Magnetic stimulation of the facial nerve

Abstract

Intracranial activation of the facial nerve and the face-associated motor cortex are now possible with noninvasive magnetic stimulation techniques. Compound muscle action potentials (CMAPs) and the Blink reflex, in response to magnetic stimulation, were investigated. Subjects were 10 normal controls and 2 Bell's palsy patients. CMAPs were elicited in the orbicularis oris muscle by magnetic stimulation at the parieto-occipital skull and stylomastoid foramen. Furthermore, CMAPs were evoked by a magnetic coil oriented over the cortex. CMAP recording was possible with magnetic stimulation and the latencies of CMAPs at the parieto-occipital skull were slightly greater than those at the stylomastoid foramen. In 10 normal subjects, the mean onset latency following transcranial magnetic stimulation of the facial nerve at the parieto-occipital skull was 5.07 msec (SD = 0.40), while transcutaneous latency at the stylomastoid foramen was 2.77msec (SD = 0.539). In the blink reflex, R1 latency was 10.99 msec (SD = 1.27), ipsilateral-R2 latency was 37.46 msec (SD = 2.57), and contralateral-R2 latency was 38.925 msec (SD = 3.20). The blink reflex thus had a configuration similar to that evoked by conventional electrical stimulation. In the patients with Bell's palsy, CMAPs elicited by magnetic stimulation were of low amplitude with normal latency. However, in the blink reflex, only a contralateral R2 response could be recorded, and R1 and ipsilateral-R2 showed no response to stimulation at the affected side. Investigation of patients with Bell's palsy using this technique may therefore prove useful in the evaluation of peripheral facial nerve disorders.