Magnetic stimulation of the facial nerve

Electrical testing of the facial nerve has evolved into an important means of assessing neural injury. However, the inability to stimulate the intratemporal facial nerve electrically results in a delay in diagnosis, because axonal degeneration must progress distal to the stylomastoid foramen before testing can be meaningful. To circumvent this problem, we began an investigation of magnetic stimulation of the facial nerve, because pulsed magnetic fields can pass unattenuated through all body structures, including bone. Normal volunteers and one patient with acute facial paralysis were studied with both magnetic and electric stimulation of the facial nerve. The results indicate that (1) magnetic stimulation was more comfortable because high current levels were not required at the skin surface to assure indepth stimulation, (2) magnetic and electric stimulation of the extratemporal facial nerve resulted in nearly identical compound muscle action potentials, indicating that the sites and mechanisms of neural depolarization are similar, and (3) transtemporal magnetic stimulation appears to allow depolarization of the proximal intratemporal nerve. These preliminary results are encouraging and indicate that magnetic stimulation of the facial nerve warrants further investigation.