Effects of local injections of botulinum toxin on electrophysiological parameters in patients with hemifacial spasm: role of synaptic activity and size of motor units.

Ten patients with typical hemifacial spasm were examined before and after treatment with local injections of botulinum toxin type A. After a mean follow-up period of 38 days there was a reduction of the compound muscle action potential (CMAP) of the injected orbicularis oculi muscle of 40%. Ephaptic transmission studied by selective stimulation of facial nerve branches revealed a preserved delayed response of the affected mentalis muscle. However, no delayed response could be recorded in the injected orbicularis oculi muscle in nine patients. The discrepancy between complete loss of the delayed (ephaptic) response and only moderate reduction of the CMAP amplitude of the direct response may be explained by preferential uptake of botulinum toxin type A by hyperactive synapses involved in ephaptic transmission.