The lateral spread (LS) response, which can be elicited in muscles innervated by other branches of the facial nerve, is electromyographically specific for patients with hemifacial spasm (HFS), occurring about 10 ms after stimulus. The F-wave in facial muscles, which is a late response that antidromicaly propagates to the facial motonucleus and returns orthodromicaly down the same axon, revealed a trend toward enhancement in patients with HFS. The LSs were facilitated by repetitive stimulation during the microvascular decompression (MVD) operation, which has proved to be a successful treatment, and the F-waves were also facilitated by repetitive stimulation on the spasm side more than on the normal side. Greater facilitation of these responses was in direct proportion to higher stimulation rates and greater numbers of stimulations. The repetitive stimulation of the facial nerve may result in activation of the motoneuron pool and in the lowering of the threshold of somatic membranes. These results support the hypothesis that hemifacial spasm is caused by hyperexcitability of the facial motonucleus, which is increased by antidromic repetitive stimulation.
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