The cochlear histopathology of chronic intracochlear implantation

Abstract

A fundamental question regarding the feasibility of artificial electrical stimulation of auditory nerve in cases of sensory deafness was the fate of that nerve after electrode implantation. The results of chronic scala tympani electrode implantation in normal and neomycin-deafened cats indicated that the vast majority of primary auditory neurons survived the initial surgical implantation and the long-term interface with the molded silastic electrode for periods of at least 30 mo. (the longest normal-implant survivors). While there was histologic evidence that some neurons were lost in the basal coil, especially in neomycin-deafened animals, the majority of spiral ganglion cells and their radial fibers 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. There was little apparent difference in nerve survival between the implanted and unimplanted ears of the neomycin animals for periods up to 5 mo (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.