Effects of chronic implantation and long-term stimulation of a cochlear implant in the partial hearing cat model

Abstract

The expansion of criteria for cochlear implantation has resulted in increasing numbers of cochlear implant subjects having some level of residual hearing. The present study examined the effects of implantation surgery and long-term electrical stimulation on residual hearing in a partially deafened cat model. Eighteen animals were partially deafened, implanted and chronically stimulated. Implantation resulted in a pronounced loss evident 2-weeks post implantation of up to 30–40 dB at 4 & 8 kHz which was statistically significant (2-way RM ANOVA (Time, Frequency): p(Time) = 0.001; p(Frequency) < 0.001; p(Time x Frequency) < 0.001)). Chronic stimulation resulted in a significant (RM ANOVA: p(Time) = 0.030) ongoing hearing loss, with 5 animals (∼30%) exhibiting an increase in threshold of 20 dB or more. Different loss profiles were evident with peripheral and central hearing assessments suggests that changes in ‘central gain’ may be occurring. Despite significant loss of hair cells and spiral ganglion neurons and distinct fibrous tissue growth in the scala tympani following implantation and long-term electrical stimulation, there were no significant correlations with any histological measures and ongoing hearing loss. The partially deafened, chronically stimulated cat model provides a clinically relevant model in which to further investigate the cause of the delayed hearing loss following cochlear implant surgery and use.