Electrophysiological study of ephaptic axono-axonal responses in hemifacial spasm.
One of the classic features of hemifacial spasm (HFS) is spread of the blink reflex responses to muscles other than the orbicularis oculi. The pathophysiological mechanisms underlying the generation of such abnormal responses include lateral spread of activity between neighboring fibers of the facial nerve and hyperexcitability of facial motoneurons. In this report we present evidence for another mechanism that can contribute to the generation of responses in lower facial muscles resembling the R1 response of the blink reflex. In 13 HFS patients, we studied the responses induced in orbicularis oris by electrical stimuli applied at various sites between the supraorbital and zygomatic areas. We identified responses with two different components: an early and very stable component, with an onset latency ranging from 10.5 to 14.8 ms, and a more irregular longer-latency component. Displacement of the stimulation site away from the supraorbital nerve and towards the extracranial origin of the facial nerve caused a progressive shortening of response latency. These features indicate that, in our patients, the shortest latency component of the orbicularis oris response was likely generated by antidromic conduction in facial nerve motor axons followed by axono-axonal activation of the fibers innervating the lower facial muscles. Our results suggest that motor axono-axonal responses are generated by stimulation of facial nerve terminals in HFS.
Montero
J
J
Neurology Department, Hospital de Bellvitge, Feixa Llarga s/n L'Hospitalet del Llobregat, 08907 Barcelona, Spain. jmonterohoms@hotmail.com
Junyent
J
J
Calopa
M
M
Povedano
M
M
Valls-Sole
J
J
D000328:Adult; D000368:Aged; D001767:Blinking / Q000502:physiology* / Q000528:radiation effects; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography / Q000379:methods; D005152:Facial Muscles / Q000294:innervation* / Q000503:physiopathology; D005154:Facial Nerve / Q000503:physiopathology*; D005260:Female; D019569:Hemifacial Spasm / Q000473:pathology* / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D011930:Reaction Time / Q000502:physiology / Q000528:radiation effects