Hemifacial spasm: location of the lesion by electrophysiological means.

In hemifacial spasm a late abnormal motor response was elicited by the antidromic volley after stimulating a peripheral branch of the facial nerve. The site of the lesion was calculated from the latency of that motor response and the conduction velocity. In 5 patients the distance was long, and corresponded with a tumor or a vascular anomaly at the cerebellopontine region in 3 of them. An artery sling entering the internal acoustic meatus was found in 2 out of 3 patients with short distances. It is concluded that hemifacial spasm results from a nerve lesion that may be located as far peripherally as the internal acoustic meatus. The theory of facial nuclear irritation is incompatible with such a peripheral lesion.