The Journal of
Laryngology and Otology (Founded in 1887 by MOREIX MACKENZIE and NORRIS WOLFENDEN)
February ipj6 Electrocochleography in the diagnosis of acoustic neuroma* By W. P. R. GiBsoNf and H. A. BEAGLEYJ (London) Introduction the term used for the clinical measurement of electrical output of the cochlea. It may be used clinically in two separate ways. Firstly, as an objective test of auditory acuity in which the cochlear and auditory nerve function is measured. This allows the auditory threshold to be determined with an acceptable degree of accuracy. Secondly, it may be used as a method of determining the pathology underlying auditory dysfunction, in a neuro-otological context. As an objective auditory test, electrocochleography has several important advantages over other methods of electric response audiometry. It is extremely reliable and the responses are easily reproduced and there is rarely any difficulty of interpretation. No masking of the non-test ear is required and finally, the responses are not affected by general anaesthetic agents. These properties combine to produce a robust test of peripheral auditory function for sub j ects, such as disturbed and multiply-handicapped young children, who cannot co-operate in other methods of hearing assessment (Beagley et al., 1974). The second neuro-otological application of electrocochleography is yet to be established on such a firm clinical basis. It is hoped that analysis of the responses obtained in patients with known neuro-otological disorders ELECTROCOCHLEOGRAPHY is
* (Paper read at the short papers meeting, Section of Otology, Feb 1975). t Senior E.N.T. Registrar, London Hospital and Honorary Senior Registrar, Royal National Throat, Nose and Ear Hospital. \ Consultant Otologist, Royal National Throat, Nose and Ear Hospital. 127
W. P. R. Gibson and H. A. Beagley will provide the necessary information to develop electrocochleography as a diagnostic test in this area. This paper describes our preliminary findings as a result of using electrocochleography as an investigative procedure in a series of forty-five cases of proven acoustic neuroma. Method
We have used the trans-tympanic method of electrocochleography as described by Portmann et al. (1967). The advantage of this method is that the potentials obtained are large and therefore easily read. It is possible to record the potentials from more distant sites such as the tympanic membrane, external acoustic meatus, mastoid or ear lobe but since the electrical activity conducts relatively easily through the cochlear fluids but is rapidly attenuated by the more electrically resistant tissues such as bone and skin, these more distant sites yield only very small responses. When the electrodes are placed on the mastoid or earlobe and the vertex, it is possible to obtain several brainstem responses in addition to the auditory nerve action potential and so it is likely that additional valuable neurological information can be gained (Jewett et al., 1970; Sohmer and Feinmesser, 1967; Sohmer et al., 1974; Coles and Thornton, 1975)We test co-operative adult subjects under local anaesthesia. The drum is first inspected and then sprayed lightly with cetacaine (Cetylite Industries Inc.). No antiseptic or other fluids are used to cleanse the meatus. A slender needle electrode is inserted through the tympanic membrane midway between the umbo and the postero-inferior rim of the annulus so that its point rests on the promontory of the cochlea anterior to the round window niche. Crowley et al. (1975) in a review of several thousand cases have shown the technique to be very safe. The recordings
Using a trans-tympanic electrode, various electrical cochlear potentials were recorded in response to sound stimuli, using an Amplaid-Medelec ERA/ECochG apparatus. These potentials consist of cochlear microphonics, summating potentials and the auditory nerve action potentials. We have not at this stage attempted to analyse the summating potentials systematically. The cochlear microphonics are generated by the cochlear hair-cells and these responses closely resemble the waveform of the stimulus. The action potentials are derived from the auditory nerve fibres. Using trans-tympanic electrocochleography it is not possible to record action potentials from individual nerve fibres, but only from the whole of the auditory nerve. Usually a stimulus with an abrupt onset such as a click is used so that the maximum number of individual nerves fire synchronously and in this way the whole nerve action potential is recorded. Since it is the nerves from the basal coils that fire with the greatest degree of 128
Electrocochleography in the diagnosis of acoustic neuroma synchrony, it is inevitable that the major element of the whole nerve action potential is derived from this area. A much smaller and more poorly synchronized potential is also obtained from the middle and apical turns of the cochlea due to the relatively slow travelling wave responsible for the stimulation of the hair cells in these regions (Eggermont, 1974). At present the information obtained by trans-tympanic electrocochleography is effectively restricted to the basal, and beginning of the middle coil of the cochlea and hence to the higher frequencies of hearing. At high intensities of stimulation, the action potential is usually obscured by the cochlear microphonics (Fig. 1). If one wishes to analyze the action potential alone, it is usual to remove the cochlear microphonics from the trace. This may be done by alternately inverting the stimulus. When the responses to the successive stimuli are added electronically, the action potential continues to summate since its form is unaltered by stimulus inversion but the cochlear microphonics, which faithfully follow the form of the stimulus, alternately invert and therefore cancel out. By an adaptation of this method, the cochlear microphonics may be retained and the action potential rejected (Fig. 1). The normal electrocochleogram (ECochG)
The characteristics of the ECochG which are obtained from the normally hearing ear are shown in Fig. 2. The size or amplitude of the response diminishes as the intensity of the stimulus reduces until the response becomes unidentifiable at a level close to the subjective hearing threshold. The timing or latency of the response grows longer as the stimulus intensity decreases. These changes may be represented on an input/output graph (Fig. 3). At high intensities, the response is monophasic and consists essentially of a single negative peak (Ni). As the intensity diminishes, a second negative peak appears (N2) and at 60 dB the response is ' W shaped. It appears to be this latter part of the response that is traced down towards the hearing threshold. It is likely that each of the negative peaks represents a different group of hair cells and it has even been suggested that these are the inner and outer groups of hair cells, although the evidence is incomplete in some respects. Conductive hearing losses
The ECochG that is obtained from a patient with a conductive hearing pathology is essentially the same as that obtained from a normally hearing subject except that higher intensities of stimulation are needed to evoke the responses. The input/output functions show a 'shift to the right' similar to that seen with the speech audiogram. Cochlear hearing losses
Patients with cochlear pathology may be distinguished on audiological testing by the phenomenon of recruitment. The ECochG in such cases 129
W. P. R. Gibson and H. A. Beagley shows several interesting characteristics (Fig. 4). The amplitude of the response diminishes in a more uniform manner towards the hearing threshold and there is little change in the latency. The findings are clearly shown
C O C H L E A R
C O C H I. E A R
M I C R O P H O N I C
M I C R O P H O N I C
a n d
V I I I
only
NERVE
( x 2 )
A C T I O N
P O T E N T I A l
S
V I M
N E R V E A C T I O N
P O T E N T I A l
H O d B
W lD E BAN 0
2 0 m s
A N A U
SIS
A C O U S T I
only
C
CtICK
T I M E
FIG. I.
The electrical potentials obtained by electrocochleography.
on the input/output graph (Fig. 5). The response at high intensities is often diphasic in shape. Analysis of these results reveals that the changes are due to the non-appearance of the second negative deflection (N2) and, perhaps, this indicates a loss of outer hair cells which certainly may occur in many ears which show recruitment. The whole explanation, however, is probably more complicated than this. 130
Electrocochleography in the diagnosis of acoustic neuroma Retro-cochlear (or neural) hearing losses
We have been fortunate in being able to investigate a large number of patients with retro-cochlear hearing losses which have been referred to Mr. Andrew Morrison at the London Hospital. Forty-five of these cases have been proven to be acoustic neuromata and several other cases were shown to have other pathologies affecting the auditory nerve within the internal acoustic meatus. The following account is a progress report of our findings in these cases. E.C.O.G. NORMAL EAR
dB 110 HI 100 90
II
80 70 60
10ms scan 2ms delay Wide band click
50 40 30
xlO
20 xlO
10 x 10
Subjective hearing threshold 10dB FIG. 2. Normal ear: The action potentials.
Typical acoustic neuromata
The typical case of acoustic neuroma shows no recruitment on audiological testing. The ECochG obtained from such a typical case is shown in Fig. 6. In particular, it is the up-going limb of the first negative deflection
W. P. R. Gibson and H. A. Beagley
I 10 20 30 iO 50 60 70 80 90 100 110 INTENSITY da ISO FIG. 3. Normal ear: Input/output functions.
E.C.O.G. RECRUITING EAR
100
II
10m i s c a n 2ms d e l a y W i d e b a n d click
Subjective h e a r i n g t h r e s h o l d
SOdB
FIG. 4. Recruiting ear: The action potentials.
132
100 90 80 70 60 50 40 30 20 10 10 20 30 40 50 60 70 80 90 100 110 INTENSITY dRISO Ri^itearo
Leftearx
FIG. 5. Recruiting ear: The input/output functions. TYPICAL RIGHT ACOUSTIC NEUROMA
xlO
xlO
xlO
20
10ms scon 2ms d e l o y
•I!
Wideband clicks
BB d Agt 31
6. Typical acoustic neuroma: The action potentials. FIG.
133
W. P. R. Gibson and H. A. Beagley (Pi) that is affected. We have tested twenty-four cases of acoustic neuroma, one case of a meningioma and one case in which an ectatic basilar artery was pressing on the auditory nerve (Gibson and Wallace, 1975). All these cases showed a non-recruiting hearing loss audiometrically. Two of the neuroma cases and the meningioma patient failed to show a clear widening of the response on electrocochleography. Atypical acoustic neuromata
Approximately 10 per cent of ears affected by an acoustic neuroma and a higher proportion in cases of deafness associated with multiple neurofibromatosis give audiological results that are consistent with a recruiting (sensory) pathology. The smaller the tumour, the more likely it is to show this atypical audiological picture (Morrison, 1974). Obviously it is desirable to diagnose tumours at this early stage since the operative mortality and morbidity associated with the removal of small tnmours is considerably less than in larger tumours. We have tested eight such patients. In all but one case, which has not yet been confirmed surgically, the electrocochleogram gave results similar to that shown in Fig. 6. Large acoustic neuromata
In many cases patients with large acoustic neuromata are either completely deaf or severely deafened in the affected ear. Under such circumstances it is usually not possible to obtain any action potentials but we have often noticed that, using the same amplification, the cochlear microphonics are still visible even when using high frequency filtered clicks that stimulate mainly the hair cells at the basal end of the cochlea. We have encountered fifteen such cases and an example of such a patient is shown in Fig. 7. We believe that this finding may be explained by a lesion that destroys the nerve but spares the blood supply of the cochlea sufficiently to allow some of the hair cells to survive and to produce a cochlear microphonic potential in response to acoustic stimulation. Furthermore, we have also seen several cases in which a small distorted action potential is still obtained even though the ear was subjectively totally deaf. The clearest example was seen on testing a patient with a brainstem astrocytoma (Fig. 8), in whom the distorted action potential was evident to stimulation of less than 40 dB H.L. It would seem that in such a case the nerve has been damaged to such an extent that no meaningful message can pass along it to the higher centres whilst the most peripheral part of the nerve is still able to produce electrical activity. Patients with acoustic neuromata but apparently normal electrocochleogram
Four patients in this series gave results which at the time of testing were interpreted as being normal. Retrospectively, we now feel that there 134
Electrocochleography in the diagnosis of acoustic neuroma were some features of the responses which were suggestive of a loss of recovery of the positive limb of the potential (Pi). Fig. 9 shows one such patient in whom it appears that, if the analysis time is doubled to twenty milliseconds, the response has not recovered completely until approximately fifteen milliseconds have elapsed and at this point a small myogenic response is evident. Three of the patients were apparently in this category whilst the remaining case gave a high P i which was followed by a definite trough. We now feel that greater clinical significance should be assigned to such changes. PATIENT WITH LEFT ACOUSTIC NEUROMA (NO SUBJECTIVE HEARING) J1 dB 110 HL
,00
90
HI No action potential
10ms scon 2ms delay Wideband clicks P r e s e n t c o c h l e a r microphonics GR
O* Age 5 0
7. Large acoustic neuroma with no subjective hearing: No action potential but cochlear microphonic intact. FIG.
Patients with no acoustic neuroma but positive ECochG changes
Although we have met only four patients in our series of forty-nine tumours who gave possibly normal responses, we have encountered several patients who gave electrocochleographic findings that mimicked those found in the tumour group. Ozsahinoglu and Harrison (1974) state that in their series less than 50 per cent of patients presenting with non-recruiting hearing loss were subsequently found to have a tumour. Thus it is likely that there are various pathologies that can affect the auditory nerve which are not acoustic neuromata or even tumours, and indeed we have identifield a number of such cases. At the London Hospital, a number of patients were referred with suggestive audiological or radiological findings, in whom 135
W. P. R. Gibson and H. A. Beagley no lesion in the internal acoustic meatus was detected on Myodil cisternography. Electrocochleography was used routinely in the investigation of all these patients. RIGHT EAR
10ms scan
lOpv
2ms delay W i d e b a n d clicks GJ 6 Age 54 Kt
512
1124 2041 4«M
l»2 Hi
M
i " S « o .s
n * u
I
1 110 121
FIG. 8. Brainstem astrocytoma with no subjective hearing: Distorted action potentials are obtained.
The most difficult group to distinguish from the tumours using ECochG proved to be patients with Meniere's syndrome. Figure 10 shows the responses obtained from such a patient. Myodil cisternography failed to reveal a tumour and later a saccus drainage operation caused the hearing to improve dramatically. It is disappointing that ECochG cannot differentiate clearly between some cases of Meniere's syndrome and acoustic neuromata. The problem is probably not so difficult in cases of bilateral Meniere's syndrome, where, in addition to the overall clinical picture, the ECochG findings may show bilateral distortion of the ECochG tracing. 136
Electrocochleography in the diagnosis of acoustic neuroma Such a case is unlikely to be confused with acoustic neuroma, but a unilateral case could be more difficult. One possible help is that the cochlear microphonic response is usually readily obtainable in cases of acoustic neuroma whilst in Meniere's syndrome the cochlear microphonics are sometimes quite difficult to obtain. Perhaps this is due to the increased endolymphatic pressure damping the vibrations of the basilar membrane. Unfortunately the size of the cochlear microphonics tend to vary so widely from one patient to the next that there may be a considerable overlap between the tumour group and the Meniere's group. Nevertheless we believe that this observation is worthy of close attention. LEFT EAR
100
2 0 m s scoo 2ms d e l a y W i d e b a n d clicks
10 2ml delay
1
+ | FS 9 As* 44
g. Acoustic neuroma with near-normal action potentials. FIG.
Conclusions
Table I shows the results obtained on testing ninety-one patients that had been referred specifically for investigation of suspected acoustic neuromata. The electrocochleogram was reported as either being positive or negative for a retro-cochlear (or neural) lesion immediately after the test and before the myodil radiography. Retrospectively, we feel that we may now be more accurate in interpreting the results, especially those cases with slight but significantly delayed recovery of the up-going limb of the Ni as described earlier.
W. P. R. Gibson and H. A. Beagley Several other patients with Meniere's syndrome were tested who gave a broad response on ECochG in whom the diagnosis was not in doubt and who were not investigated as possible tumour cases. These cases are not included in Table I. RIGHT EAR
100
xlO
70 x 10
60 xlO
50 .10
IOJJV
10 ms sea n
ii
2 ms delay W i d e b a nd click FIG. IO.
Meniere's disease: The action potential mimicks that of an acoustic neuroma.
Electrocochleography does seem to be of value in the diagnosis of acoustic neuroma, especially in small tumours which show atypical results with other forms of testing. It does not provide a cast-iron diagnosis but in our series it correctly identified forty-five out of forty-nine (88 per cent) of patients with a tumour, although the seven false positive due to unilateral Meniere's syndrome remains a problem. It seems likely that electrocochleography will prove to have clinical uses in addition to the diagnosis of purely cochlear disorders. 138
Electrocochleography in the diagnosis of acoustic neuroma TABLE I. Tumour No present tumour 45 7 ECochG positive ECochG negative 4 35 (In respect of a diagnosis of a retro-cochlear, or neural, lesion)
Acknowledgements
We would like to thank Mr. Andrew Morrison and Mr. Tom King who kindly asked us to investigate this very interesting series of patients. We would like also to thank Mr. Connolly and the Department of Clinical Photography of the Institute of Laryngology and Otology for providing the illustrations. REFERENCES BEAGLEY, H. A., HUTTON, J. N. T. and HAYES, R. A. (1974) The Journal of Laryngo-
logy and Otology, 88, 993. COLES, R. R. A. and THORNTON, A. R. T. (1975) Personal communication. CROWLEY, D., DAVIS, H. and BEAGLEY, H. A. (1975) Annals of Otology, Rhinology
and Laryngology, 84, 1, EGGERMONT, J. J. (1974) Ada Oto-laryngologica, supplementum 316, 7. GIBSON, W. P. R. and WALLACE, D. (1975). The Journal of Laryngology and Otology, 89, 721. JEWETT, D., ROMANO, H. N. and WILLISTON, J. S. (1970) Science, 167,1517.
MORRISON, A. W. (1974) The British Journal of Audiology, 8, 86. OZSAHINOGLU, C. and HARRISON, M. S. (1974) The British Journal of Audiology, 8, 61. PORTMANN, M., L E BERT, G. and ARAN, J-M. (1967) Revue de Laryngologie, Otologie
et Rhinologie, 88,157. SOHMER, H. and FEINMESSER, M. (1967) Annals of Otology, Rhinology and Laryngology, 76,427. SOHMER, H., FEINMESSER, M. and SZABO, G. (1974) Electroencephalography and
Clinical Neurophysiology, 37, 663. The Nuffield Speech and Hearing Centre, The Royal National Throat, Nose and Ear Hospital, Gray's Inn Road, London, WCi, and The London Hospital, Whitechapel, London, E i .
139
Laryngology and Otology (Founded in 1887 by MOREIX MACKENZIE and NORRIS WOLFENDEN)
February ipj6 Electrocochleography in the diagnosis of acoustic neuroma* By W. P. R. GiBsoNf and H. A. BEAGLEYJ (London) Introduction the term used for the clinical measurement of electrical output of the cochlea. It may be used clinically in two separate ways. Firstly, as an objective test of auditory acuity in which the cochlear and auditory nerve function is measured. This allows the auditory threshold to be determined with an acceptable degree of accuracy. Secondly, it may be used as a method of determining the pathology underlying auditory dysfunction, in a neuro-otological context. As an objective auditory test, electrocochleography has several important advantages over other methods of electric response audiometry. It is extremely reliable and the responses are easily reproduced and there is rarely any difficulty of interpretation. No masking of the non-test ear is required and finally, the responses are not affected by general anaesthetic agents. These properties combine to produce a robust test of peripheral auditory function for sub j ects, such as disturbed and multiply-handicapped young children, who cannot co-operate in other methods of hearing assessment (Beagley et al., 1974). The second neuro-otological application of electrocochleography is yet to be established on such a firm clinical basis. It is hoped that analysis of the responses obtained in patients with known neuro-otological disorders ELECTROCOCHLEOGRAPHY is
* (Paper read at the short papers meeting, Section of Otology, Feb 1975). t Senior E.N.T. Registrar, London Hospital and Honorary Senior Registrar, Royal National Throat, Nose and Ear Hospital. \ Consultant Otologist, Royal National Throat, Nose and Ear Hospital. 127
W. P. R. Gibson and H. A. Beagley will provide the necessary information to develop electrocochleography as a diagnostic test in this area. This paper describes our preliminary findings as a result of using electrocochleography as an investigative procedure in a series of forty-five cases of proven acoustic neuroma. Method
We have used the trans-tympanic method of electrocochleography as described by Portmann et al. (1967). The advantage of this method is that the potentials obtained are large and therefore easily read. It is possible to record the potentials from more distant sites such as the tympanic membrane, external acoustic meatus, mastoid or ear lobe but since the electrical activity conducts relatively easily through the cochlear fluids but is rapidly attenuated by the more electrically resistant tissues such as bone and skin, these more distant sites yield only very small responses. When the electrodes are placed on the mastoid or earlobe and the vertex, it is possible to obtain several brainstem responses in addition to the auditory nerve action potential and so it is likely that additional valuable neurological information can be gained (Jewett et al., 1970; Sohmer and Feinmesser, 1967; Sohmer et al., 1974; Coles and Thornton, 1975)We test co-operative adult subjects under local anaesthesia. The drum is first inspected and then sprayed lightly with cetacaine (Cetylite Industries Inc.). No antiseptic or other fluids are used to cleanse the meatus. A slender needle electrode is inserted through the tympanic membrane midway between the umbo and the postero-inferior rim of the annulus so that its point rests on the promontory of the cochlea anterior to the round window niche. Crowley et al. (1975) in a review of several thousand cases have shown the technique to be very safe. The recordings
Using a trans-tympanic electrode, various electrical cochlear potentials were recorded in response to sound stimuli, using an Amplaid-Medelec ERA/ECochG apparatus. These potentials consist of cochlear microphonics, summating potentials and the auditory nerve action potentials. We have not at this stage attempted to analyse the summating potentials systematically. The cochlear microphonics are generated by the cochlear hair-cells and these responses closely resemble the waveform of the stimulus. The action potentials are derived from the auditory nerve fibres. Using trans-tympanic electrocochleography it is not possible to record action potentials from individual nerve fibres, but only from the whole of the auditory nerve. Usually a stimulus with an abrupt onset such as a click is used so that the maximum number of individual nerves fire synchronously and in this way the whole nerve action potential is recorded. Since it is the nerves from the basal coils that fire with the greatest degree of 128
Electrocochleography in the diagnosis of acoustic neuroma synchrony, it is inevitable that the major element of the whole nerve action potential is derived from this area. A much smaller and more poorly synchronized potential is also obtained from the middle and apical turns of the cochlea due to the relatively slow travelling wave responsible for the stimulation of the hair cells in these regions (Eggermont, 1974). At present the information obtained by trans-tympanic electrocochleography is effectively restricted to the basal, and beginning of the middle coil of the cochlea and hence to the higher frequencies of hearing. At high intensities of stimulation, the action potential is usually obscured by the cochlear microphonics (Fig. 1). If one wishes to analyze the action potential alone, it is usual to remove the cochlear microphonics from the trace. This may be done by alternately inverting the stimulus. When the responses to the successive stimuli are added electronically, the action potential continues to summate since its form is unaltered by stimulus inversion but the cochlear microphonics, which faithfully follow the form of the stimulus, alternately invert and therefore cancel out. By an adaptation of this method, the cochlear microphonics may be retained and the action potential rejected (Fig. 1). The normal electrocochleogram (ECochG)
The characteristics of the ECochG which are obtained from the normally hearing ear are shown in Fig. 2. The size or amplitude of the response diminishes as the intensity of the stimulus reduces until the response becomes unidentifiable at a level close to the subjective hearing threshold. The timing or latency of the response grows longer as the stimulus intensity decreases. These changes may be represented on an input/output graph (Fig. 3). At high intensities, the response is monophasic and consists essentially of a single negative peak (Ni). As the intensity diminishes, a second negative peak appears (N2) and at 60 dB the response is ' W shaped. It appears to be this latter part of the response that is traced down towards the hearing threshold. It is likely that each of the negative peaks represents a different group of hair cells and it has even been suggested that these are the inner and outer groups of hair cells, although the evidence is incomplete in some respects. Conductive hearing losses
The ECochG that is obtained from a patient with a conductive hearing pathology is essentially the same as that obtained from a normally hearing subject except that higher intensities of stimulation are needed to evoke the responses. The input/output functions show a 'shift to the right' similar to that seen with the speech audiogram. Cochlear hearing losses
Patients with cochlear pathology may be distinguished on audiological testing by the phenomenon of recruitment. The ECochG in such cases 129
W. P. R. Gibson and H. A. Beagley shows several interesting characteristics (Fig. 4). The amplitude of the response diminishes in a more uniform manner towards the hearing threshold and there is little change in the latency. The findings are clearly shown
C O C H L E A R
C O C H I. E A R
M I C R O P H O N I C
M I C R O P H O N I C
a n d
V I I I
only
NERVE
( x 2 )
A C T I O N
P O T E N T I A l
S
V I M
N E R V E A C T I O N
P O T E N T I A l
H O d B
W lD E BAN 0
2 0 m s
A N A U
SIS
A C O U S T I
only
C
CtICK
T I M E
FIG. I.
The electrical potentials obtained by electrocochleography.
on the input/output graph (Fig. 5). The response at high intensities is often diphasic in shape. Analysis of these results reveals that the changes are due to the non-appearance of the second negative deflection (N2) and, perhaps, this indicates a loss of outer hair cells which certainly may occur in many ears which show recruitment. The whole explanation, however, is probably more complicated than this. 130
Electrocochleography in the diagnosis of acoustic neuroma Retro-cochlear (or neural) hearing losses
We have been fortunate in being able to investigate a large number of patients with retro-cochlear hearing losses which have been referred to Mr. Andrew Morrison at the London Hospital. Forty-five of these cases have been proven to be acoustic neuromata and several other cases were shown to have other pathologies affecting the auditory nerve within the internal acoustic meatus. The following account is a progress report of our findings in these cases. E.C.O.G. NORMAL EAR
dB 110 HI 100 90
II
80 70 60
10ms scan 2ms delay Wide band click
50 40 30
xlO
20 xlO
10 x 10
Subjective hearing threshold 10dB FIG. 2. Normal ear: The action potentials.
Typical acoustic neuromata
The typical case of acoustic neuroma shows no recruitment on audiological testing. The ECochG obtained from such a typical case is shown in Fig. 6. In particular, it is the up-going limb of the first negative deflection
W. P. R. Gibson and H. A. Beagley
I 10 20 30 iO 50 60 70 80 90 100 110 INTENSITY da ISO FIG. 3. Normal ear: Input/output functions.
E.C.O.G. RECRUITING EAR
100
II
10m i s c a n 2ms d e l a y W i d e b a n d click
Subjective h e a r i n g t h r e s h o l d
SOdB
FIG. 4. Recruiting ear: The action potentials.
132
100 90 80 70 60 50 40 30 20 10 10 20 30 40 50 60 70 80 90 100 110 INTENSITY dRISO Ri^itearo
Leftearx
FIG. 5. Recruiting ear: The input/output functions. TYPICAL RIGHT ACOUSTIC NEUROMA
xlO
xlO
xlO
20
10ms scon 2ms d e l o y
•I!
Wideband clicks
BB d Agt 31
6. Typical acoustic neuroma: The action potentials. FIG.
133
W. P. R. Gibson and H. A. Beagley (Pi) that is affected. We have tested twenty-four cases of acoustic neuroma, one case of a meningioma and one case in which an ectatic basilar artery was pressing on the auditory nerve (Gibson and Wallace, 1975). All these cases showed a non-recruiting hearing loss audiometrically. Two of the neuroma cases and the meningioma patient failed to show a clear widening of the response on electrocochleography. Atypical acoustic neuromata
Approximately 10 per cent of ears affected by an acoustic neuroma and a higher proportion in cases of deafness associated with multiple neurofibromatosis give audiological results that are consistent with a recruiting (sensory) pathology. The smaller the tumour, the more likely it is to show this atypical audiological picture (Morrison, 1974). Obviously it is desirable to diagnose tumours at this early stage since the operative mortality and morbidity associated with the removal of small tnmours is considerably less than in larger tumours. We have tested eight such patients. In all but one case, which has not yet been confirmed surgically, the electrocochleogram gave results similar to that shown in Fig. 6. Large acoustic neuromata
In many cases patients with large acoustic neuromata are either completely deaf or severely deafened in the affected ear. Under such circumstances it is usually not possible to obtain any action potentials but we have often noticed that, using the same amplification, the cochlear microphonics are still visible even when using high frequency filtered clicks that stimulate mainly the hair cells at the basal end of the cochlea. We have encountered fifteen such cases and an example of such a patient is shown in Fig. 7. We believe that this finding may be explained by a lesion that destroys the nerve but spares the blood supply of the cochlea sufficiently to allow some of the hair cells to survive and to produce a cochlear microphonic potential in response to acoustic stimulation. Furthermore, we have also seen several cases in which a small distorted action potential is still obtained even though the ear was subjectively totally deaf. The clearest example was seen on testing a patient with a brainstem astrocytoma (Fig. 8), in whom the distorted action potential was evident to stimulation of less than 40 dB H.L. It would seem that in such a case the nerve has been damaged to such an extent that no meaningful message can pass along it to the higher centres whilst the most peripheral part of the nerve is still able to produce electrical activity. Patients with acoustic neuromata but apparently normal electrocochleogram
Four patients in this series gave results which at the time of testing were interpreted as being normal. Retrospectively, we now feel that there 134
Electrocochleography in the diagnosis of acoustic neuroma were some features of the responses which were suggestive of a loss of recovery of the positive limb of the potential (Pi). Fig. 9 shows one such patient in whom it appears that, if the analysis time is doubled to twenty milliseconds, the response has not recovered completely until approximately fifteen milliseconds have elapsed and at this point a small myogenic response is evident. Three of the patients were apparently in this category whilst the remaining case gave a high P i which was followed by a definite trough. We now feel that greater clinical significance should be assigned to such changes. PATIENT WITH LEFT ACOUSTIC NEUROMA (NO SUBJECTIVE HEARING) J1 dB 110 HL
,00
90
HI No action potential
10ms scon 2ms delay Wideband clicks P r e s e n t c o c h l e a r microphonics GR
O* Age 5 0
7. Large acoustic neuroma with no subjective hearing: No action potential but cochlear microphonic intact. FIG.
Patients with no acoustic neuroma but positive ECochG changes
Although we have met only four patients in our series of forty-nine tumours who gave possibly normal responses, we have encountered several patients who gave electrocochleographic findings that mimicked those found in the tumour group. Ozsahinoglu and Harrison (1974) state that in their series less than 50 per cent of patients presenting with non-recruiting hearing loss were subsequently found to have a tumour. Thus it is likely that there are various pathologies that can affect the auditory nerve which are not acoustic neuromata or even tumours, and indeed we have identifield a number of such cases. At the London Hospital, a number of patients were referred with suggestive audiological or radiological findings, in whom 135
W. P. R. Gibson and H. A. Beagley no lesion in the internal acoustic meatus was detected on Myodil cisternography. Electrocochleography was used routinely in the investigation of all these patients. RIGHT EAR
10ms scan
lOpv
2ms delay W i d e b a n d clicks GJ 6 Age 54 Kt
512
1124 2041 4«M
l»2 Hi
M
i " S « o .s
n * u
I
1 110 121
FIG. 8. Brainstem astrocytoma with no subjective hearing: Distorted action potentials are obtained.
The most difficult group to distinguish from the tumours using ECochG proved to be patients with Meniere's syndrome. Figure 10 shows the responses obtained from such a patient. Myodil cisternography failed to reveal a tumour and later a saccus drainage operation caused the hearing to improve dramatically. It is disappointing that ECochG cannot differentiate clearly between some cases of Meniere's syndrome and acoustic neuromata. The problem is probably not so difficult in cases of bilateral Meniere's syndrome, where, in addition to the overall clinical picture, the ECochG findings may show bilateral distortion of the ECochG tracing. 136
Electrocochleography in the diagnosis of acoustic neuroma Such a case is unlikely to be confused with acoustic neuroma, but a unilateral case could be more difficult. One possible help is that the cochlear microphonic response is usually readily obtainable in cases of acoustic neuroma whilst in Meniere's syndrome the cochlear microphonics are sometimes quite difficult to obtain. Perhaps this is due to the increased endolymphatic pressure damping the vibrations of the basilar membrane. Unfortunately the size of the cochlear microphonics tend to vary so widely from one patient to the next that there may be a considerable overlap between the tumour group and the Meniere's group. Nevertheless we believe that this observation is worthy of close attention. LEFT EAR
100
2 0 m s scoo 2ms d e l a y W i d e b a n d clicks
10 2ml delay
1
+ | FS 9 As* 44
g. Acoustic neuroma with near-normal action potentials. FIG.
Conclusions
Table I shows the results obtained on testing ninety-one patients that had been referred specifically for investigation of suspected acoustic neuromata. The electrocochleogram was reported as either being positive or negative for a retro-cochlear (or neural) lesion immediately after the test and before the myodil radiography. Retrospectively, we feel that we may now be more accurate in interpreting the results, especially those cases with slight but significantly delayed recovery of the up-going limb of the Ni as described earlier.
W. P. R. Gibson and H. A. Beagley Several other patients with Meniere's syndrome were tested who gave a broad response on ECochG in whom the diagnosis was not in doubt and who were not investigated as possible tumour cases. These cases are not included in Table I. RIGHT EAR
100
xlO
70 x 10
60 xlO
50 .10
IOJJV
10 ms sea n
ii
2 ms delay W i d e b a nd click FIG. IO.
Meniere's disease: The action potential mimicks that of an acoustic neuroma.
Electrocochleography does seem to be of value in the diagnosis of acoustic neuroma, especially in small tumours which show atypical results with other forms of testing. It does not provide a cast-iron diagnosis but in our series it correctly identified forty-five out of forty-nine (88 per cent) of patients with a tumour, although the seven false positive due to unilateral Meniere's syndrome remains a problem. It seems likely that electrocochleography will prove to have clinical uses in addition to the diagnosis of purely cochlear disorders. 138
Electrocochleography in the diagnosis of acoustic neuroma TABLE I. Tumour No present tumour 45 7 ECochG positive ECochG negative 4 35 (In respect of a diagnosis of a retro-cochlear, or neural, lesion)
Acknowledgements
We would like to thank Mr. Andrew Morrison and Mr. Tom King who kindly asked us to investigate this very interesting series of patients. We would like also to thank Mr. Connolly and the Department of Clinical Photography of the Institute of Laryngology and Otology for providing the illustrations. REFERENCES BEAGLEY, H. A., HUTTON, J. N. T. and HAYES, R. A. (1974) The Journal of Laryngo-
logy and Otology, 88, 993. COLES, R. R. A. and THORNTON, A. R. T. (1975) Personal communication. CROWLEY, D., DAVIS, H. and BEAGLEY, H. A. (1975) Annals of Otology, Rhinology
and Laryngology, 84, 1, EGGERMONT, J. J. (1974) Ada Oto-laryngologica, supplementum 316, 7. GIBSON, W. P. R. and WALLACE, D. (1975). The Journal of Laryngology and Otology, 89, 721. JEWETT, D., ROMANO, H. N. and WILLISTON, J. S. (1970) Science, 167,1517.
MORRISON, A. W. (1974) The British Journal of Audiology, 8, 86. OZSAHINOGLU, C. and HARRISON, M. S. (1974) The British Journal of Audiology, 8, 61. PORTMANN, M., L E BERT, G. and ARAN, J-M. (1967) Revue de Laryngologie, Otologie
et Rhinologie, 88,157. SOHMER, H. and FEINMESSER, M. (1967) Annals of Otology, Rhinology and Laryngology, 76,427. SOHMER, H., FEINMESSER, M. and SZABO, G. (1974) Electroencephalography and
Clinical Neurophysiology, 37, 663. The Nuffield Speech and Hearing Centre, The Royal National Throat, Nose and Ear Hospital, Gray's Inn Road, London, WCi, and The London Hospital, Whitechapel, London, E i .
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