Detection of intracochlear damage during cochlear implant electrode insertion using extracochlear measurements in the gerbil

Objectives/Hypothesis: An intraoperative monitoring algorithm during cochlear implant electrode insertion could be used to detect trauma and guide electrode placement relative to surviving hair cells. The aim of this report was to assess the feasibility of using extracochlear recording sites to monitor acoustically evoked responses from surviving hair cells and neural elements during implantation in an animal model. Study Design: Animal experiments. Methods: The normal-hearing gerbil was used. Two recording methods, one using a lock-in amplifier and another using Fourier analysis of recorded signals, were used to obtain frequency-specific information about the responses to tones. Amplitude and threshold determinations were made at the round window and at three extracochlear sites. To induce intracochlear damage, a platinum-iridium wire was inserted through the round window. The wire was advanced, and changes in the potentials were correlated with cochlear contact. Anatomic integrity was assessed using cochlea whole mount preparations. Results: In general, the lock-in amplifier showed greater sensitivity and lower thresholds at higher frequencies relative to the Fourier method. Also, the lock-in amplifier was more resistant to masking effects. Both systems were able to detect loss of cochlear potentials secondary to intracochlear trauma. Histologic damage was seen in all cases and corresponded to electrophysiologic changes. Conclusions: Impact of electrodes on cochlear structures affecting cochlear performance could be detected from several extracochlear sites. The lock-in amplifier demonstrated greater sensitivity and resistance to noise when compared to the fast Fourier transform recording paradigm. The latter showed greater flexibility of detecting and separating hair cell and neural potentials.