Mechanisms of electrically induced damage after cochlear implantation.

This report is the fourth in a series of parametric studies designed to evaluate and define conditions that may produce histological damage by means of electrical stimulation from cochlear prostheses. Earlier studies established damage thresholds for both acute (400 microA rms or 70 microC/cm(2)0) and chronic (100 microA rms or 15 to 20 microC/cm(2)0) stimulation with continuous sinusoidal current of 1,000 Hz. In a subsequent study, a tenfold reduction in the stimulation frequency (to 100 Hz) resulted in a 50% reduction in the acute damage threshold (200 microA rms), which was a smaller reduction than anticipated if damage is dependent only on charge density. This finding and the damage patterns observed in the preceding studies suggested that multiple mechanisms are responsible for the sensory and supporting cell degeneration induced by the electrical stimulus. A similar pattern of structural changes has been observed by other investigators after acoustic stimulation of the cochlea, suggesting that common mechanisms may be involved. With electrical stimulation, electrophoretic effects have been implicated; like acoustic trauma, however, mechanical, biochemical, and metabolic processes may also be involved. This investigation was designed to identify and analyze better the damage mechanisms active in acute stimulation. Sixteen normal guinea pigs were implanted and stimulated using an interrupted 1,000-Hz signal at 500 microA for three hours. Duty cycle was reduced to 50% and signal periodicity ranged from 100 to 1,000 ms.(ABSTRACT TRUNCATED AT 250 WORDS)