Objective: To determine whether monitoring facial nerve motor evoked potentials (FNMEPs) elicited by transcranial electrical stimulation during microvascular decompression (MVD) for hemifacial spasm (HFS) is useful for predicting postoperative outcome.
Methods: We analysed FNMEP findings in 25 patients with HFS. Corkscrew electrodes positioned at C3 or C4 and Cz were used to deliver supra-maximal stimuli (152- 450 V). FNMEPs were recorded from the orbicularis oculi, mentalis and oris muscles. Differences in amplitude or response duration between the final (at dural closure) and baseline FNMEP were evaluated. Final-to-baseline FNMEPs ratios (shown as percentages) obtained from the 3 muscles were examined.
Results: In the orbicularis oculi muscles, both duration and amplitude in the final FNMEP were significantly reduced compared to the baseline FNMEP (p < 0.001 and p < 0.0005, respectively). The FNMEP ratio obtained from the orbicularis oculi muscle (44.8 ± 25.4%) was significantly lower than the other 2 muscles (95.8 ± 51.2% for the mentalis muscle and 84.5 ± 34.6% for the orbicularis oris muscle; p < 0.0001). In 1 patient who showed the same degree of symptoms immediately after surgery as in the preoperative state, neither LSRs nor FNMEP from the orbicularis oculi muscle changed.
Conclusions: The FNMEP amplitude from the orbicularis oculi muscle decreased after MVD in HFS patients whose symptoms were resolved postoperatively, thus suggesting normalization of facial nerve excitability. FNMEP monitoring during MVD surgery as well as LSR monitoring could predict postoperative outcome in HFS patients.