Localization of nerve depolarization with magnetic stimulation

The specific location on the magnetic stimulation (MS) coil that may correspond to the area of nerve depolarization has not been determined. In order to localize such an area, MS with 9-cm and 5-cm diameter coils was compared with conventional percutaneous electric stimulation (ES). On the 9-cm coil the distribution of points of nerve depolarization corresponded to that quarter of the coil which was placed over and parallel to the median nerve, whereas on the 5-cm coil, this area also extended outside the coil. The points of median nerve depolarization with MS were distributed over a distance of 7 cm on the stimulator head and was nearly identical for the 2 coil sizes at the wrist and elbow. Ulnar nerve costimulation was less frequent with the smaller coil at the wrist. A calculated reference point on the coil is suggested for more accurate NCV determinations.