Spinal somatosensory evoked potentials following segmental sensory stimulation. A direct measure of dorsal root function.

Dorsal root function cannot presently be measured directly. The H-reflex is an indirect measure of dorsal root function but only for the S1 root. Spinal somatosensory evoked potentials (SEPs) following dermatomal stimulation of the legs have the potential of providing direct data reflecting dorsal root function but have not been reliably recorded in normal subjects. We have developed a reliable technique for recording SEPs at the lumbar root entry zone following segmental sensory stimulation of the legs. The saphenous, superficial peroneal, and sural nerves were stimulated representing the L3/L4, L5 and S1 roots respectively. Reproducible responses (N-wave) were recorded over the lumbar spine in all 60 normal limbs examined. The N-wave peak latency was significantly correlated with lower limb length. The conduction velocities from the stimulation sites to the lumbar spine were similar to published values for peripheral conduction velocities in these nerves. The mean inter-limb latency differences for the N-wave peak were: L3/L4 0.61 msec; L5 0.35 msec; and S1 0.57 msec. The mean N-wave amplitudes were: L3/L4 0.11 microV; L5 0.28 microV; and S1 0.23 microV. This technique is a direct measure of dorsal root integrity. Unlike scalp recorded SEPs, the lumbar N-wave is not state-dependent and is unaffected by lesions within the brain and rostral cord.