Use of cerebral evoked potentials to evaluate spinal somatosensory function in patients with traumatic and surgical myelopathies

Cerebral somatosensory evoked potentials (SEP's) were elicited by electrical stimulation of the median nerve in the arm (SEPA) and of the posterior tibial nerve in the leg (SEPL) in 23 patients with incomplete localized lesions (including traumatic injuries, neoplasms, vascular malformations and infarcts) of the low cervical, thoracic, or lumbar spinal cord. In eight of 46 attempts (left and right sides), SEPL could not be recorded. Of the remaining 38 sides, spinal somatosensory conduction velocity (SSCV, indirectly estimated) was abnormally slow (<35 m/sec) in 20, and the amplitude of SEPL relative to SEPA (L:A ratio) was abnormally low (<0.5) in 20 (p<0.001 in each case, compared to normal controls). All three criteria yielded a combined 72% incidence of abnormality, correlating best with impairment of joint position sense. Serial postoperative studies in four cases documented an increase in the SSCV and L:A ratio following spinal decompression. These results demonstrate that the latency and amplitude characteristics of the cerebral SEP's from arm and leg permit quantitative evaluation of the functional status of the spinal somatosensory system.