Short latency vestibular potentials evoked by electrical round window stimulation in the guinea pig

Investigation of Balance Function Using Dynamic Posturography under Electrical-Acoustic Stimulation in Cochlear Implant Recipients

Introduction. The purpose of the present study is to investigate the effect of electrical-acoustic stimulation on vestibular function in CI patients by using the EquiTest and to help answer the question of whether electrically stimulating the inner ear using a cochlear implant influences the balance system in any way. Material and Methods. A test population (n = 50) was selected at random from among the cochlear implant recipients. Dynamic posturography (using the EquiTest) was performed with the device switched off an switched on. Results. In summary, it can be said that an activated cochlear implant affects the function of the vestibular system and may, to an extent, even lead to a stabilization of balance function under the static conditions of dynamic posturography, but nevertheless also to a significant destabilization. Significant improvements in vestibular function were seen mainly in equilibrium scores under conditions 4 and 5, the composite equilibrium score, and the vestibular components as revealed by sensory analysis. Conclusions. Only under the static conditions are significantly poorer scores achieved when stimulation is applied. It may be that the explanation for any symptoms of dizziness lies precisely in the fact that they occur in supposedly noncritical situations, since, when the cochlear implant makes increased demands on the balance system, induced disturbances can be centrally suppressed.

Sound-evoked neurogenic responses with short latency of vestibular origin

OBJECTIVE

In ABR recording, a large negative deflection with a latency of 3 ms (N3) has been recorded in patients with peripheral profound deafness. It has been suggested that N3 might be of vestibular origin. So far, N3 has been recorded only in patients with peripheral profound deafness. If we can record N3 potentials in subjects with preserved hearing, recording N3 potentials might be a new clinical test of the vestibular system. To record neurogenic potentials (N3) of vestibular origin in healthy volunteers and patients with vestibular disorders.

METHODS

Twelve healthy volunteers (10 men and two women, aged 23-37 years) and 12 patients with vestibular disorders (6 men and 6 women, aged 29-71 years) were enrolled in this study. To record responses, surface electrodes were placed on the ipsilateral mastoid and the vertex. An electrode on the nasion served as the ground. Recording was performed using an auditory evoked potential recording system with a mini-mixer and a stereo-amplifier. Signals at the vertex to the ispilateral mastoid were amplified and bandpass filtered (100-3000 Hz). One thousand-hertz short tone bursts (1 kHz STB; rise/fall time=0.5 ms, plateau time=1 ms) were presented to either ear through a headphone with or without white noise (WN) ipsilateral to the stimulated ear. The stimulation rate was 10 Hz, and the analysis time was 10 ms. The responses to 500 stimuli were averaged twice.

RESULTS

When 1 kHz STB (95 dBnHL, equivalent to 130 dBSPL) were presented with 100 dBSPL WN (ipsilateral to the stimulated ear), a negative peak with 3-4 ms latency (N3) was observed in 23 of the 24 ears (95.8%) with reproducibility in healthy subjects. Without WN, N3 was observed in 17 of the 24 ears (70.8%). The threshold of N3 was 90.2 dBnHL on the average. The presence of N3 in the patients was in agreement with the presence of the VEMP, which were also recorded.

CONCLUSIONS

Using techniques of WN exposure ipsilateral to the stimulated ear, we recorded N3 in healthy subjects and in vestibular disorder patients with preserved hearing. This negative peak is likely to be of vestibular origin.

SIGNIFICANCE

N3 may be measured from subjects who cannot contract neck muscles due to their ages, mental states, or consciousness disorders. In other words, N3 may be measured from subjects from whom VEMP cannot be recorded. In combination with VEMP, N3 may be useful for the detection of lesion sites.

Electrically evoked auditory potentials: comparison between transtympanic promontory and round-window stimulations

OBJECTIVE

Electrically evoked auditory potential (eEAP) techniques have been developed in order to assess electrical excitability of auditory nervous pathways in deaf patients before cochlear implantation. The aim of this study was to compare eEAPs recorded after a transtympanic promontory stimulation with those obtained after a round-window stimulation during a surgical approach.

DESIGN

eEAPs were recorded after promontory and round-window stimulation in 10 deaf subjects (i.e. 14 ears) who were candidates for cochlear implantation.

RESULTS

eEAPs were successfully recorded in 13 out of 14 (93%) and 11 out of 14 (79%) ears after promontory stimulation and round-window stimulation, respectively. Similar waveforms, latencies, thresholds and eV amplitudes were observed in both cases.

CONCLUSIONS

Both tests provide objective and useful information for the patient selection process before cochlear implantation. Transtympanic stimulation should be preferred in clinical practice as it is less invasive than round-window stimulation and does not require surgical exposure.

Changes in electrovestibular brainstem responses after aminoglycoside intoxication in guinea pigs

OBJECTIVE

To verify the usefulness of short-latency vestibular responses evoked by a combination of round window electrical stimulation and sinusoidal rotation (electrovestibular brainstem responses; EVBRs) as a new monitoring tool of the vestibular function in animal experiments.

METHODS

EVBRs were obtained before, during, and after treatment with aminoglycosides, along with compound action potential (CAP) audiograms. The changes in EVBRs were compared with morphological changes observed by scanning electron microscopy.

RESULTS

EVBR amplitudes did not change in the group of guinea pigs treated with amikacin, but markedly decreased in the streptomycin and gentamicin- treated groups. CAP audiograms indicated a significant threshold elevation in the amikacin group, a moderate elevation in the gentamicin group, and no change in the streptomycin group. Under scanning electron microscopy, the loss of the sensory hair cells observed in the cristae ampullares was slight to moderate in the amikacin group, moderate to severe in the streptomycin group, and severe in the gentamicin group.

CONCLUSION

EVBRs reflect overall pathological changes undergone by vestibular hair cells, and support the vestibular specificity of EVBRs.

Vestibular stimulation by multichannel cochlear implants

The recipient of a Nucleus 22 multichannel cochlear implant began to experience severe vestibular stimulation related to the implant. This patient's experience initiated a study with the objective of determining the frequency of implant-related vestibulo-ocular stimulation. Subjects consisted of 17 randomly selected patients who use cochlear implants. Included in the study were 14 Nucleus 22 and three Med-El Combi 40-devices. Stimulation of the implants was performed both by individual channel and with sound field broad-band 80-dB noise using the users' normal device settings. Eye movements were monitored with infrared videonystagmography. Only one subject, who used a Med-El Combi 40, showed a consistent and strong ocular response to cochlear stimulation but had no subjective symptoms. The authors conclude that vestibulo-ocular activation is possible with multichannel cochlear implants but is infrequent and may not be clinically significant.

Electric response audiometry in young children before cochlear implantation: a short latency component

OBJECTIVE

Evaluation of a short latency component (SLC) observed in profoundly deaf young children during recordings of the auditory brain stem response (ABR) before cochlear implantation.

DESIGN

Seventy young children (ages 2 to 11 yr) were investigated as part of their routine audiologic and diagnostic assessment, before cochlear implantation. The ABR was evoked using click stimuli and tone pips (500 Hz and 1 kHz) at intensity levels up to 105 dB nHL. The incidence of the SLC arising at a latency of 3 msec with high level click stimuli, in addition to residual ABR waves, was assessed.

RESULTS

An SLC was observed in a total of 18 of the 70 children investigated. It was present in 15 out of 31 congenitally deaf children (48%) compared with only 2 out of 33 children deafened after meningitis (6%). A chi-squared test employing a 2 x 2 contingency table shows that this dependency on etiology of deafness is highly statistically significant (p < 0.001). The presence of the SLC in the congenitally deaf children is related to age at the time of the Electric Response Audiometry test and is more likely to be observed in the younger child (p < 0.01).

CONCLUSIONS

Interpretation of the ABR in profoundly deaf children should take into consideration the possible presence of the SLC. An evoked potential arising from stimulation of the vestibular system, particularly the vestibular nuclei, is proposed as a likely origin for this component. Damage to sensory cells in the vestibular portion of the labyrinth might explain why an SLC is seen rarely in cases of deafness after meningitis.

Adaptation in the compound action potential response of the guinea pig VIIIth nerve to electric stimulation

An experimental study, carried out in guinea pigs, was designed to investigate whether forward masking measured psychophysically in 3M-House cochlear implant users might have a correlate in VIIIth nerve activity. The study was based on electrically evoked VIIIth nerve compound action potentials (ECAPs), using a masking paradigm comparable to the one used in the psychophysical study. Trains of 50 maskers with inter-masker-intervals of 509 ms appeared to induce a long-term fatigue effect that could influence the recovery from adaptation measurements. Fatigue stabilized within about 1 to 3 min when masker trains were repeated with intervening silent intervals of 10.5 s. The change in amplitude of probe-evoked ECAPs with increasing masker-probe delays was determined within the steady fatigue state. The recovery-from-adaptation functions obtained from these measurements resembled the forward masking functions found in 3M-House cochlear implant users. No correlate of psychophysical backward masking was found at the VIIIth nerve level. To examine whether hair cells were involved in fatigue and recovery from adaptation, the measurements described above were carried out in intact cochleas and in cochleas without hair cells. Results were essentially the same in the different preparations. The results suggest that processes at the level of the VIIIth nerve could, at least partly, account for forward masking found in 3M-House cochlear implant users. Backward masking must be attributed to mechanisms located centrally to the VIIIth nerve.

Differential sensitivity to rotation measured on potentials evoked by electrical stimulation of the guinea-pig ear

Responses to electrical stimulation of the ear applied between round-window and vertex electrodes were recorded in awake guinea-pigs from the same electrodes or from separate vertex/mastoid subdermal needle electrodes. They were averaged during opposite phases of sinusoidal rotation or before and after constant velocity rotation. In both cases the responses were subtracted from each other and yielded differential per- or post-rotatory "electrovestibular" responses. For comparison, responses were also recorded in the same animals and conditions of electrical stimulation during silence and during silence and during presentation of a broad-band noise. The difference yielded "electroacoustic" responses. In round-window records, electrovestibular and electroacoustic responses presented typical compound nerve action potential patterns. Electrovestibular responses could be recorded for head angular velocities as low as 3 degrees sec-1 at 0.1 Hz. Response amplitude showed a logarithmic relation to head velocity. Changes in amplitude, as a function of time after rotation, were comparable to those reported for vestibular nerve fibre responses. In vertex/mastoid records, electroacoustic responses presented a sequence of peaks similar to the click-evoked auditory brain-stem responses, and electrovestibular responses presented two peaks, presumably representing contributions of central vestibular structures. Such "electrovestibulography" permits the study of an individual ear and makes available to the investigator a large range of vestibular stimulation conditions.