Skin and epidural recording of spinal somatosensory evoked potentials following median nerve stimulation: correlation between the absence of spinal N13 and impaired pain sense.

A clinical lesion study and intraoperative epidural recordings were made to test the origin and clinical significance of the spinal N13 and P13 of somatosensory evoked potentials (SEP) that follow median nerve stimulation. Intraoperatively, the respective peak latencies of spinal P13 and N13 coincided with those of the N1 component of the dorsal cord potential and its phase reversed positivity. On both the ventral and dorsal sides of the cervical epidural space, maximal amplitude was at the C5 vertebral level to which nerve input from the C6 dermatome is the main contributor. The modality of sensory impairment in the hand dermatome was examined in selected patients with cervical lesions, who showed such normal conventional SEP components as Erb N9, far-field P9, P11, P14, N18 and cortical N20, with or without loss of spinal N13. Statistically, the loss of spinal N13 was associated with decrease of pain sensation in the C6 dermatome. This was interpreted as being due to damage to the central grey matter of the cord, including the dorsal horn. Our results suggest the spinal N13 and P13 originate from the same source in the C6 spinal cord segment and that they are good indicators for the detection of centromedullary cervical cord damage.