Cochlear implant effects on the spiral ganglion.

To evaluate the effects of chronic intrascalar implants on spiral ganglion cells, we studied 15 monkey cochleas that had had implants for periods ranging from 1 to 28 months. All cases exhibited loss or ongoing degeneration of cells. Cell loss typically was greater in the basal turn, where the implants were located, than at more apical locations. Increasing apical damage was a function of postimplant survival time. Preimplant local treatment of the inner ear with neomycin did not influence the loss of spiral ganglion cells. Osteoneogenesis occurred in the majority of cases, appearing in the basal turn with occasional extension into middle and apical turns. Cell loss was inconsistently associated with new bone formation. Electrical stimulation had no obvious influence on cell survival.