Hemifacial spasm is characterized by involuntary, tonic, and clonic contraction of muscles on one side of the face with synkinesis, which typically begins in the inferior orbicularis oculi muscle.

Routine electrodiagnostic study, simultaneous monitoring of motor unit activities in 2 facial muscles, and recordings of the late response in orbicularis oculi and mentalis were performed before operation for patients with hemifacial spasm. In 85 patients, consecutive intraoperative recordings of late response were made from the mentalis muscles during microvascular decompression for hemifacial spasm. The major results of this study were as follows:

1) The ages of the 85 patients with hemifacial spasm (15 men, and 70 women) ranged from 28 to 66 years, with a mean of 47 years.

2) The duration of symptoms before surgery was 6 months to 25 years, with mean 8.2 years.

3) In all of the patients studied, antidromic stimulation of the zygomatic branch produced a late response of mean 8.96±0.72 msec latency in the mentalis, and intraoperative EMG monitoring observing this late response was performed.

5) The amplitude of late response diminished significantly after the dura incised in 11 cases.

6) When the offending vessel was held away from root exit zone of the facial nerve, the late response disappeared totally in 32 patients, and much reduced in 41 patients.

7) At the end of operation, the late response disappeared totally in 34 patients, and much reduced in 44 patients.

8) In the clinical results of the 85 patients treated with microvascular decompression for HFS at 7 days after the operation, 47 were free from spasm, 32 had occasional minimal spasm, and 6 were slightly improved.