Chronic intracochlear electrode implantation: cochlear pathology and acoustic nerve survival

The temporal bones of ten cats implanted with intracochlear electrodes for three to 117 weeks were stained with hematoxylin and eosin and examined with light microscopy. The electrodes were embedded in Silastic® which was molded to fill the most basal 9 mm of the scala tympani. They were inserted directly into the scala through the round window. Among our observations were the following: 1) All or nearly all hair cells were lost in the basal coil during the first several weeks after implantation. Some, but not all, supporting cells were also lost. There was extensive hair cell loss in the middle and apical turns, although some hair cells were seen there in all examined cats. 2) There was evidence of degeneration of spiral ganglion cells in the basal cochlea in several animals, but most primary auditory neurons including (with two exceptions) most of those in the region directly over the electrode, survived implantation in every cat. The radial nerve fibers of the spiral ganglion cells also survived long-term implantation. The functional viability of remaining spiral ganglion cells was confirmed in acute neurophysiological experiments conducted just before the animals were sacrificed. 3) More severe degeneration was seen in two cats in which the electrode perforated the basilar partition. In these animals, there was loss of many spiral ganglion cells, and evidence of new bone growth in the region of the perforation. 4) The appearance of the stria vascularis and spiral ligament in some implanted animals paralleled their descriptions following occlusion of the cochlear vein. 5) Connective tissue formed around the electrode surfaces, apparently displacing perilymph and sealing the electrode into the scala tympani. There was no evidence of perilymph fistula in any animal. 6) There was little evidence of progressive degeneration of the organ of Corti or spiral ganglion from three to 34 weeks after implantation. Some of the implications and limitations of these findings are discussed.