Morphological changes following cochlear implantation in the animal model.

Examination of the effects of cochlear implantation in the animal model has made it increasingly clear that biological changes may be induced by the device. In a preliminary study we established a damage threshold at 400 microA RMA (70 microCoul/cm2/phi) after a single three-hour exposure to continuous 1 kHz sinusoidal stimulation. By systematic modification of stimulation parameters in the present study we have sought to establish damage thresholds and patterns of histological change in the chronically stimulated animal and further elucidate the mechanisms underlying the histopathological change in a second acute animal preparation. In part one, chronic stimulation at 1 000 Hz for a total of 12 hours distributed over a four-week period resulted in a lower damage threshold at 15-20 microCoul/cm2/phi, the lowest value tested. In addition to sensory and supporting cell degeneration osteoneogenesis and spiral ganglion cell degeneration were also observed. These findings are of some relevance under conditions of prolonged implantation and stimulation in the human. In part two of the study, acute stimulation for a three-hour period at 100 Hz reduced the damage threshold from 400 microA to 200 microA RMS. This inverse relationship between threshold and frequency suggests that electrophoretic effects are, at least in part, responsible for the histological changes observed. The threshold shift was less than predicted, however, making it less likely that a single factor is responsible for the stimulation-induced damage.