Cochlear nerve action potential monitoring with the microdissector in vestibular schwannoma surgery

We developed a cochlear nerve action potential (CNAP) monitoring technique using a microdissector and compared the results of CNAP and auditory brainstem response (ABR) monitoring. Thirty-six patients underwent vestibular schwannoma resection via the retrosigmoid approach to preserve hearing. Both CNAP with the microdissector and surface ABR were recorded during the operation. We used the microdissector as an intracranial electrode for CNAP monitoring. The CNAP waveform was classified into four types: triphasic, biphasic, positive, and flat. At the completion of the tumor resection, the triphasic waveform was observed in 11 patients and the biphasic waveform was observed in 11 patients. Hearing function was preserved in all of them, although it was preserved in only two patients with other CNAP waveform types. The prognostic value of CNAP is significantly higher than that of ABR. We found that although CNAP with a microdissector does not provide real-time monitoring, with the classification of waveforms it can be used as predictable tool for postoperative hearing more accurately than ABR.