title: The excitation site of the trigeminal nerve to transcranial magnetic stimulation varies and lies proximal or distal to the foramen ovale: an intraoperative electrophysiological study in man.
abstract: The excitation site of the trigeminal nerve using transcranial magnetic stimulation (magStim) was analyzed in 5 patients in whom the trigeminal nerve was surgically exposed in the posterior fossa during microvascular decompression of the facial nerve for hemifacial spasm. The trigeminal nerve was stimulated (1) magnetically immediately prior to craniotomy, and (2) electrically near the root exit zone (elREZ) of the nerve from the brainstem. Mean latency differences (delta) of masseter compound muscle action potentials (CMAPs) (delta elREZ minus magStim) were 0.7 (range: +0.3 to +1.3) ms (P less than or equal to 0.05, Wilcoxon-test). From these results, an analysis of anatomical data, and using a trigeminal nerve conduction velocity (NCV) of 50 m/s as reported in the literature, the following conclusions were drawn: the excitation site to magStim (1) is variable among individuals, (2) is located 3.4 (1.6-6.5) cm distal to the trigeminal REZ, and (3) which corresponds to segments of the nerve that are located either within or outside the cerebrospinal fluid (CSF), either proximal or distal to the foramen ovale. These findings are in contrast to those we obtained in a previous study of the facial nerve in which the excitation site was found to be constant among subjects and restricted to the location on the nerve where it exists the high conductivity CSF to enter the high-resistance petrous bone.
authors.affiliation: Department of Neurological Surgery, University of Pittsburgh School of Medicine, PA 15213.
authors.forename: U D
authors.initials: UD
authors.lastname: Schmid
authors.forename: A R
authors.initials: AR
authors.lastname: Møller
authors.forename: J
authors.initials: J
authors.lastname: Schmid
mesh_terms: D000200:Action Potentials; D000328:Adult; D000368:Aged; D004558:Electric Stimulation / Q000379:methods; D004594:Electrophysiology; D005260:Female; D006801:Humans; D007432:Intraoperative Period; D008297:Male; D008875:Middle Aged; D009132:Muscles / Q000502:physiology; D012886:Skull; D013100:Sphenoid Bone; D050781:Transcranial Magnetic Stimulation; D014276:Trigeminal Nerve / Q000502:physiology*