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Transcranial magnetic stimulation excites the labyrinthine segment of the facial nerve: An intraoperative electrophysiological study in man

Authors
Journal
Neuroscience Letters
0304-3940
Publisher
Elsevier
Publication Date
Volume
124
Issue
2
Identifiers
DOI: 10.1016/0304-3940(91)90111-6
Keywords
  • Facial Nerve
  • Neurography
  • Transcranial Magnetic Stimulation
  • Electrical Stimulation
Disciplines
  • Medicine

Abstract

Abstract The site where transcranial magnetic stimulation (magStim) depolarizes the facial nerve was investigated in 6 patients who underwent surgery of the cerebellopontine angle (CPA). The facial nerve was stimulated (1) magnetically prior to craniotomy, (2) electrically near the brainstem (elREZ), (3) at the exit from the CPA into the facial canal (elPorus), and (4) in the stylomastoid fossa (elStylo). The range of latency differences (Δ) of compound muscle action potentials (CMAPs) recorded from the ipsilateral mentalis muscle were as follows: ΔelREZ-magStim: +0.5 to +1.1 ms ( P⩽0.03, Wilcoxon test); ΔelPorus-magStim: +0.2 to +0.5 ms ( P⩽0.03); ΔelStylo-magStim: +0.8 to +1.0 ms ( P⩽0.03). On the basis of anatomical data and a facial nerve conduction velocity of 33–46 m/s in these patients, it was concluded that transcranial magnetic stimuli depolarized the facial nerve at a location 10–15 mm distal to its entrance into the facial canal. This corresponds to the end of the labyrinthine segment of the facial nerve, i.e. the transit zone where the nerve ceases to be surrounded by cerebrospinal fluid (CSF) with its high electrical conductivity and enters the high-resistance tissue of the petrous bone.

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