The intraoperative relationship between intracochlear electrical field and excitability of the auditory nerve

A limiting factor of cochlear implant (CI) technology is the electrode-contact overlapping spread of the electrode-generated intracochlear electrical field (EF). While the extent of the spread can be reduced with intracochlear ground electrodes, the stimulation level must be increased to reach similar loudness as with monopolar stimulation utilizing an extracochlear ground. In this study, we investigated the relationship between the monopolar intracochlear EF and the minimum stimulation level required for a measurable neural response assessed with electrically evoked compound action potential (eCAP) thresholds in intraoperative settings. Also, the effect of cochlear diameter on the intracochlear EF was evaluated, as narrower intracochlear EFs were found from larger than smaller cochleae in an earlier study. A total of 171 ears of severely-to-profoundly hearing-impaired patients (ages 0.7–89 years; 42.5 ± 27.8 years, mean ± SD) implanted with a Cochlear Nucleus CI522 or CI622 implant and Slim Straight electrode array or with a Med-El Synchrony implant and Flex 28 electrode array were included in the study. Normal anatomy was established and cochlear diameter was measured for all patients from preoperative imaging. Intraoperative intracochlear EF and eCAP threshold measurements were measured for both Cochlear and Med-El devices with the CIs’ back-telemetry options, and EF and eCAP were compared for Cochlear devices. The peak and width of the intracochlear EF correlated with each other (r = 0.46, p < 0.001), and both had an inverse relationship with eCAP thresholds (r = –0.41, p < 0.001 and r = –0.29, p < 0.001, respectively). The peak amplitudes of the intracochlear EF increased towards the apical part of the electrode array with both Cochlear (r = 0.97, p < 0.001) and Med-El (r = 0.80, p = 0.002) devices. The peak amplitudes of the intracochlear EF were shallower across the electrode array in large than in small cochleae (p < 0.05). Our results indicate that the responsiveness of the cochlear nerve is not only dependent on neural health but is also affected by the physical environment of the electrode array, which can be assessed by measuring the intracochlear EF. Further studies are warranted to investigate the detailed characteristics of the intracochlear current spread in CI recipients with varying anatomical features of the cochlea and with electrode arrays with different locations in the scalae or related to the modiolus in the cochleae.