COCHLEAR PATHOLOGY FOLLOWING CHRONIC ELECTRICAL-STIMULATION USING NONCHARGE BALANCED STIMULI

During the course of a chronic intracochlear electrical stimulation study using charge balanced biphasic current pulses, one animal inadvertently received a short period of direct current (DC) stimulation at a level of approximately 1-mu-A. Subsequent, the animal was chronically stimulated using a poorly charge balanced waveform that produced a DC level of approximately 2-mu-A. Extensive pathological changes were observed within the cochlea. These changes included widespread spiral ganglion cell loss and new bone growth that extended throughout all turns of the cochlea. Significant changes in the morphology of the electrically evoked auditory brainstem response (EABR) were associated with these pathological changes. EABRs recorded prior to the DC stimulation exhibited a normal waveform morphology. However, responses recorded during the course of the DC stimulation were dominated by a short latency response believed to be vestibular in origin. The response thresholds were also significantly higher than levels recorded before the DC stimulation. In contrast, the contralateral cochlea, stimulated using charge balanced stimuli, showed no evidence of adverse pathological changes. Furthermore, EABRs evoked from this cochlea remained stable throughout the chronic stimulation period. Although preliminary, the present results illustrate the adverse nature of poorly charge balanced electrical stimuli. These results have important implications for both the design of neural prostheses and the use of DC stimuli to suppress tinnitus in patients.