[Intraoperative Monitoring during Microvascular Decompression for Hemifacial Spasm]

Brainstem auditory evoked potential(BAEP)and abnormal muscle response(AMR)monitoring are widely used during microvascular decompression(MVD)for hemifacial spasm(HFS). In BAEP monitoring, the intraoperative findings of wave V do not necessarily predict postoperative hearing function. However, if a warning sign as significant as wave V change appears, the surgeon must abort the operation or inject artificial cerebrospinal fluid into the VIII nerve. During MVD for HFS, BAEP monitoring must be performed to preserve hearing function. AMR monitoring is useful in identifying the offending vessels compressing the facial nerve and confirming the completion of decompression intraoperatively. During the operation of the offending vessels, AMR sometimes changes its onset latency and amplitude in real time. These findings allow surgeons to identify the offending vessels. Even if the AMRs remain after completion of decompression, a decrease in amplitude of more than 50% compared to baseline is predictive of postoperative loss of HFS in long-term outcomes. When the AMRs disappear after dural opening, the AMR monitoring should be continued because the AMRs sometimes reappear.