Magnetic stimulation of motor cortex and motor roots for painless evaluation of central and proximal peripheral motor pathways. Normal values and clinical application in disorders of the lumbar spine.

Magnetic stimulation of the motor cortex, motor roots, and proximal nerve trunks was performed in 46 healthy adults and in 73 consecutive patients with disorders of the lumbar spine. In combination with neurography and F-wave recordings, the fractionated stimulation of the motor pathways allowed calculation of conduction times of the pyramidal tract fibers, of the motor roots (ie, caudal fibers), and of the motor fibers of the lumbosacral plexus. Normal values for motor conduction times to the quadriceps, anterior tibial, and extensor digitorum brevis muscles were established. Patients had clinical and radiologic diagnoses of spinal stenosis (n = 43) and nerve root compression syndromes (n = 30). Motor conduction times to lower limb muscles were significantly delayed (above mean normal value +/- 2 x SD) in 65% of the patients with spinal stenosis and 50% of the patients with nerve root compression syndromes. Conduction slowing could be localized within the motor root and caudal fiber segment of the motor pathways in 80% of the patients in whom F-waves could be recorded. This method can be used to verify, quantify, and locate lesions of the motor pathways in conditions such as compression of the spinal cord, the caudal fibers (spinal stenosis), or the motor root passing through the intervertebral canal.