Ephaptic transmission is the origin of the abnormal muscle response seen in hemifacial spasm

•Abnormal muscle responses result from antidromic motor impulses rather than trigeminal sensory input.•Slow conducting motor fibers traveling at 33 m/s give rise to lateral spread, inducing orthodromic impulses.•Ephaptic transmission takes place at the site of vascular compression rather than the facial nerve nucleus.

ObjectiveIn patients with hemifacial spasm, stimulation of a branch of the affected facial nerve elicits an abnormal response in the muscles innervated by another branch. We tested the hypothesis that this anomaly results from lateral spread of impulses from one motor axon to another at the site of the nerve compression by the offending artery.MethodsIn a preoperative study of 21 patients, we delivered a series of stimuli, in short increments, successively distally along the temporal branch of the facial nerve to record abnormal muscle responses from the orbicularis oculi and mentalis muscles. In intraoperative monitoring of 10 patients during microvascular decompression, we monitored propagating nerve action potentials with a handheld electrode placed on the facial nerve 3 mm distal to the vascular compression site.ResultsWith incremental shifts of stimulating points distally, the latency of abnormal muscle responses increased by 0.3 ± 0.1 ms/cm. This finding implicates the antidromic motor impulse as the trigger for lateral spread. The nerve action potentials recorded during surgery comprised the initial antidromic signal followed by one or more additional peaks. The latter immediately abated, together with abnormal muscle responses, after microvascular decompression. Thus, the secondary peaks must represent the orthodromic impulses generated by ephaptic transmission. An average inter-peak interval of 1.1 ms between the first and secondary peaks is consistent with the estimated conduction time from the stimulation point to the site of vascular compression but not to the facial nucleus and return.ConclusionAn abnormal muscle response results from lateral spread of impulses between motor axons at the site of vascular compression rather than at the facial nucleus.SignificanceThis study establishes the mechanism of lateral spread responsible for abnormal muscle responses and contributes to the understanding of pathophysiology underlying hemifacial spasm.