Somatosensory evoked potentials from dermatomal stimulation as an indicator of L5 and S1 radiculopathy

Dermatomal somatosensory evoked potential (DSEP) results for L5 and S1 were contrasted with electromyography (EMG) results for 50 patients referred to the electrodiagnosis laboratory to document the presence of radiculopathy. Stimulation sites were over the dorsum of the foot at the distal fifth metatarsal for the S1 dermatome and at the web space of the first and second toe for the L5 dermatome. Recordings were made at PZ reference to FZ. Spinal cord or cauda responses could not be detected. Both EMG and DSEP were contrasted to myelography or lumbar computerized tomography results on 31 patients. Side-to-side amplitude differences proved too variable to be of use. Sixty-five percent of abnormal DSEP results were on the basis of side-to-side latency criterion, and 35% were on the basis of an absent unilateral response. When compared to EMG and anatomic studies DSEP showed less accuracy and sensitivity. The specificity of the two tests was similar. Using both positive EMG and anatomic studies to define radiculopathy, there were 27% false negative tests and 9% false positive tests. An 86% root level correlation was found between EMG and DSEP. Dermatomal somatosensory evoked potential studies add little to the diagnosis of radiculopathy.