title: Ephaptic transmission is the origin of the abnormal muscle response seen in hemifacial spasm.
abstract: OBJECTIVE
  In 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.
  
  
  METHODS
  In 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 3mm distal to the vascular compression site.
  
  
  RESULTS
  With incremental shifts of stimulating points distally, the latency of abnormal muscle responses increased by 0.3±0.1ms/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 1.1ms 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.
  
  
  CONCLUSIONS
  An abnormal muscle response results from lateral spread of impulses between motor axons at the site of vascular compression rather than at the facial nucleus.
  
  
  CONCLUSIONS
  This study establishes the mechanism of lateral spread responsible for abnormal muscle responses and contributes to the understanding of pathophysiology underlying hemifacial spasm.
authors.affiliation: Department of Functional Neurosurgery, Nishi-Niigata Chuo National Hospital, Niigata, Japan. Electronic address: kameyama@masa.go.jp.
authors.forename: Shigeki
authors.initials: S
authors.lastname: Kameyama
authors.affiliation: Department of Functional Neurosurgery, Nishi-Niigata Chuo National Hospital, Niigata, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Masuda
authors.affiliation: Department of Functional Neurosurgery, Nishi-Niigata Chuo National Hospital, Niigata, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Shirozu
authors.affiliation: Department of Functional Neurosurgery, Nishi-Niigata Chuo National Hospital, Niigata, Japan.
authors.forename: Yosuke
authors.initials: Y
authors.lastname: Ito
authors.affiliation: Department of Functional Neurosurgery, Nishi-Niigata Chuo National Hospital, Niigata, Japan.
authors.forename: Masaki
authors.initials: M
authors.lastname: Sonoda
authors.affiliation: Division of Clinical Electrophysiology, Department of Neurology, University of Iowa Health Care, Iowa City, Iowa, USA.
authors.forename: Jun
authors.initials: J
authors.lastname: Kimura
mesh_terms: D000328:Adult; D000368:Aged; D004576:Electromyography; D005152:Facial Muscles / Q000294:innervation*; D005154:Facial Nerve / Q000503:physiopathology*; D005260:Female; D019569:Hemifacial Spasm / Q000503:physiopathology* / Q000601:surgery; D006801:Humans; D008297:Male; D061145:Microvascular Decompression Surgery; D008875:Middle Aged; D016343:Monitoring, Intraoperative; D008991:Monitoring, Physiologic; D009431:Neural Conduction / Q000502:physiology*
keywords: Abnormal muscle response; Ephaptic transmission; Hemifacial spasm; Lateral spread of impulses; Microvascular decompression; Nerve action potential