Auditory brainstem prosthesis: Biocompatibility of stimulation

As part of a program to develop a central auditory system prosthesis, we have examined the reaction of tissue to different levels of stimulation from electrodes chronically implanted in the cochlear nucleus of the guinea pig. Tolerance and histologic reaction to 20 hours of periodic electrical stimulation of the cochlear nucleus were analyzed. Intraoperative and postoperative electrically evoked middle latency responses were monitored during stimulus trials. The threshold necessary to generate the middle latency responses was frequently below 50 μA. In the animals that received 50 and 100 μA of biphasic charge-balanced stimulation (corresponding to approximately 200 abd 400μC/cm² phase), adverse tissue reaction was minimal, and glial proliferation along the electrode tract never exceeded 25 μm in width. Stimulation intensities of 150 and 200 μA (approximately 600 abd 800 μC/cm² phase) produced significant tissue response at the site of the electrode terminus, with necrosis, cell loss, and reactive cells present. Therefore neuronal damage was observed to occur at an intensity far greater than that required for eliciting an electrophysiologic response.