Enhancing surgical precision: a novel electromyography finding for confident identification of the root exit zone during microvascular decompression surgery

OBJECTIVE The lateral spread response (LSR) is an important electrophysiological sign that predicts successful decompression in patients undergoing microvascular decompression (MVD) for hemifacial spasm (HFS). However, LSRs do not consistently correlate with clinical outcomes, and there are cases in which LSRs are absent. In this study, the authors identified a unique pattern on facial nerve electromyography (EMG) when the root exit zone (REZ) is touched. This distinctive EMG pattern, which the authors coined the "Sang-ku sign" (SKS), could aid in identifying the offending vessel on the REZ, even in the absence of LSRs. METHODS Between November 2022 and August 2023, the authors analyzed EMG findings from 185 patients undergoing MVD for HFS. Bipolar stimulation electrodes were placed at the marginal mandibular branch, and recordings were obtained from the frontalis and orbicularis oris muscles. Additionally, stimulation electrodes were placed at the temporal branch, and recordings were obtained from the oris and mentalis muscles. The authors statistically analyzed the presence of SKS and its association with demographic, surgical, clinical outcomes, and anatomical factors. RESULTS The authors observed a brief, burst-like wave pattern arising from all recorded muscle branches when Teflon felt was placed between the REZ and the facial nerve. This EMG pattern, the SKS, was found in 164 patients (88.6%). Among the factors analyzed, only patient age showed a significant association with the presence of the SKS in univariate analysis (p = 0.007). The presence of the SKS was significantly associated with predicting the disappearance of LSRs (p = 0.045). Clinical outcomes were favorable (99% at the last follow-up) in all cases; thus, no positive correlation was observed in the existence of the SKS and LSRs. CONCLUSIONS The SKS could serve as an excellent guide for the facial nerve REZ during surgery. Given that HFS results from abnormal vascular contact on the REZ, this finding plays a crucial role in ensuring surgical success. Alongside LSRs, the SKS could provide valuable insights for neurosurgeons.