Intraoperative Monitoring Using Cochlear Microphonics in Cochlear Implant Patients With Residual Hearing

Objectives: To describe a technique for the intraoperative monitoring of residual hearing during a cochlear implant (CI) procedure and, thus, to identify the time of occurrence and surgical steps leading to hearing loss. Study Design: Prospective audiologic analysis in a patient series. Setting: Tertiary referral center. Subjects: Patients with residual hearing subjected to cochlear implantation. Interventions: During cochlear implantation, cochlear microphonics (CMs) in response to frequency-specific stimuli were recorded in patients with residual hearing. Thresholds were determined before and after opening of the cochlea, with a limited portion of the electrode array inserted, and after full insertion. Results: Monitoring of the hearing state using CMs was quick, reliable, and capable of detecting an intracochlear trauma. In a first series of patients, thresholds were preserved in all patients after opening of the cochlea. Thresholds were preserved in 5 of 6 patients after limited insertion and half of the patients after full insertion of the electrode array. Despite threshold preservation until the end of surgery, the residual hearing was lost in patients with deep insertions 1 week postoperative. Conclusion: Intraoperative monitoring of CM thresholds may be valuable for identifying the exact point of time at which residual hearing is affected in CI patients. Opening of the cochlea itself seems to be unrelated to hearing loss. A significant proportion of patients may have hearing loss caused by secondary effects rather than a direct trauma.