Pathophysiology of hemifacial spasm: I. Ephaptic transmission and ectopic excitation.

We studied 62 patients with hemifacial spasm to test the presence of ephaptic transmission and ectopic excitation. The zygomatic and mandibular branches of the facial nerve were stimulated separately, recording simultaneously from the orbicularis oculi and mental muscles. Antidromic impulses were transmitted bidirectionally between the two branches. Transmission took place in a fraction of slow conducting motor nerve fibers. After-activity and late-activity were recorded as single potentials or trains, suggesting autoexcitation of fibers. The interspike frequency was 250 to 350 Hz. Hyperventilation produced synchronous clonic-tonic activity, suggesting ectopic excitation caused by hypocalcemia. Ectopic excitation and ephaptic transmission are important pathophysiologic factors in hemifacial spasm.