Bilocated Intracochlear Electrocochleography During Cochlear Implantation to Provide Surgical Feedback

Introduction: Recent advancements toward minimizing surgical trauma and preserving residual hearing during cochlear implantation have promoted the use of intracochlear electrocochleography (ECochG) for intraoperative surveillance. However, variations in the distribution of response generators throughout the cochlea complicate the interpretation of signal changes during electrode array insertion. This exploratory study aimed to investigate whether simultaneous recordings from 2 locations within the cochlea could address this issue. This was done by (1) comparing recordings acquired simultaneously from an apical and a more basal electrode contact during atraumatic electrode array insertions; and (2) comparing these response patterns with preoperative pure-tone audiograms. Materials and methods: In 10 standard CI recipients, ECochG recordings were obtained during stepwise insertion of a short-temporary electrode array. Simultaneous intracochlear recordings were acquired from 2 contacts separated by 4.2 mm in response to 500 Hz tone bursts. For both electrode contacts, the differences between responses to alternating-polarity stimuli, hereafter named “ECochG responses,” were derived. After completion of the recordings, the temporary electrode array was removed and a standard CI electrode array inserted. Results: In 6 of 10 participants, ECochG response amplitude decreases of ≥3 dB were recorded at the more basal electrode contact. In 4 cases, these amplitude drops were preceded by amplitude drops recorded at the apical electrode contact, with both occurring no more than 1.7 mm apart along the cochlear duct. Trauma would be expected to produce a simultaneous amplitude drop—at different recording locations—for both electrodes. In contrast, a drop at the basal electrode that is preceded by a drop in the ECochG recording at the apical electrode, with both drops occurring at approximately the same location, is likely atraumatic. These atraumatic drops may also be associated with large phase shifts. Overall, the ECochG response tracks recorded at the apical and more basal contact were similar, while the ECochG response patterns and audiogram did not show a strong resemblance. Conclusion: The proposed approach could facilitate the detection of ECochG response changes relevant for predicting hearing preservation during cochlear implantation. By distinguishing between simultaneous and sequential amplitude drops, this method could provide additional insights into the atraumatic nature of certain ECochG response changes.