ABSTRACT
Hemifacial spasm is a relatively uncommon but distressing condition characterized by insiduous development of paroxysmal, involuntary, unilateral hyperkinetic facial movement. In patients with hemifacial spasm, there is an abnormality in the facial nerve or its nucleus which allows the stimulus applied to one branch of the facial nerve to spread to other branches of that same nerve. This lateral spread response is presumed to be due to cross transmission of the antidromic activity in the branch of the facial nerve, and intraoperative monitoring of lateral spread responses provides a useful way of confirming complete facial nerve decompression. In 17 consecutive patients, intraoperative electromyographic(EMG) recordings were made from facial muscles during microvascular decompression for hemifacial spasm. At the beginning of the operation, electrical stimulation of the temporal or zygomatic branch of the facial nerve gave rise to electrically recordable activity in the mentalis muscle(lateral spread), with a latency of about 10msec, that in ten patients, lasted until the facial nerve was decompressed; In seven patients, however, it disappeared when the arachnoid membrane was opened. Early disappearance of lateral spread was frequently occurred in single offending vessel or cases where there was loose compression. Postsurgically the 16 patients in whom the lateral spread response disappeared totally were free from spasm after the operation and in the remaining patient, there was much improvement. These results support the use of lateral spread response monitoring during decompression surgery for hemifacial spasm, and provide strong circumstantial evidence that vascular cross-compression is an important etiologic factor in hemifacial spasm. During a decompression operation for hemifacial spasm, the authors now routinely monitor facial EMG response.