International Journal of Pediatric Otorhinolaryngology, 15 (1988) 107- 116 Elsevier

107

PGR 00489

John Graham University College Hospitni, London (UK.)

(Received 22 June 1987) (Revised version received 2 October 1987)

(Accepted 15 October 1987)

Key work: Congenital deafness: Cochkar implant; Transtympanic electrocochleography; Electrical stimulation; Neural survival

Cochlear implants now play a standard role in the management of adults with acquired profound sensorineural deafness. Their role in children is more controver- sial. However, as demonstrated by a 1979 report on childhood deafness in the European Communities, there is no reason for complacency in the present manage- ment of profoundly deaf children. Transtympanic electrocochleography has been used as a method of estimating neural survival in deafened adults being assessed for the U.C.H./R.N.I.D. single channel extracochlear implant and also in a parallel group of profoundly deaf children referred by a paediatric hearing assessment clinic. In adults the effectiveness of electrocochleography in judging neural survival was monitored by electrical stimulation of the cochlea and found to be significantly effective. This result was extrapolated to the paediatric group to estimate the proportion of congenitally deaf children wh9 might benefit from a cochlear implant. It is suggested that in a programme designed to implant such children at 2 years of age, techniques should be chosen that neither damage the cochlea nor the middle ear sound conducting mechanism.

CaMear implants in children. Physiol consi&rations

Since they were developed 23 years ago, cochlear implants have won an accepta- ble role in the rehabilitation of adults with acquired profound deafness. At Univer-

Correspondence: J. Graham, 16 Upper Wimpole Street, London WI, U.K.

0165~5876/88/$03.50 0 1988 Elsevier Science Publishers B.V. (Biomedical Division)

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&y college Hospital, London, with the Royal National Institute for the Deaf and in mnjunction with Addenbrookes Hospital, Cambridge, we have performed 20 co&ear implant operations on deafened adults and are currently using our own single channel, extmcochl~ device. We have put this device into 14 of an initial run of 50 adults. The electrode passes through a generous posterior tympanotomy to lie against the round window membrane.

A few centres have put implants in children, notably the House Ear Institute. Most teams, however, have displayed a more cautious approach. Should we, or should we not do implants on children? If we should, how ought we to go about it?

The use of battery-powered hearing aids by deaf children has reduced the size of the ‘deaf and dumb’ booty by giving usable hearing to the moderately severely and severely deaf. There remains, however, a huge population of profoundly deaf children for whom thin8s are not satisfactory.

The Co~ssion of the Eur*~e~ Co~u~ti~ produced a report in 1979 called Childhood Deafness in the European Community’ [l]. This was a study, performed in 1977, of all the deaf children in the European Community who had been born 8 years earlier during 1969. To be included in the study a child had to have an average loss of 50 dB or worse, for the 3 pure tone frequencies 500, 1000 and 2000 Hz, affecting the better ear. Three of the findings in this report are of particular interest.

(1) There was a predominance of severely and profoundly deaf children. Thirty- three percent of the deaf children in the study had a hearing loss of 100 dB or more (mean loss in dB nHL at 500,lOOO and 2000 Hz for the better ear).

(2) Of all the deaf children studied, one in three, while wearing their aids, could not hear more than a loud shout at 3 m and had no speech aviation while wearing their aids (Fig. 1).

(3) Only SO% of the children in the study had speech that could be understood by people outside their own family (Fig. 1).

There is therefore a huge pop~ation that is not helped by existing arrangements. What should be done?

In many countries there is a strong anti-oral&t movement. This points out the clear fails of aiding for many profo~dly deaf chiklren and the poor quality of language achieved in the purely oral management of the profoundly deaf. Members of this movement would prefer simply to abandon their wrecked auditory pathway and concentrate from an early age on s@n language, which gives them a usable language, lively communication with their peers and, as they put it, “makes them first class citizens in the deaf community rather than third class members of the hearing population”.

For a m~~ately severely deaf child, a hearing aid brings previously inaudible sounds within reach of the central auditory pathways. If a cochlear implant were able to do the same for some profoundly deaf children, it seems logical to assume that some of these children might derive a modest degree of benefit both in their perception and understanding of sounds and in their production of speech, and that this would be a good thing,

The population under discussion is not a uniform one. To begin with it is hard to decide how to categozise children both for selection and assessment in an implant

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Fig. 1, Aided performance of a cohort of deaf children aged 9 years in the European Community.

programme. There is little general agreement even about the simple terms pre-, peri- and post-lingual. The precise time of onset of deafness may be doubtful: in a c identified as being deaf at the age of 2 or 3, it is often bard to be certain whether deafness is congenital, perinatal or acquired later in the first year of life. Table shows these two groups of categories side by side. It is probably best to put groups (a) and (b) in this table together for purposes of selection and bab~~~on but to separate the 3 groups for purposes of assessment, once implanted. These ~st~ctio~s

TABLE f

Catqories of deafness by rime of onset

a. ~~~: ~ong~~-P~a~ b. Perilingual: Aquired c. Postlinguak Aquired

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are further clouded by progresaive deafness which may happen, for example, in’ Rubella.

Assuming that at first one would only want to implant profoundly deaf children, it may also be hard to draw firm conclusions about the severity of hearing loss in the first two years of life. Electric response audiometry may be of assistance but does not reliably exclude quite moderate degrees of residual hearing in the low frequencies. Often it is the absolute failure of a child to respond to a trial with powerful hearing aids and intense habilitation that finally confirms profound deafness.

Matutwtion Animal experiments su st that auditory input is needed for the auditory

pathway and centers to mature. Unfortunately, there does not seem to be any hard evidence that in profound deafness electrical stimulation will accomplish the same goal. However, on the same principle that deaf children should be aided early, it can be argued that these profoundly deaf children should receive their implants as young as possible. Technically this poses few problems, but audiologists point out the benefit of a period of one year of conventional habilitation to confirm failure to benefit from hearing aids, to give both parents and teachers a chance to make

TABLE II

Pas&gnal acquired deafness

SeWion criteria

Arbitrary minimum age: 4 years Bilateral profound sensorineural loss No demonstrable benefit from powerful aids and habilitation Additional handicap? Good support from family and local habitation service

TABLE III

Pr&guuk Congen&i- Perinaral Perilhgual: A_

Sekchn criteria I

Age of onset Duration of deafness Biitual profound scnsorineural deafness Additional handicnp? support No prostega with conventional habitation Pathology and site of lesion

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informed decisions about the operation and to establish a base line fro cb to judge a child’s performance once implanted [2].

The selection of children deafened at a relatively late age is more or less Ost Of the criteria are a matter of common sense (Fable II). Each for itself the minimum age and whether they will accept a child

with other handicaps as well as deafness. In selecting children from the much larger group of those born deaf or becoming

deaf in approximately the first year of life, some possible selection criteria are shown in Table III. Some aspects of the last item in the table will be discussed.

Transtympanic electrocochleography (ECochG) is routinely used as a reasonably effective way of measuring the physiological function of the co&lea and its nerve in response to sound. We routinely use ECochG in implant selection protocol for adults, first to confirm the degree of deafness, second to try and assess the site of lesion and third, by passing an electric current through the same needle electrode, to check that this gives the patient a sensation of s

ECochG is also well establi as a way of measuring th deaf children. In addition to threshold it gives some about peripheral auditory function. This qualitative in in a series of 170 children tested by ECochG and found to have a bearing loss.

Table IV shows those ears with hearing thresholds The group of 156 ears with thresholds of IO0 d or worse represents the population of profoundly deaf ears from which one d try to select suitable cases for cochlear implantation.

Where there is some residual hearing, the ability of electrocochleography to determine the main site of lesion may be limited. Attempts have been made, on empirical grounds, to associate certain types of pathology with certain types of abnormally shaped action potentials (A.P.). There is also a characteristic broad potential, probably a combination of A.P. and summating potential that is ass ated with deafness from kernicterus, anoxia and in premat low birth weight infants. This leaves a substantial population in whom the . is more or less normal in appearance and in whom it is not possible to make any clzar j about the site of lesion.

However, in cases of profound deafness, with thresholds of 100 d separation into retrocochlear and cochlear deafness seems clearer (Table V), these

TABLE IV

Electrocochleographic thresholds in 170 children with hearing loss

Better than 100 dB 100 dB or worse

59 Children: 29 bilateral 113 children: 43 bilateral 30 unilateral 70 unii~teral

88 ears 156 ears

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TABLE V

Qualitative results of ekctracochleogmphy

Better than 100 dB 100 dB or worse

RGtKMXChleU Cochlearfretro Not clear Wide D.C. potential

15 : 17% 102 : 65% 19 : 22% 51: 33% 29 : 33% 25:28% 3: 2%

88ears 156 ears

cases are precisely the ones that may be selected for cochlear implantation. Where the& is no detectable nerve A.P. but a large cochlear microphonic (C.M.) greater in &e than 1 ‘PV and traced to a threshold better than 85 dB, it is assumed that the main site of the deafness is retrocochlear. Conversely, where there is no trace either of a CM, or A.P. in response to stimuli of more than 100 dB, it is assumed that the patient has a cochlear lesion with or without retrocochlear involvement.

This distinction relies on the qualitative use of the C.M. Since the threshold of the C.M. from ECochG is not a true one, but simply a threshold of detectability, the value of CM. for quantitative measurements is limited. However, for qualitative purposes, the only relevant aspect of the C.M. is whether it is present or absent. There remains, however, one further problem. If the auditory stimuli are delivered from a loudspeaker, the time taken for the sound to reach the ear eliminates electromagnetic artefacts from the recording but there may be a vibration artefact mimicking a true CM. This artefact occurs particularly at high intensity and the cut-off point for the artefact has been taken as 85 dB nHL. (A subsidiary purpose of the present study, as will be seen later, was to try to validate the likely threshold of such an artefact by comparing electrocochleography with electrical stimulation of the cochlea in a group of adult patients.)

Of the 156 ears with profound deafness of 100 dB or worse, only 3 ears showed a widened D.C. potential and all the rest could be divided into the ‘retrocochlear’ and ‘cochlear with or without retrocochlear involvement’ groups. Fig. 2 shows the relative proportions of these categories in the main diagnostic groups of children. Fii 3 shows the apparent preponderance of ‘retrocochlear’ deafness in this group of 156 profoundly deaf ears.

In theory, when cochlear function is clearly demonstrated and where there is still profound deafness with no sign of a nerve A.P. it might be expected that the VIIIth nerve was too severely damaged for a cochlear implant to be of much use. On comparing these children with a parallel group of adults with profound acquired deafness who had passed through our cochlear implant selection protocol (Fig. 4), there is a larger proportion of children with retrocochlear lesions than adults, in whom ‘co&k’ lesions are more common.

HOVWWX, the practical implications of this finding may be modified by looking in more detail at the behaviour of the adults. In our protocol we routinely record whether or not the adults get a sensation of sound from electrical stimulation, using

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Retrocochleor

Cochleor

f Retro-

RUBELLA MENINGITIS PRWTURE MIX,

32

24 25

56 ears 14 ears 20 ears 63 eors Fig. 2. Site of lesion determined by electrocochleography in profoundly deaf children, grouped by

pathology.

RETROCOCHLEAR

COCHLEAR k RETRO-

63

100 oB + < 100 DB

153 EARS 34 EARS

Fig. 3. Site of lesion clearly defined by electrocochleography. In the profoundly deaf (HKI dB or more) the defiitions ‘cmhlear f reirocochlear’ and ‘retrocuchlear’ were easier to achieve.

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COChlCW f Retro-

63 ears

58 eors

82X

Chl ldren Adults

00, of @US

Children Adui ts

Fig, 4, Site of lesion, determined by ~~t~~by, in p~fo~dly deaf children and ~rofo~dly deaf adults. Numbers of ears are shown in the left hand blocks, the right hand blocks show these

numbers expressed as percentages.

a sinusoidal rtltematiq current of up to 100 pA. It could be predicted that the Wroeo&learP group would get little or no sound sensation while in the ‘cochlear’ group a sensation of sound in response to electrical stimulation wuld be reasonable evidence of surviw VII&h nerve fibres,

Fig. S shows that the situation is not as clear-cut as this, although in the koehlear’ group the majority did get a sensation of sound while in the ‘retro- eochlear’ group a barfly high proton failed to hear ~~ in response to electrical stimulation. Even this, however, is probably not the whole story since a proportion of those who do not have a sensation of sound with promontory stimulation may still gain a sensation of hearing from a cochlear implant, whose electrical impedance is better than that of a ~~st~p~c needle electrode.

In spite of the above qualifications it seems reasonable to suggest that if one is go& to implant children with profound deafness, whether congenital or acquired at a very early age, one should start by selecting them from the group with mainly

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X2 0,025

Fig. 5. Profoundly deaf adults who

Retrocochlear C,M, threshold 'C,M.' threshold

< 85 dB 85 dB+ did or did not have a sensation of sound with A.C. elec&al

stimulation using a fine needle electrode resting on the promontory. The blocks show the difference between those with a mainly retrocochlear lesion, determined by electrocochl aphy and those with a

mainly cochlear lesion.

cochlear deafness. It will be important to check the response to electrical stimula- tion using some form of evoked neurogenic or myogenic potential. profoundly deaf children may be suitable for cochlear implant expected, this still leaves a very large number of suitable cbildr the number of possible adult patients with profound acquired deafness.

es ve!rs

In adults it seems likely that multichannel, intracochlear implants give better results than extracochlear e channel ones. The other determining factors for the success of a cochlear imp the quantity and quality of swing neural tissue and the intelligence, aptitude and motivation of the patient. For an 01 acquired deafness a multichannel intracochlear electrode may be reasonable choice. However, for the main group of younger present an extracochlear implant should be used. In a twti- be an unsuspected amount of residual hearing and ins distance into the cochlea carries a risk of destroying all re

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An extracochlear electrode will not only avoid the fear of destroying an island of residual hearing but will also allow a more flexible approach to habilitation and assessment. It would even make it possible to compare the performance of the implant with that of a conventional hearing aid in the same ear.

A last, important point is about conductive hearing loss. In a child with severe sensorineural deafness the extra conductive hearing loss imposed by secretory otitis media may shift the hearing beyond the reach of the child’s hearing aids. Any implant technique, whether intra- or extracochlear, that involves destruction or removal of parts of the middle ear and the introduction of a conductive deafness must certainly be avoided until the child has reached the age at which one can be certain that there is no useful residual hearing.

Acknowledgement

The U.C.H./R.N.I.D. Cochlear Implant Programme is supported by a grant from the Jules Thome Foundation.

References

1 EUR 6413, Childhood deafness in the European Community (1979). Luxemburg: Office for Official Publications of the European Communities. Botie Postale 1003, Luxembourg.

2 Northern, J.L., Black, F.O., Brimecombe, J.A., Cohen. N.L., Eisenberg, L.S., Kuprenas, S.V.. Martinez, S.A. and Mischke, R.E., Selection of children for cochlear implantation. In DJ. Mecklen- burgh (Ed.), Seminars in Hearing, 1986, Vol7, no 4.. Thieme, New York, pp. 341-347.