Correlation of early auditory potentials and intracochlear electrode insertion properties: an animal model featuring near real-time monitoring.

OBJECTIVE: The goal of this work was to assess electrophysiologic response changes to acoustic stimuli as an intracochlear electrode impacted cochlear structures in an animal model of hearing preservation cochlear implantation. The ultimate goal is to develop efficient procedures for assessing the status of cochlear physiology for intraoperative use.
METHODS: Sixteen gerbils and 18 ears were tested. A rigid electrode was inserted through a basal turn cochleostomy and directed toward the basilar membrane/osseous spiral lamina complex. We recorded acoustically evoked early auditory potentials including cochlear microphonics (CMs) and compound action potentials (CAPs) to a short stimulation sequence consisting of one stimulus frequency and intensity as the electrode was advanced. A microendoscope was used to visualize the electrode insertion progress and to identify the site of electrode impact. After each experiment, the site of intracochlear trauma was confirmed using whole mount preparations.
RESULTS: Electrophysiologic changes correlated well with the degree and location of trauma. We observed four distinct patterns. In addition, the endoscope in conjunction with the short recording sequence allowed for the detection of response changes that were reversible when the electrode was retracted. These cases were associated with less than full-thickness damage on histology.
CONCLUSION: The short recording sequence to obtain acoustically evoked intracochlear potentials and the microendoscope allowed us to detect various levels of cochlear trauma including minor and reversible damage. Recordings of this type are potentially available using current implant technology. Future improvements in the measurements can be expected to improve the efficiency of the recording paradigm to produce a clinically useful tool.