Is complete elimination of lateral spreading response essential in microvascular decompression for hemifacial spasm?

Objective: The objective was to investigate whether complete intraoperative elimination of the lateral spread response (LSR) is essential during microvascular decompression (MVD) for hemifacial spasm (HFS) and to identify a quantitative intraoperative biomarker predictive of favorable outcomes.

Methods: The authors retrospectively analyzed 208 adult patients who underwent MVD for primary HFS. Intraoperative neurophysiological monitoring (IONM) included LSR recordings from three facial muscles. Based on clinical outcomes at the 6-month postoperative follow-up, patients were independently categorized into two binary outcome groups-completely resolved (CR) versus non-CR, and clinically improved (CI) versus non-CI-for separate analyses. The final-to-baseline amplitude change ratio (FBCR) of LSR amplitude was calculated. Predictive thresholds were identified using machine learning models including random forest and decision trees.

Results: LSR was most frequently observed in the mentalis (96.2%) and orbicularis oris (92.3%). Complete disappearance of LSR was not a prerequisite for achieving either CI or CR outcomes. FBCR ≥ 86.5% in the mentalis muscle predicted CR with 88% accuracy, 99% sensitivity, and 47% specificity. FBCR ≥ 48.5% predicted CI with 98% accuracy and 91% specificity. Multivariate models did not significantly improve prediction compared to mentalis FBCR alone.

Conclusions: Complete elimination of LSR is not essential for clinical success in MVD for HFS. A quantitative reduction in LSR amplitude, especially in the mentalis muscle, provides a robust and practical intraoperative predictor of both objective and subjective outcomes. These findings advocate for a shift toward a muscle-specific, threshold-driven strategy for intraoperative neurophysiological monitoring in HFS surgery.