Comparison of somatosensory evoked responses from root and cord recorded by skin and epidural electrodes using stimulation of the median nerve in cervical radiculopathy and radiculomyelopathy.

Somatosensory evoked potentials (SEPs) were recorded by stimulating the median nerve at the wrist from the skin and epidural space of the 7th cervical spine in patients suffering from cervical radiculopathy or radiculomyelopathy. The patients were divided into four subgroups according to the severity of the disease. Skin and epidural SEPs were calculated and compared with each other and with control values. Usually only one negative potential N13 was identified in the skin recording, but two potentials N11 and N13 occurred in the epidural recording. Lower amplitudes were obtained from the skin than from the epidural space. In the skin SEPs the mean of the central latency of N13 was significantly prolonged in the severe radiculomyelopathy groups, while the mean of the amplitude N13 showed only a tendency to decrease. In contrast, in the epidural SEPs a significant decrease in the mean of the N11 and N13 amplitudes together with a significant prolongation in the mean of the central latency of N13 could be found. In the epidural recording the amplitude changes in particular increased with the severity of the disease, but the highest number of abnormalities (61%) could be seen in the central latency of N13.