Hemifacial spasm is characterized by spontaneous clonic and tonic muscle spasm on one side of th face with synkinesis.

Electrophysiologic recordings were made from 24 patients with hemifacial spasm and 20 healthy control group. Routine electrodiagnostic study, simultaneous monitoring of motor unit activities in 4 facial muscles, and recordings of delayed response in orbicularis oculi and mentalis were performed.

The major results were as follows:

1) The mean age of patients were 47 years.

2) Among the 24 patients were 47 years.

3) The mean duration of symptoms from onset to study was 9.4 years.

4) The involuntary, irregular, intermittent motor unit activities were recorded synchronously in four facial muscles in all patients on four channel recording.

5) In the control group and on the unaffected side of patients with hemifacial spasm, stimulation of the mandibular or zygomatic branches never evoked an action potential in the distant muscle, innervated by another branch of the nerve.

6) On the affected side of patients with hemifacial spasm, there was a delayed response in the orbicularis oculi and in the mental muscle in all 23 patients after stimulation of the mandibular and zygomatic branches, respectively.

7) The mean latency of the delayed response was 9⁑1.09 msec in orbicularis oculi, and 9.76⁑1.1 msec in mentalis.