Assessment of Facial Weakness After Microvascular Decompression: Morphology of Lateral Spread Responses and Sequential Changes in Compound Muscle Action Potentials

BACKGROUND AND OBJECTIVES: Assessing facial nerve injury in hemifacial spasm (HFS) remains clinically challenging; although delayed facial palsy (DFP) frequently resolves without treatment, anticipating its occurrence is important for patient counseling. However, reliable predictors remain incompletely established, and early recognition of corneal complications secondary to facial weakness with timely referral for appropriate ophthalmological care is essential. The aim of this study was to investigate the utility of lateral spread response (LSR) waveform morphology and compound muscle action potentials (CMAPs) as indicators of postoperative facial weakness. METHODS: We retrospectively analyzed 140 patients with HFS who underwent primary microvascular decompression (MVD) between 2015 and 2023. LSR waveforms were classified as monophasic or polyphasic. CMAP amplitudes were measured pre- and postoperatively, and the CMAP amplitude ratio of the affected to the unaffected side was calculated to evaluate sequential changes in facial nerve function. RESULTS: In the polyphasic LSR group, CMAP ratios showed consistent improvement from 1 week to 1 year postoperatively. By contrast, the monophasic group exhibited a transient reduction in CMAP ratio 1 month after surgery, followed by gradual recovery (P < .05 at 1 week and 1 month). CMAP recovery patterns differed depending on the presence of preoperative facial paresis. Notably, in the monophasic group, CMAP ratios did not significantly differ between patients with and without preoperative paresis, suggesting that monophasic waveforms may reflect more severe axonal injury. DFP occurred in 10 patients (7.7%). Patients who developed DFP demonstrated significantly lower CMAP ratios 1 week postoperatively, indicating that early CMAP decline may serve as a predictive marker for DFP. CONCLUSION: LSR waveform morphology and postoperative CMAP ratio dynamics are promising electrophysiological indicators for evaluating facial nerve injury and predicting DFP in patients with HFS undergoing MVD.