Transcranial magnetic stimulation of the facial nerve: Intraoperative study on the effect of stimulus parameters on the excitation site in man

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Abstract
Magnetic stimulation (magStim) of the intracranial facial nerve is performed in clinical and research settings, but the activation site is a matter of controversy. Latencies of nasalis muscle responses to magStim were, therefore, compared with those obtained by direct electrical stimulation of the facial nerve (a) at the root exit zone (REZ); (b) at the porus of the facial canal; and (c) in the stylomastoid fossa during microvascular decompression operations in the cerebellopontine angle (CPA). Measurements of latencies of the nasalis muscle response, obtained while the stimulating coil was placed over the parieto‐occipital area of the scalp, indicated that it was the labryinthine segment of the facial canal, 5 to 16 mm distal to the CPA, that was activated. This would be in agreement with studies of physical models reported in the literature that showed (a) the strength of the electrical current generated by a magnetic field is particularly high close to a nerve foramen; and (b) excitation to magStim is most likely to occur where the induced electrical field changes rapidly over distance, i.e., at anatomical boundaries between media of high and low specific resistance. These characteristics are found at the end of the labyrinthine segment of the facial canal, where the facial nerve leaves the low‐resistance cerebrospinal fluid and enters the high‐resistance petrous bone. The site of neural excitation is robust and unaffected by stimulus intensity and current direction within a wide range, or by large changes in location of the coil. The practical implication of this study is that transcranial magStim does not reflect nerve conduction in segments of the facial nerve that are located proximal to the facial canal, which accounts for about 40% of the nerve length between the REZ and the stylomastoid fossa.