[Investigation of sites of magnetic coil placement and its influence on CMAPs]

Varying CMAP results with changing coil location during magnetic stimulation of facial nerve have been noted in the literature. In order to investigate parameters for clinical application, 20 normal subjects were tested with Magstim Model 200 Magnetic Stimulator synchronized with Nicolet Viking II with 9 designed points of coil location at the parietal-occipital-temporal area. The amplitude and latency were recorded and analyzed statistically. Results demonstrated that the maximal amplitude was mainly obtained from following 3 points: C (midpoint between vertex and external auditory canal), V-2 (2 cm inferior to C) and H-2 (2 cm posterior to C), which showed the best location of coil for magnetic stimulation among the tested points. The maximal amplitude was also observed in points of H-4 (4 cm posterior to C) and V+2 (2 cm superior to C) in 4 subjects. In contrast to the varying amplitude, latency obtained from each stimulating point was constant, so we postulate that the same segment of facial nerve was excited regardless of moving magnetic coil around those points.