Intraoperative free-running electromyography as a dynamic adjunct during microvascular decompression for hemifacial spasm

To characterize intraoperative free-running electromyography (EMG) patterns during microvascular decompression (MVD) for hemifacial spasm (HFS) and to evaluate their potential clinical significance, including their distribution across key surgical steps, their utility in distinguishing operation-related nonspecific interference from pathologic facial nerve hyperactivity, and their possible value in reflecting facial nerve irritation and decompression adequacy. This single-center retrospective observational study included consecutive patients with primary HFS who underwent MVD with intraoperative neurophysiological monitoring. Free-running EMG activities were reviewed and classified according to waveform morphology. Their occurrence and dynamic changes were correlated with predefined surgical maneuvers, including arachnoid dissection, facial nerve exploration, offending-vessel manipulation, Teflon insertion/adjustment, and adequate decompression. Operation-related nonspecific EMG changes induced by bipolar coagulation, triggered lateral spread response (LSR) stimulation, and irrigation were analyzed separately. Associations between free-running EMG changes, LSR status, and postoperative outcomes were also evaluated. A total of 108 patients were included in the final analysis, and abnormal free-running EMG activity was detected in 102 patients (94.4%). The recorded abnormalities were classified into reproducible spike, burst, and sinusoidal patterns. Spike waves were the most common waveform type, followed by burst waves and sinusoidal waves of types A–C. Abnormal free-running EMG activity occurred most frequently during facial nerve exploration, conflict localization, and offending-vessel manipulation and became less frequent after adequate decompression. During decompression, increased activity predominated during Teflon placement, whereas attenuation or disappearance became more common during Teflon adjustment and after adequate decompression. Bipolar coagulation and forceful irrigation produced distinct nonspecific interference patterns that were distinguishable from structured spontaneous EMG abnormalities. Improvement of free-running EMG was more frequent in patients with LSR disappearance and in those with favorable postoperative outcomes. Intraoperative free-running EMG demonstrates reproducible waveform patterns and dynamic changes associated with specific surgical maneuvers during MVD for HFS. When interpreted in the context of operative events, free-running EMG may help distinguish nonspecific interference from clinically meaningful abnormal facial nerve activity and may serve as a useful adjunct to conventional LSR monitoring for real-time assessment of facial nerve irritation and decompression adequacy.