Although SCC electrode implantation caused severe hearing loss in 4 out of 6 animals, the remaining 2 animals retained hearing to the same degree in the implanted ear as in the sham surgery control ear as measured at 4 weeks post-implantation. These results confirm that it is possible to implant vestibular prosthesis electrodes within the labyrinth without compromising hearing as measured a month later.
The only significant threshold changes associated with sham surgery were the small increases of 8.5 dBSPL above normative values seen at 2 kHz and 8.7 dBSPL above normative values seen at 6 kHz. This increase might reflect a conductive rather than a sensorineural hearing loss; a bone conduction ABR (not available on our system) would be necessary to confirm this. Identification of a conductive component to the hearing loss we observed using free-field ABR would further support the conclusion that SCC electrode implantation can be achieved without loss of cochlear function.
Hearing outcome did not correlate with the order in which the chinchillas were implanted (i.e., with experience of the surgeon who performed all implantations), amount of intraoperative anesthesia, or time elapsed between implantation and ABR testing. Future studies will further explore variations in method to identify specific factors that do contribute to hearing loss. Electrode design and surgical technique will be refined to maximize hearing preservation.
It is likely that the hearing loss we observed is due to mechanical disruption of the membranous labyrinth during insertion of electrodes into the ampullae. Alternatively, the loss might be related to other factors, such as the materials used. However, neither the electrode materials (Pt/Ir wire, Teflon, silicone) nor the dental restoration material we used are known to be neurotoxic. Since all electrode insertions involved the same materials, we would expect a neurotoxin to affect all 6 of the implanted ears, but we observed that 2 of the implanted ears had preserved hearing. We might also expect to see a positive correlation between degree of hearing loss and length of exposure time to the cement. The time elapsed between implantation and ABR testing ranged from 3 to 9 weeks, and we did not see more severe hearing loss with longer exposure time to cement. The 2 hearing-preserved animals were both tested 4 weeks after implant. Of the 4 animals that lost hearing (all to a similar degree), 2 were tested at 3 weeks, 1 at 4 weeks, and 1 at 9 weeks after implant.
It is possible that damaging effects of electrode implantation could evolve slowly, resulting in the onset of hearing loss months or years after implantation. Similarly, it is possible, though it seems unlikely, that animals with severe hearing loss at 4 wks postoperatively might exhibit recovery of cochlear function later. Future studies could address this issue by following cochlear function out to longer post-operative durations. The present study tested hearing changes due to the surgical trauma of implantation. Therefore, although pulsatile electrical stimuli were delivered postoperatively via each electrode to confirm proper placement of electrodes (as indicated in each animal by reflexive eye movements in phase with pulse rate modulation), we did not deliver prosthetic electrical stimuli during ABR measurements. Another way that a vestibular prosthesis could affect hearing is through spurious excitation of the auditory nerve by prosthetic electrical stimuli. We will address this question in future studies by measuring ABR thresholds in the setting of concurrent prosthetic electrical stimulation via the implanted electrodes.
Risk of hearing loss will be an important determinant of the clinical utility of vestibular prostheses intended for treatment of patients with profound loss of vestibular sensation but intact hearing. Whereas cochlear implants can be inserted into the scala tympani and thus kept within the perilymphatic compartment of the inner ear, we expected that ampullary electrode implantation would be impossible without violation of the membranous labyrinth and consequent disruption of the endolymph-perilymph electrochemical gradient upon which normal cochlear function depends. Our results show that even in the chinchilla, which has inner ear dimensions much smaller than do humans, functional vestibular prosthesis electrodes can be implanted without significant cochlear injury.