Cochlear pathology following chronic electrical stimulation of the auditory nerve: II. Deafened kittens

The present study examines the effects of long-term electrical stimulation of the auditory nerve on cochlear histopathology and spiral ganglion cell survival in young sensorineural deafened cats. Eight kittens were deafened using kanamycin and ethacrynic acid, and implanted with bipolar or monopolar scala tympani electrodes. Following recovery from surgery the animals were unilaterally stimulated using charge balanced biphasic current pulses for 450-1730 hours over implant periods of up to four months. Charge densities varied from 0.6-0.9 microC.cm-2 geom. per phase for monopolar electrodes to 12-26 microC.cm-2 geom. per phase for the bipolar electrodes. Electrically-evoked auditory brainstem responses (EABRs) were periodically monitored during stimulation to confirm that the stimulus levels were above threshold, and to monitor any change in the response of the auditory nerve. Following completion of the stimulation program cochleae were prepared for histological examination. EABRs exhibited relatively stable thresholds for both stimulated and implanted, unstimulated control cochleae for the stimulus duration. While the growth in response amplitude as a function of stimulus current remained stable for the bipolar control and monopolar stimulated cochleae, the five cochleae chronically stimulated using bipolar electrodes exhibited a moderate to large increase in response amplitude. These increases were associated with a more widespread fibrous tissue response which may have altered the current distribution within these cochleae. Implanted control cochleae exhibited significantly less tissue response within the scala tympani. Importantly, we observed no statistically significant difference in the spiral ganglion cell density associated with chronic electrical stimulation when compared with unstimulated control cochleae. While the present study supports the safe application of cochlear implants in young profoundly deafened children, it does not corroborate previous studies that have reported electrical stimulation providing a trophic effect on degenerating auditory nerve fibres.