A Phase-Based Framework for Interpreting Intraoperative Lateral Spread Response During Microvascular Decompression for Hemifacial Spasm

BACKGROUND AND OBJECTIVES: Intraoperative lateral spread response (LSR) monitoring during microvascular decompression (MVD) for hemifacial spasm (HFS) is often summarized as disappearance or persistence at the end of surgery. However, serial LSR behavior may follow multiple intraoperative courses that are not fully captured by a binary final-status label. This study organized serial intraoperative LSR behavior into phase-based patterns and descriptively summarized postoperative outcomes within this framework. METHODS: We retrospectively analyzed a prospectively maintained single-center cohort of 398 consecutive patients who underwent primary MVD for HFS. LSR monitoring followed a standardized institutional dual-pair protocol. Serial LSR behavior was classified at the patient level into 6 predefined patterns: nonelicitable LSR, preseparation disappearance, immediate postseparation disappearance, delayed postseparation disappearance, reappearance, and persistent end-of-procedure LSR. Outcomes were assessed within 1 week, at 1 month, and at 1 year. RESULTS: All cases were classified into 1 of the 6 phase-based patterns. Immediate postseparation disappearance was the most common pattern, occurring in 203 patients (51.0%); however, 195 patients (49.0%) followed other intraoperative courses, including nonelicitable LSR in 28 (7.0%), preseparation disappearance in 81 (20.4%), delayed postseparation disappearance in 43 (10.8%), reappearance in 33 (8.3%), and persistent end-of-procedure LSR in 10 (2.5%). At 1 year, overall favorable outcome was observed in 376 of 398 patients (94.5%). Among LSR-elicitable patients, 1-year nonfavorable outcome occurred in 16 of 327 patients (4.9%) with LSR-end-absent status and in 6 of 43 patients (14.0%) with LSR-end-present status. CONCLUSION: Serial intraoperative LSR behavior during MVD for HFS can be organized into phase-based patterns that describe how final LSR status is reached. This framework may help standardize intraoperative interpretation, communication, and reporting, while supporting the use of LSR as contextual electrophysiological information rather than as a standalone operative mandate.