Acute Morphological Changes in Guinea Pig Cochlea following Electrical Stimulation

A major area of concern in the development of the cochlear prosthesis involves the effects of the implant on inner ear anatomy and function. This study examined one aspect of host-implant interaction — the effects of cochlear implantation with and without electrical stimulation on cochlear morphology in the guinea pig. To accomplish this, a series of normal guinea pigs were acutely implanted bilaterally with a scala tympani multiple electrode prosthesis. One ear was stimulated with continuous 1 kHz sinusoidal current of constant intensity for a period of three hours. The contralateral control ear was not stimulated. Current intensities tested ranged from 0.1 mA to 1 mA rms. After stimulation the animals were sacrificed, perfused with fixative and the temporal bones were microdissected for examination using the scanning electron microscope. Morphological changes observed in the stimulated ear ranged from hair cell and supporting cell degeneration to complete destruction of the basilar membrane and organ of Corti overlying the electrode. These changes occurred at current intensities ranging from 0.4 mA to 1 mA and were conspicuously absent in the implanted but unstimulated control ears. The current intensities employed in this experiment were within operating ranges presently used in long-term behavioral studies in other animal models. To what degree the morphological changes observed in this study may effect the function of the prosthesis or host responses on a long-term basis cannot be determined from this experiment. However, our observations do demonstrate that electrical stimulation of the cochlea is not benign and point out the need for further evaluation to help define safe standards for use of the prosthesis.

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