Magnetic stimulation of the motor cortex in cervical spondylosis.

We report a new technique of transcranial magnetic stimulation of the motor cortex to measure conduction within central motor pathways of 67 patients with cervical spondylosis or disk herniation. There were upper motor neuron signs in 34 patients (51%) and x-ray evidence of cervical cord compression in 44 (66%). Muscle action potentials (MAPs) to cortical stimulation were abnormal in 84% of patients with, and 22% of those without, radiologic signs of cervical cord compression. Median nerve somatosensory evoked potentials were altered in only 25% of patients. The frequency of MAP alterations correlated with upper motor neuron signs. In 5 (11%) of the 44 patients with x-ray evidence of cervical cord compression, subclinical cord compression was disclosed by cortical stimulation. In 10 patients restudied 3 months after surgical decompression, normalization of central motor conduction time did not occur, indicating permanent damage to the cervical cord.