Monitoring of Cochlear Function During Cochlear Implantation

OBJECTIVE: To report the feasibility of monitoring cochlear function during cochlear implantation. STUDY DESIGN: Case report. SETTING: Tertiary care referral center. METHODS: A child with audiologic features typical of bilateral auditory neuropathy underwent cochlear implantation. The scala tympani was entered inferior and slightly anterior to the round window membrane margin and smooth electrode insertion was achieved. Using single polarity click stimuli, the cochlear microphonic was measured at several steps during surgery. RESULTS: Cochlear microphonics were present at all stages during the implantation process and were clearly distinguished from neural responses by stimulus polarity inversion and constant latencies, despite changes in stimulus level. With the electrode in situ, amplitudes were smaller but persisted until the final measurement at 10 minutes after insertion. At follow-up 2 weeks after surgery, behavioral audiometry results indicated profound hearing loss in the operated ear. CONCLUSIONS: This paper demonstrates the feasibility of monitoring cochlear function during cochlear implantation. The routine surgical approach did not appear to adversely affect the functional measurements. Standard size, full electrode insertion did diminish the amplitude of the cochlear microphonics, possibly as a result of intracochlear mechanical impairment. Ultimately, profound hearing loss was documented, indicating that factors other than immediate changes induced by electrode insertion were likely responsible for the loss of cochlear function.