Electrocochleography and brain-stem potentials in Ramsay Hunt syndrome.

Audiometric investigations and electrophysiologic recordings of cochlear and brain-stem auditory evoked potentials (BAEPs) were performed in 13 patients to elucidate further the type of hearing disorders in Ramsay Hunt syndrome. Transtympanic electrocochleography showed no enhancement of summating potential and did not suggest secondary endolymphatic hydrops. The recording of BAEPs was clearly abnormal in several of the 13 patients. The striking feature of the abnormalities in these patients was the prolongation of the latencies of waves III and V with the preservation of wave I, which clearly suggests retrocochlear involvement. In all the patients tested, abnormalities of the BAEPs were present only on the affected side. It is possible, on the basis of BAEP findings, to suggest that in Ramsay Hunt syndrome both cochlear and retrocochlear involvement may occur.

Diagnostic significance of transtympanic electrocochleography in Menière's disease.

Transtympanic electrocochleography (ECoG) was performed on 32 normal ears, 40 ears affected by hair cell damage without any evidence of endolymphatic hydrops (sensory damage) and 44 ears affected by established Menière's disease. The amplitude of the summating potential (SP) and the amplitude of the action potential (AP) were measured at a click stimulus intensity level of 100 dB HL. The SP amplitude was expressed as a percentage of the AP amplitude. In normal ears, the mean SP/AP ratio was 25% (range 10%-63%). In sensory damage, the SP/AP ratio was 13% (range 0%-29%), and in Menière's ears, the SP/AP ratio was 51% (range 29%-89%). In this series, an SP/AP ratio of 29% provided a diagnostic dividing mark between the sensory damage and Menière's-affected ears. Although this precise division was probably fortuitous, it does suggest that ECoG is a useful tool in the differential diagnosis of these two types of cochlear disorders.

Auditory function in hereditary motor and sensory neuropathy (Charcot-Marie-Tooth disease).

Fourteen patients with hereditary motor and sensory neuropathy (HMSN), 12 of Type I and 2 of Type II, were assessed for auditory dysfunction. Five patients complained of hearing loss and all had pure-tone audiograms outside the normal range, while one patient who did not complain of hearing impairment also had an abnormal pure-tone audiogram. Assessment of loudness function, speech audiometry and brainstem auditory evoked potentials (BAEP) suggested that the hearing loss was the result of VIII nerve dysfunction, a conclusion supported by the abnormality of the electrocochleogram (ECochG) in one patient.

Latency of contralateral sound-evoked auditory efferent suppression of otoacoustic emissions.

The suppression of transiently evoked otoacoustic emissions by contralateral sound stimulation is thought to occur as a result of the action of the efferent pathway from the superior olivary complex to the cochlea via the medial olivo-cochlear neurons. The purpose of this study was to determine the time taken for this pathway to activate the suppressive mechanism in response to contralateral sound in normal human subjects. The time for onset of suppression was found to be between 7 and 20 ms.

Effects of olivocochlear bundle section on otoacoustic emissions in humans: efferent effects in comparison with control subjects.

The effects of contralateral acoustic stimulation on evoked otoacoustic emissions (OAE) were examined in three subject groups in order that the impact of efferent olivocochlear bundle section (as a consequence of vestibular neurectomy) could be compared with normal findings, and with a control surgical population. Results demonstrated that the inhibitory effect of contralateral noise on OAE amplitude was absent from the cochlea with severed efferent fibers. These findings appear to be independent of acoustic reflex activity, as suppression was absent despite normal reflexes. Inter-aural suppression of emissions recorded from the patients' intact cochleae act as a control and show a clear reduction in amplitude during contralateral stimulation in a frequency specific pattern consistent with normal findings. Patients who had undergone a similar surgical approach for vascular decompression of the VIIIth nerve without vestibular nerve section, were studied in order to assess the impact of retrolabyrinthine surgery on inter-aural suppression. Inhibition of OAE amplitude was maintained in all control cases in both the operated and intact sides, and was consistent with suppression observed in normal subjects, suggesting that the surgical procedures had not disturbed inter-aural suppression of otoacoustic emissions. It is concluded that the olivocochlear efferent system, when activated by low level contralateral acoustic stimulation, has an inhibitory role in controlling the cellular mechanisms responsible for the generation of otoacoustic emissions in humans. OAE techniques in conjunction with contralateral acoustic stimulation may thus prove to be of value in providing a rapid and non-invasive clinical test of efferent function and offer a means of investigating the functional significance of the efferent auditory system in humans.

Mechanisms of hearing loss in acoustic neuroma: an otoacoustic emission study.

Evoked otoacoustic emissions (EOAE) are active mechanical responses from the cochlea which provide information about the integrity of the preneural cochlear receptor mechanisms. It may be hypothesised, therefore, that if a hearing impairment is neural in origin, normal EOAEs may be obtained from the cochlea, which, although dissociated, is functioning normally. This study examined the status of the cochlea with EOAE in patients with cochlear (Meniere's disease) and neural (surgically proven acoustic neuroma) disease. In patients with presumed cochlear lesions, no emissions were present with mean hearing worse than 40 dB across a frequency range of 0.5 to 4 kHz. Similarly, an EOAE was not present in any of the 26 acoustic neuroma patients studied when the average (0.5 to 4 kHz) hearing was greater than 40 dB. We conclude that dissociation of the cochlea in patients with acoustic neuroma appears to be rare and, in fact, cochlear involvement occurs in most cases. Possible mechanisms responsible for the effect on the cochlea in this group include degenerative changes due to chronic partial obstruction of the blood supply by the tumour, biochemical alterations in the inner ear fluids, loss of efferent control of active mechanical tuning, and hair cell degeneration secondary to neuronal loss in the eighth nerve.

A microprocessor controlled multi-channel evoked potential data acquisition system.

A stored program control system is described for the acquisition of evoked potential (EP) data. It is a small, portable and flexible system with a variety of input and output possibilities. The system presents a low cost solution to the problem of acquisition of evoked potential data. A novel method of converting and reducing sixteen channels of EEG analogue data to their digital equivalent is shown. It is adaptable for the inter active control of experiments and is sophisticated enough to deal with the complexities of experimental control as well as data manipulation. One of the major advantages of the system is that both the sequencing of the functions and their detailed make-up can be readily altered by programming to meet the individual requirements of any given situation.

The influence of stimulus spectral content on rise time effects in cortical-evoked responses.

A tone burst with a fast rise time elicits a large-amplitude cortical response which reduces markedly as rise time is increased. This effect is not observed with white-noise bursts. It is suggested that where changes in rise time affect the spectral content of the stimulus, a change in response amplitude is encountered. The increase in the latency of the response with increase in rise time is found to be the same for both tone and noise bursts, hence independent of spectral composition.

Brain stem auditory evoked potentials: significant latency differences between ipsilateral and contralateral stimulation.

A number of electrical potentials can be recorded from the human scalp following acoustic stimulation. The potentials which occur within 10 msec of the stimulus onset have been termed the brain stem auditory evoked potentials (BAEPs). Latency appears to be the most stable measure and in consequence knowledge of the exact limits of normal latency of each wave is important. In this study the effects of ipsilateral and contralateral stimulation on BAEP latencies have been investigated in 23 normal subjects. The exact limits of normal latency of each wave have been established. It has been shown that significant latency differences exist between ipsilateral and contralateral stimulation. Possible hypotheses are put forward to explain the findings which demonstrate that different neural pathways are followed by ipsilateral and contralateral stimuli and that their respective responses can be investigated separately in man using BAEP recordings.

Brain stem auditory evoked potentials: a comparative study of monaural versus binaural stimulation in the detection of multiple sclerosis.

As the latency of these responses is the most stable measure in normal subjects, the present study examines the latency of the BAEP wave forms in multiple sclerosis (MS) with a view to comparing a monaural stimulation with binaural. In previous studies binaural stimulation has been used to detect MS in clinically 'definite' cases. In this study both clinically definite and possible MS cases were examined and it was shown that in the clinically definite MS category the detection rate improved from 70% to 90% with ipsilateral/contralateral stimulation. In the possible multiple sclerosis group the detection rate was negligible with binaural stimulation compared to 67% with monaural stimulation. Both inter-wave latency and the repeatability of the response in the different groups are discussed in detail.

The effect of acoustic neuroma removal on hearing in the contralateral ear.

22 patients with unilateral acoustic neuromas were assessed audiometrically pre- and post-operatively. A high proportion (36%) were found to have a hearing loss of 10 dB or more (6 frequency average) in the contralateral ear in the post-operative period. The evidence points to a cochlear site of the loss, and serial audiograms suggest that the hearing deteriorates in the immediate post-operative period. Follow-up audiometry at three months or more showed that the hearing recovered to pre-operative levels. The possible mechanism of the hearing loss is discussed.

Effect of simulated bilateral cochlear distortion on speech discrimination in normal subjects.

Bilateral sensorineural hearing loss may introduce grossly dissimilar cochlear distortion at the two ears, causing abnormal demands to be made upon the cortical analytical centres which normally receive congruent information. As a result, the prescription of binaural hearing aids may be a handicap rather than a help. In order to explore this possibility, 10 normal subjects were presented with simulated, dissimilar cochlear distortion at the two ears. Discrimination scores with binaural presentation were poorer than the best monaural score and there were clear indications that in the former, subjects selectively attended to one ear and neglected the other. In contrast, binaural presentation of the same simulated distortion resulted in a significant improvement, compared with the monaural discrimination score. Inability of the cortex to contend with discongruent speech input from the two ears may be a factor contributing to the rejection of binaural hearing aids in some individuals.

Phase reversal in brain stem responses: its use in the detection of asymmetry in the auditory pathways.

Electrode placement is an important consideration in the recording of a large-amplitude stable response. In this study, brain stem auditory-evoked potentials (BAEP) were obtained from subjects and patients under two electrode configurations, namely 'vertex-mastoid' and 'mastoid-mastoid'. The BAEP waveforms to ipsilateral, contralateral and binaural stimulation were examined for their phase relation with respect to stimulation. In the mastoid-mastoid recording mode, a complete polarity reversal was shown upon changing stimulation from the ipsilateral (with respect to the active input) to the contralateral ear. On simultaneous binaural stimulation, the response was shown to summate to zero. This observation has led to the objective assessment of asymmetry of hearing. Several patients exhibiting recruitment were tested under the mastoid-mastoid recording configuration with binaural stimulation. The results show that a summated 'null' response only occurs at an intensity level where loudness equality is attained at the two ears and recruitment is complete. Applied in this way the technique has promising application in the determination of the presence or absence of recruitment and hence in the differential diagnosis of cochlear and retrocochlear lesions.

Brainstem auditory-evoked potentials and electrocochleography: comparison of different criteria for the detection of acoustic neuroma and other cerebello-pontine angle tumours.

Different means of identifying the presence of cerebello-pontine angle (CPA) tumours using brainstem auditory-evoked potentials (BAEP) were evaluated. The results from 33 ears affected by tumours were compared with BAEP from 50 normal ears and 23 ears affected by Menière's disease. Measurement of inter-aural latency differences correctly identified 97% of tumours from the normal group, but only 81% of tumours from the Menière group. Comparison of different interwave interval measures showed that the NI to NV interval was the best criterion and correctly identified 90% of the tumours from both the normal and the Menière groups. The use of transtympanic electrocochleography (ECochG) is discussed. Used alone, ECochG was of limited value in detecting CPA tumours except in five out of nine deaf ears where no BAEP were recorded but 'disconnection' action potentials were encountered. By combining BAEP and ECochG, the NI to NV interval could be identified with certainty. Combined BAEP and ECochG recordings were only undertaken when NI was unclear on BAEP recordings or when the ECochG was indicated clinically for the identification of endolymphatic hydrops. In this series, the combined use of BAEP and ECochG resulted in a tumour detection rate of 100% with a false positive rate of zero. The BAEP from the unaffected ear recorded from the ipsilateral and contralateral sides were compared. Significant NI to NV delay was noted, especially on recording from the contralateral side, when there was distortion of the brainstem due to a large tumour. The article concludes with an appendix of six illustrative case reports.

The selective effects of central masking on brain stem potentials.

The effect of contralateral white noise stimulation on ipsilaterally presented click evoked brain stem potentials was examined. Continuous contralateral white noise does not influence the brain stem response components but pulsed white noise simultaneously presented with the clicks produced a central masking effect which was observed as an amplitude reduction confined to Wave V. Our findings confirm those of Zwislocki (1971) that pulsed noise is a more effective central masker than continuous noise. A computer addition of the responses to pulsed white noise and clicks recorded independently revealed no reduction in Wave V amplitude but the response to the two stimuli presented simultaneously showed a specific reduction in Wave V amplitude. No change in the latency of Wave V was observed to suggest any 'cross-over' masking of the click stimulus. Patients with total unilateral deafness did not exhibit this phenomenon. The effect of a time delay of the masker to one ear in relation to the click presented to the other ear reduced the amplitude of Wave V less than when the two stimuli were presented simultaneously. The reduction in the amplitude, being specific to Wave V, suggests that the effect is central and that the particular locus for this aspect of central masking is at the level of the inferior colliculus.

Latency variability and temporal interrelationships of the auditory event-related potentials (N1, P2, N2, and P3) in normal subjects.

Peak latency variation and the temporal interrelationships of the auditory event-related potential were investigated in 12 normal adults (ages 28-42). Measures of variation were based on both conventional averages and single trials. Estimates of N1, P2, N2 and P3 latencies were made on a trial-by-trial basis to target stimuli recorded from Fz, Cz and Pz scalp locations. Results showed that single-trial latency variability of the auditory ERP differed both among the various components and between subjects. Larger standard deviations were measured for the later N2 and P3 components than the earlier N1 and P2 components. Regression analyses between various component latencies indicated a strong covarying relationship between N2 and P3, with N2 accounting for up to 61% of the variance of P3 latency at Pz. Earlier N1 and P2 components added little to the overall prediction of either P3 or N2. For the other components, P2 accounted for 9-16% of the variance of N2, while N1 accounted for approximately 1% of the variance of N2; N1 accounted for 8-10% of the latency variation of P2. The correlations between single-trial peak latencies and RTs were positive but of low magnitude. The highest correlations between peak latency and RT were found for N2 (r = 0.33) and P3 (r = 0.24). The low correlations between the single-trial latencies of N1 and P3 suggest that the processes reflected by these components are independent and support a distinction between the earlier and the later components of the ERP. The close temporal coupling between N2 and P3 suggests that N2 may reflect cognitive properties in common to P3 in stimulus evaluation processes.

Effects of contralateral acoustic stimulation on otoacoustic emissions following vestibular neurectomy.

This study demonstrates that, following unilateral vestibular neurectomy, the inhibitory effect of contralateral acoustic stimulation on evoked otoacoustic emissions is absent. The patient acts as her own control in that the unoperated side shows normal suppression of otoacoustic emission amplitude with contralateral acoustic stimulation. The lack of interaural suppression of otoacoustic emissions on the sectioned side, in the presence of normal acoustic reflex threshold levels, provides evidence that observed phenomena are not merely a function of middle ear reflex activity. It is concluded that the lack of inhibition in the operated ear is due to the sectioning of the olivocochlear bundle within the inferior vestibular nerve, removing the efferent control of the receptor cells. Otoacoustic emissions recorded during contralateral acoustic stimulation may thus provide a rapid, non-invasive means of investigating the functional of the efferent auditory system.

Presence of tinnitus indicated by variable spontaneous otoacoustic emissions.

Spontaneous otoacoustic emissions (SOAE) have been widely studied in normal subjects, and there is evidence of their high frequency stability in repeated recordings. A study to determine the frequency stability of SOAE in 53 of 100 consecutive patients, who presented with tinnitus and in whom SOAE were recordable, was undertaken. Patients were divided into five aetiologically homogeneous subgroups: (i) those with normal hearing and no identified pathology, (ii) those with sensorineural hearing loss of unknown origin, (iii) those with normal hearing, but complaining of tinnitus related to head injury, (iv) those with endolymphatic hydrops, and (v) those with noise exposure. The control group consisted of 20 subjects, selected on the basis of recordable SOAE from 38 volunteers with normal hearing and no tinnitus. The prevalence of SOAE and their inter-session frequency stability (reproducibility and relative frequency shift) were analysed. In contrast to the controls, the tinnitus group had significantly increased frequency variability of SOAE (lower reproducibility and increased relative frequency shift). The prevalence of subjects with SOAE was not notably different between the controls and subjects with tinnitus, if the tinnitus group was considered in toto, but a striking 100% prevalence of bilaterally present SOAE was observed in the tinnitus subgroup with head injury.