Cochlear Microphonics: A Promising Tool for Hearing Monitoring in Cerebellopontine Angle Surgery

Background: Postoperative hearing impairment in acoustic neuroma surgery (ANS) is frequently due to intraoperative alteration of cochlear blood supply. Cochlear microphonics (CM) is a cochlear electrical response to sound that can be measured noninvasively and should be sensitive to changes in cochlear blood flow. Our purpose was to evaluate CM reliability to detect intraoperative cochlear blood flow modifications. Material: A total of 25 patients underwent a cerebellopontine angle (CPA) surgery by retrolabyrinthine approach in four French hospital centers (Reims, Lyon, Clermont-Ferrand and Paris) from May 2012 to May 2013. Thirteen patients had an ANS; 11 patients in whom the acoustic nerve was not manipulated (vestibular neurotomy, microvascular decompression of a cranial nerve) served as controls. All patients had a continuous intraoperative hearing monitoring by CM. Results: The control group kept a stable intraoperative CM signal and unchanged postoperative hearing thresholds. In the ANS group, progressive or abrupt CM disappearance was associated with postoperative cophosis. CM significant intraoperative disturbances were always followed by postoperative hearing degradation. CM stability throughout surgery came with preserved postoperative hearing thresholds, except when the cochlear nerve had been injured, which is undetectable by CM and resulted in postoperative cophosis. Conclusion: CM is a reliable intraoperative hearing monitoring tool in CPA surgery.