Hemifacial spasm (HFS) patients occasionally present with preoperative facial weakness (PFW) or develop delayed facial palsy (DFP) after microvascular decompression (MVD). This study is aimed to evaluate the neurophysiology underlying facial nerve motor dysfunction in HFS patients preoperatively and postoperatively. In all, 54 HFS patients without prior botulinum toxin injection who underwent MVD were retrospectively reviewed. The compound muscle action potential (CMAP) amplitude ratios of the affected and unaffected facial nerves, measured at 4 time points from preoperation to 1 year post-surgery, were aggregated. Clinical outcomes and the CMAP amplitude ratios were evaluated. Six patients (11.1%) presented with PFW, which correlated with advanced age (p = 0.007) and symptom duration (p = 0.001). The average duration to achieve PFW relief was 2.67 months postoperatively. The preoperative CMAP amplitude ratios of PFW patients were lower than those of patients without PFW (85.3% vs 95.7%). The ratios showed the lowest value at 1-week post-surgery in both groups (70.3% vs 90.9%), had a tendency toward improvement at 1 month, and finally recovered to almost the same level as that before the surgery at 1 year. Three patients (5.6%), whose CMAP ratios showed a persistent decrease from 1 week (56.5%) to 1 month (31%) after MVD, developed DFP. This study illustrates PFW in HFS patients reflects facial nerve axonal stress. MVD is effective in resolving spasm and PFW, without long-term damage to the facial nerve in most patients. In DFP patients, the direct and subsequent secondary axonal disorder develops on the postoperative facial nerve.
Keywords: compound muscle action potential; delayed facial palsy; facial weakness; hemifacial spasm; microvascular decompression.