Dermatomal somatosensory evoked potentials in unilateral lumbosacral radiculopathy.

We examined scalp-recorded somatosensory evoked potentials (SSEPs) to electrical stimulation of the peroneal nerves and to stimulation in the L5 and S1 dermatomes in 19 patients with unilateral radiculopathies involving these segments. For the dermatomal studies at least two trials of 512 responses were recorded from the vertex with reference to both the midfrontal and contralateral parietal electrodes, using an averaging technique. Findings on the symptomatic and asymptomatic sides were compared in each patient. We found that peroneal SSEPs were normal in all patients. Dermatomal SSEPs correctly identified the lesion in 5 patients. In 1 patient dermatomal SSEPs lateralized the lesion correctly but localized it to the adjacent root. In 10 cases dermatomal SSEPs gave misleading information, indicating an abnormality on the asymptomatic side in 1 patient and no abnormality in 9. In the remaining 3 patients, both SSEPs and radiological contrast studies failed to identify any lesion, although the radiculopathy was confirmed by electromyography. These findings raise doubt about the ultimate utility of these evoked potential techniques in the evaluation of patients with suspected radiculopathies.