Facial Motor Neuron Excitability in Hemifacial Spasm: A Facial MEP Study

Abstract

Hemifacial spasm (HFS) may be due to peripheral axon ephapsis or central motor neuron hyperexcitability. Low facial motor evoked potential (MEP) thresholds or MEP responses to single pulse stimulation (normally multipulse stimulation is needed) may support the central hypothesis. We retrospectively compared response thresholds for facial MEPs in 65 patients undergoing surgical microvascular decompression (MVD) for HFS and 29 patients undergoing surgery for skull base tumors. Single pulse stimulation elicited facial Mep in up to 87% of HFS patients whereas only 10% of tumor patients responded to single pulse stimulation. When comparing facial MEP thresholds using multi-pulse stimulus trains the voltage required in the HFS group were significantly lower then in skull base tumor patients (p < 0.001). the MEP latencies and amplitudes at threshold stimulation were similar between the two groups. these results suggest the facial corticobulbar pathway demonstrates enhanced excitability in HFS.