Chronic electrical stimulation of auditory nerve in cat: Physiological and histological results

Cats were implanted with two-channel scala tympani bipolar electrode arrays consisting of four PtIr wires in a molded silicone rubber carrier. The electrically evoked auditory brainstem response (ABR) was recorded to monitor the physiological response to biphasic pulsatile stimulation in these chronic preparations. Baseline data were collected over a 1-6 month period. Animals were then subjected to a long period of continuous high level stimulation delivered through a system designed to insure delivery of charge-balanced biphasic waveforms. Subsequent changes in physiological response were interpreted as indicating electrically induced damage to the cochlea. Localized loss of hair cells and growth of connective tissue resulted from the implantation of scala tympani inserts. Electrically evoked ABR responses were not altered by the long-term presence of the electrode, nor by the presence of intervening connective tissue. Physiological manifestations of stimulus-induced change appeared only after hundreds of hours of continuous stimulation. Apparent functional damage was not suspended or reversed with cessation of stimulation, but rather continued for several hundred hours after the stimulation was terminated. Deterioration of physiological response was accompanied by two deleterious histological changes: (a) bone growth within the scala tympani; and (b) loss of nerve fibers and spiral ganglion cells. Both of these changes were restricted to an area corresponding to the implant intracochlear location and were most marked in the region adjacent to the chronically stimulated electrode pair. In cases where stimuli were not charge balanced or surgical trauma was incurred, bone growth was most extensive and nerve damage most pervasive. The data from cases stimulated at lower levels of charge density, i.e. 20-40 muC/cm2, suggest that these may be more feasible levels for safe chronic electrical stimulation in scala tympani.