Spinal responses to median and tibial nerve stimulation and magnetic resonance imaging in intramedullary cord lesions.

We recorded median and tibial nerve somatosensory evoked potentials (SEPs) in 18 patients with intramedullary lesions of the cord (intramedullary tumors in 14 patients and syringomyelia cavitations in 4). Patients were divided into four groups on the basis of the longitudinal extension of the lesions as revealed by magnetic resonance imaging. The lesions involved the cervical cord in nine patients (group 1), both cervical and lumbar cord ("holocord" lesion) in two (group 2), the dorsal cord in one (group 3), and the lumbar cord in six (group 4). The recording technique enabled us to analyze clearly the spinal SEPs, labeled for the median nerve as N13, and for the tibial nerve as N24. Both spinal responses are probably generated by the activation of dorsal horn cells and proved useful in revealing focal lesions of the cervical or lumbar cords. Median nerve N13 was abnormal in all group 1 and group 2 patients; in three patients, this was the only median SEP abnormality, suggesting that the dysfunction was limited to the cervical grey matter. Tibial nerve N24 was abnormal in all group 2 and group 4 patients. The abnormality of both cervical and lumbar segmental responses in patients with holocord lesions strongly suggested that both the cervical and lumbar cords were involved. The SEP study was often more effective than the clinical examination in revealing cord involvement and the actual extension of damage. The recording of the spinal SEPs is thus highly sensitive in assessing cord dysfunction in intramedullary lesions, providing that specific recording techniques are used.