2-P-E-8. Ephaptic transmission as the origin of abnormal muscle response seen in hemifacial spasm

Abstract

Objective To test a hypothesis that an abnormal muscle response (AMR) results from ephaptic transmission at the compression site in hemifacial spasm (HFS). Methods We recorded AMRs from the orbicularis oculi and mentalis muscles during intraoperative of facial nerve action potential (NAP) monitoring performed in 5 patients with HFS. Before microvascular decompression (MVD), facial nerve was stimulated to give rise, antidromic NAP and orthodromic NAP. Results Facial NAPs recorded 3 mm distal to the compression site, comprised a double peak with an average interval of 1.1 ms before MVD. The second peak abated together with AMRs immediately after MVD. An average inter-peak interval of 1.1 ms falls short of the estimated conduction time to the facial nucleus, implicating, as the origin, an ephaptic transmission at the compression site. Conclusion The lateral spread of impulse by ephapsis at the compression site gives rise to an AMR in HFS, with estimate transmission time of 1.0 ms Significance This study establishes the primary mechanism of the lateral spread responsible for generation of AMR and contributes to the understanding of the pathophysiology of HFS.