For investigating the diagnostic value of motor evoked potentials(MEP) in hemifacial spasm(HFS), we studied the facial nerves of 215 patients with HFS using transcranial magnetic stimulation from March 1992 to Dec. 1994. The mean age of patients was 47.9 years old and the mean duration of symptom was 7.0 years. For MEP study, the center of magnetic coil was placed on 3cm posterior and 6cm lateral to vertex and stimulated with 50~60% of maximal intensity. Simultaneously, compound muscle action potentials were measured with electrical stimulation at stylomastoid foramen. Recording electrodes were placed on nasalis muscles.

In the affected side, the mean latency of MEP was 4.50msec, and the mean central conduction time was 1.57msec with no statistical difference with those in normal side. However, the mean amplitude of MEP in the affected side was 3.2mV, and statistically lower than that in normal side(p<0.01). In regression analysis, the decrement of amplitudes was statistically correlated with the increment of symptom duration(p<0.01).

According to the above results, transcranial magnetic stimulation of the facial nerve seems to excite the labyrinthine segment and MEP study is a useful method to evaluate the facial weakness of central origin in HFS.