The optimal current direction for excitation of human cervical motor roots with a double coil magnetic stimulator

Upper limb muscles can easily be excited using magnetic stimulation with a circular coil over the cervical vertebrae. Double coil magnetic stimulators generate a more focal and directed form of stimulus and may offer an advantage in the more specific stimulation of individual motor roots. The aims of this study were to determine the optimal rostro-caudal locations and orientations of a double coil magnetic stimulator for excitation of the human 5th cervical to 1st thoracic motor roots. Compound muscle action potentials (CMAPs) evoked by magnetic stimuli were recorded from deltoid, brachioradialis, extensor digitorum communis and first dorsal interosseus muscles. Optimal rostro-caudal location was determined with the coil current flowing horizontally towards the midline. Optimal orientation was determined by rotating the coil in increments of 22.5 degrees about the optimal rostro-caudal location. In all subjects with all muscles examined, the best orientation was with the coil current flowing downwards towards the midline at a mean angle between 14.9 degrees and 27.6 degrees to the horizontal. Review of the anatomy of motor roots reveals a short upwardly directed segment within the intervertebral foramen and it is postulated that excitation occurs exclusively within this segment.