The cochlear histopathology of chronic intracochlear implantation

A fundamental question regarding the feasibility of artificial electrical stimulation of auditory nerve in cases of sensory deafness is the fate of that nerve after electrode implantation. The results of chronic scala tympani electrode implantation in normal and neomycin-deafened cats indicate that the vast majority of primary auditory neurons will survive the initial surgical implantation and the long-term interface with the molded silastic electrode for periods of at least 30 months (the longest normal-implant survivors). While there is histologic evidence that some neurons are lost in the basal coil, especially in neomycin-deafened animals, the majority of spiral ganglion cells and their radial fibres survived neomycin administration and surgical implantation even in this region. The implantation of electrodes in neomycin-deafened cats did not result in heavy neuronal degeneration. Indeed, there was little apparent difference in nerve survival between the implanted and unimplanted ears of the neomycin animals for periods up to five months (the longest neomycin implant survivor). Traumatic electrode insertion with injury to the boney covering of the modiolus or disruption of the basilar membrane resulted in extensive nerve loss in the traumatized region. When carefully inserted by an experienced otologist, the molded-silastic scala tympani electrode permitted discrete differential stimulation of restricted segments of auditory nerve and produced little or no neural degeneration.