MAGNETIC MAPPING OF HUMAN CERVICAL NERVE ROOTS - VARIATION IN NORMAL SUBJECTS

Cervical magnetic stimulation occurs preferentially at the neural foramen and is more effective with the induced electric field oriented transversely to the spine rather than vertically. This geometry suggests the possibility of mapping innervation from specific nerve roots. A ''butterfly'' stimulus coil, positioned horizontally, was used to determine the morphologies and amplitudes of compound motor action potentials from multiple muscles in the upper extremities of 15 normal adults. Averages of 5 responses were taken at 1 cm intervals along the cervical-thoracic spine. Needle electrodes were used to verify atypical patterns of innervation. Cervical spine X-rays allowed anatomic correlation. Rarely abductor digiti minimi was difficult to stimulate in the horizontal orientation; satisfactory responses were then obtained with the stimulator angled upward towards the side of stimulation. Averaged compound motor action potentials were reproducible, and as the stimulus coil was moved rostro-caudally they often changed morphology in well-defined patterns, suggesting an origin in different cervical nerve roots. The sequence of muscle activation in most subjects was consistent with conventional descriptions of segmental innervation, but 5 appeared to show substantial variation in the nerve roots supplying one or more muscles. Magnetic mapping may be a useful method for studying segmental innervation in human subjects.