Morphological changes in the normal and neomycin-perfused guinea pig cochlea following chronic prosthetic implantation.

The effects of chronic prosthetic implantation and interval electrical stimulation were studied in the normal and neomycin-perfused cochlea of the guinea pig. One group of guinea pigs was implanted with a multiple-electrode prosthesis in the scala tympani. During a 4-week period, the device was stimulated for 3 hours weekly with a continuous, 1 kHz sinusoidal current of constant intensity. A second group of guinea pigs underwent identical implantation and stimulation except that cochlear perfusion with .1 M neomycin was performed at the time of implantation. Current intensities ranged from .1 to .6 mA RMS. Two complementary control groups were implanted but not stimulated. The animals were sacrificed, the cochleae were perfused with a fixative, and the temporal bones were prepared for examination under a light, transmission, or scanning electron microscope. In the electrically stimulated cochleae, degenerative changes occurred in both the inner and outer hair cells and supporting elements. A decrease was apparent in spiral ganglion cell and nerve fiber populations in areas of inner hair cell depletion and did not seem to correspond to the survival of supporting cells. The electrically active electrodes were uniformly surrounded by a connective tissue matrix and areas of immature bone. These changes occurred at all the intensities tested, and did not monotonically relate in severity to current intensity. None of the changes was apparent in the normal control ears. Morphological changes induced by the ototoxic drug neomycin were so severe is both stimulated and unstimulated cochleae that comparison was not possible; this form of pretreatment is apparently unsuitable for use in studies of electrically induced damage. It appears that in the normal animal, chronic implantation with interval electrical stimulation results in a cumulative form of sensory and neural damage histologically similar to that proceeding from chronic noise exposure as well as other ototoxic agents. Such effects should be minimized if surviving sensorineural and supporting elements in the functionally compromised cochlea are to be preserved.