Early lateral spread response loss during microvascular decompression for hemifacial spasm: its preoperative predictive factors and impact on surgical outcomes.

When early lateral spread response (LSR) loss before decompression in HFS surgery happens, the value of intraoperative monitoring of LSR for locating neurovascular conflicts and confirming adequate decompression was considered to be reduced. This study aimed to identify preoperative parameters predicting early LSR loss and figure out the impact of early LSR loss on prognosis. Hemifacial spasm (HFS) patients who received microvascular decompression (MVD) under intraoperative electrophysiological monitoring during the period of March 2013-January 2021 were reviewed retrospectively. The patients were divided into two groups according to the disappearance of their LSR before or after decompression. Preoperative clinical and radiological predictors for early LSR loss were evaluated using logistic regression. The relationship between early LSR loss and surgical outcomes was statistically analyzed. A total of 523 patients were included in the study, and the disappearance of their LSR before decompression occurred in 129 patients. In the multivariate analysis, three independent factors predicting early LSR loss were identified: (1) smaller vessel compression; (2) milder nerve deviation; (3) lower posterior fossa crowdedness index (PFCI, calculated as hindbrain volume (HBV)/the posterior fossa volume (PFV) using 3D Slicer software). The median follow-up time was about five years, and no significant differences in the spasm relief and complication rates were found between the 2 groups. Smaller responsible vessels, milder nerve deviation, and more spacious posterior cranial fossa are associated with early LSR loss. However, early LSR loss seems to have no significant adverse effect on MVD outcomes in skilled hands.