Brainstem Auditory Evoked Potential Monitoring during Microvascular Decompression for Hemifacial Spasm: Intraoperative Brainstem Auditory Evoked Potential Changes and Warning Values to Prevent Hearing Loss—Prospective Study in a Consecutive Series of 84 Patients

OBJECTIVE The nerve function of Cranial Nerve VIII is at risk during microvascular decompression for hemifacial spasm. Intraoperative monitoring of brainstem auditory evoked potentials (BAEPs) can be a useful tool to decrease the danger of hearing loss. The aim of this study was 1) to assess the side effects of surgery on hearing and describe the main intraoperative BAEP changes observed in the authors' series, and 2) to define warning values beyond which the probability of hearing impairment rises significantly. These values were calculated by correlating the (possible) postoperative hearing disturbances evaluated in terms of pure tone average with intraoperative BAEP changes (especially delay in Wave V latency). METHODS This series included 84 consecutive patients affected with hemifacial spasm who underwent microvascular decompression during which BAEPs were monitored. During surgery, Wave I, I to V interpeak interval, latency, and amplitude of Wave V were recorded and measured. Auditory function was studied before and after surgery and expressed as a pure tone average in all patients. Then, correlations were made between hearing impairment after surgery and intraoperative BAEP changes in an attempt to define warning values. RESULTS Seventy-four patients (88%) had no hearing loss after surgery (Group 1). Eight patients (9.5%) had hearing impairment with a decrease in pure tone average of more than 20 dB (Group 2). Two patients (2.3%) experienced a definitive and complete hearing loss on the side operated on (Group 3). Among intraoperative BAEP changes, latency of Peak V was the most frequently observed and the most significant phenomenon, especially during cerebellar retraction and the decompression step of the microvascular decompression procedure. In the group of patients without hearing loss (Group 1), the mean delay in latency of Peak V was 0.61 millisecond (standard deviation, +/-0.36 ms); in the group with hearing decrease (Group 2), the mean delay was 1.05 milliseconds (standard deviation, +/-0.64 ms); and in the group with deafness (Group 3), Wave V was abolished. CONCLUSION From a practical standpoint, three warning values, based on delay in latency of Peak V, were established for use during surgery: an initial one at 0.4 millisecond ("watching" signal) at the safety limit; a second one at 0.6 millisecond (risk "warning" signal), which is the mean value corresponding to the group of patients without postoperative hearing loss; and an ultimate one at 1 millisecond ("critical" warning), before irreversibility. These warnings should help the surgeon to avoid or correct maneuvers that are dangerous for hearing function, which is mandatory in functional surgery.