Transcranial magnetic stimulation of the facial nerve: recording technique and estimation of the stimulated site.

We compared transcranial magnetic and conventional electrical stimulation in 20 facial nerves of 10 normal subjects. A magnetic coil was placed tangentially over T5 or T6 (10-20 electroencephalogram electrode placement system) and electrical stimulation was applied 1 cm below the anterior tragus. Compound muscle action potentials recorded from the ipsilateral nasalis muscle showed onset latencies (mean +/- SD) of 4.48 +/- 0.50 ms with magnetic stimulation and 3.15 +/- 0.40 ms with electrical stimulation, a mean difference of 1.33 +/- 0.14 ms. Stimulation of the extracranial facial nerve at two sites yielded a conduction velocity of 59.6 +/- 4.5 m/s. Based on these findings, the magnetically stimulated site was estimated to fall 79.0 +/- 8.6 mm proximal to the point of electrical stimulation. Taking the average length of the nerve trunk and histological specificity of the root exit zone into consideration, evidence indicates that the root exit zone of the facial nerve is the most likely initiating site of excitation with magnetic stimulation.