Investigation of facial motor pathways by electrical and magnetic stimulation: Sites and mechanisms of excitation

Abstract

A refined technique is described for non invasive examination of the facial motor pathways by stimulation of the extra- and intracranial segment of the facial nerve and the facial motor cortex. Surface recordings from the nasalis muscle rather than from the orbicularis oris muscle were used, since the compound muscle action potential (CMAP) from this muscle showed a more clearly defined onset. Electrical extracranial stimulation of the facial nerve at the stylomastoid fossa in 14 healthy subjects yielded a mean distal motor latency of 3.7 ms (SD 0.46), comparable with reported latencies to the orbicularis oris muscle. Using a magnetic stimulator, transcranial stimulation of the facial nerve was performed. The mechanism of transcranial magnetic facial nerve stimulation was studied using recordings on 12 patients who had facial nerve lesions at different locations, and with intraoperative direct measurements in four patients undergoing posterior fossa surgery. The actual site of stimulation could be localised to the proximal part of the facial canal, and a mean 'transosseal conduction time' of 1.2 ms (SD 0.18) was calculated. The cerebrospinal fluid (CSF) played an important role in mediating the magnetically induced stimulating currents. Finally, with transcranial magnetic stimulation of the facial motor cortex, clearly discernible CMAPs could be produced when voluntary activation of several facial muscles was used to facilitate the responses. From this, a central motor conduction time of 5.1 ms was calculated (SD 0.60, 6 subjects).