and C, begins to charge. This causes the gate voltage on F, to rise and so reduces the electrode current Row back to zero. The components are assembled on a printed circuit board and housed in any suitably insulated container.

Figure 3 shows thecurrent versus time relatioriship htween the patient electrodes. Thc intenals T, and T, arc determined by the C,R, conihinatiuii; T, is determined b) C,R,

4 E

I I- *I

T . .~. ~.~~ ~

Figiire 3 . Gnrpli illirsrrotiny rhe exponential-like rise niid full of the current betwrcii the porieiit electrodes. See fel-r for expliinorinn of TI. 7 ; arid 7 , :

The Middlesex Hospital iontophoretic device is operated in the same way as comniercially available equipment. After explaining the procedure to the patient, the ear is exmiiiiied to ensure that the external auditory canal is not blocked by uax or debris and that the tympanic meinbraiie is intact. Iontophoretic anaesthesia should not be attempted if the tympanic membrane is so retracted that it is plastered to the promontory and cannot be riiobilized by gentle use o f Siegle speculuin. A freshl) mixed soliition, at body temperature, consisting o1 1 nd of i/iOm) adrenaline and I nil of4Y; lignocaine is instilled into thc external auditory canal. The active (probe) electrode. constructed from a length o f stainless-steel tubing of suitable diameter and coin- pletely insuliited on its outer surface by heat-shrinkable poly- olefiu sleeiiiig, is gently inserted into the ear canal. The probe is held i n pl (it' with adhzsive tape with its tip immersed in the anac4lictic hL)liii¡or within the external auditory canal, hut not touching the surface o f the tyinpanic membrane. An ECG plat< elc.-iroile i> UIIYI o n the iiidiflerent electrode. This is attached tu the liireaiiii u i t h lour layers ulgsure. saturated with electrode

twtwcn the r l c c ~ i < > d r . ;tiid furcarin to avoid dircci

"?:I! l>rCdL>;l~>\? It) '$<)t >&pltj' o f KICCIriC:il COl,bCI

ni~i) I C W I I 1 hi. 1n11, I . v i i , . 01 i>ht.!iii¡n; pr~)p~'r !I.N t I I ~ , . hwii t in#!> . , . I j,, r:pl,.j 141. w h , itIv>

, i , . c . > f a . ; ; ,,,, I,), j ; , I I , . , , , . !.(.(it;., ~ :\l.:",,tt,L. ,,I.,f;tr , , , . ' , , [ , ! . 11 , . , "

L ',,> 9 ' I , I,! , , , . ' , ,:. I , I . , :, 1 . , ' . ,' : , , . /

of the dclih<r;,tcl! sIoh ri< in currmi d ,h<trt ddx) occur) bct\rcm irt ti!,^: the 'run*and n~<~\c;mc~: i , f the miii.-icr r i rd l e . Aher IO I?niiii the currtxt i, siiii~Iie2 dl ; tut~m~.~: ic. t l l> :!lid the aniiiietcr returii, tu Z C I U .

Some patimtr nix) expriencc ver) slight parae,thcai;ie or :I sensation of uarnith on thc forear?) beneath the indilleicrit electrode, but otherwise there shopld bc no discomfort a1 all.

Discussion The main purpose o f this paper is to niake available a circuit diagram and .assembly details to enable an E N T surgeon to construct, or have constructed, an iontophoretic machine without the expense and delays involved in ordering a comnicr- ciaily manulactured instrument.

The deiice descrihed in this paper has hecn assessed in clinical practice and was used by the authors for anaesthetizing the tympanic membrane in 36 patients: the majority being adults and ranging in age from 9 to 83 years.

Complications are negligible provided that strict attention is paid to details oí technique. In particular, it is essential to use a íreshly made solution oflignocaine and adrenaline and to place moist gauze between the indifferent electrodeand the skin ofthe forearm to avoid 'hot spots'ofelettrical contact. The skin ofthe external auditory canal is not anacsthetized by iontophoresis so care must be taken not to tou$h it during any subsequent procedure. If the 'run' button is,inadvertently pressed during preparation ofthe patient the current must be allowed to return to zero before making the electrical connections, otherwise a minute electric shock will be experienced at tht moment of contact.

From a purely technical point oí view the Middlesex Hospital iontophoretic apparatus performed reliably and ef- ficiently and was found to be convenient to use. The accurate assemnent ofany pain experienced during a procedure such as myringotomy is difficult, but, nevertheless, an acceptable l e d of anaesthesia was obtained in all but one patient. I n this case iontophoresis apparently failed completely and the proposed procedure on the tympanic membrane was abandoned. This occurred early in the series and there was almost certainly an error in the preparation of the anaesthetic solution.

Two further complicatioiis occurred, The most serious o f there wa, a partial thickness burn in the forearm o f a child and although the burn healed quickly a small scar resulted. In this case the nieta1 foreann electrodeliad been applied directly to the skin, arid even though the current used is only O-SimA this is sufficient to produce a burn if not adequalel) distrihiited oyer a wide ;,red b) iiiterposiiig gauze moistened with electrode jelly between the electrode plate arid the skin. One patierit developed nauseii, vertigo and nystagmus during the procedure but this settled spontaneously and myrüngotoniics were carried out withuiit p:iiri or further priihlcm

\e scn\:iti(w, such as pain. is notw i, ,uJ) diliiculi hut i l i i leicl oldnne'lheria obtained in our se& ~u:wsi> th,ii thr de>ign and function of the Middlesex Hoipii;tl iont i iphoic i i t appai;itus compares lavourahly with siin1I:rr ~ w w ~ u c i a l n\:ichiiie\.

Th*: 111c:i . ~ I ~ C I I I C I I ~ of a siibje

BIBLIOGRAFIA

1.- Borg,E.: On The neuronal. organization of the acoustic middle ear reflex. A physiological and anatomical study.

2.- iJix,M., and Hood,J.: Modern developments in pqre tone audiome - try and their application to the clinical diagnosis of end-oggaa deafness.3.

3.- Otorrinolaringologia de Boles.: Enfermedades del oído,vías nasa - les y laringe. lnteramericana.

L

?- 4.- Manual de locopedia.: Dialux,Datat, Fress, Legent, toray-masson i

c - - - w-.

L

c

r i

P

universidad148.206.53.84/tesiuami/uam20557.pdf · sonidos complejos ..... 23 ... audhome$rlas de...

Documents