Sensory nerve somatosensory evoked potentials (SEP) in the evaluation of patients with sciatica: false P1 latency prolongation may be due to admixture of dermatomal SEP.

Patient-reported stimulus-related radiating sensory symptoms within the territory of the stimulated nerve have been used to verify stimulation in sensory nerve scalp recorded somatosensory evoked potentials (SEP). The main aim of the present study of false positive P1 latency prolongation in lumbosacral sensory nerve SEP was to investigate whether elicitation of such symptoms secures adequate sensory nerve stimulation. Nerve roots were studied on the asymptomatic side in 64 patients with unilateral sciatica. Saphenous (L4), superficial peroneal (L5), and sural (S1) nerve SEP were registered in all patients. Pretibial dermatomal SEP were registered in ten of them. Stimulation was equidistant from the registration electrode in all SEP registrations. The false positive rate was lower in saphenous nerve SEP with than without verified supramaximal stimulation (1/30 vs. 6/22, P = 0.03) in spite of radiating stimulus-related sensory symptoms in both groups. This difference was not caused by subclinical myelographic nerve root compression or general peripheral nerve dysfunction. The P1 latency was longer in the pretibial dermatomal SEP than in the saphenous and superficial peroneal nerve SEP with the same conduction distance (mean difference 4.7 (95% CI = 3.8 to 5.6) and 4.4 ms (95% CI = 3.4 to 5.4), respectively). It is concluded that dermatomal SEP have longer P1 latency than sensory nerve SEP. Verified supramaximal nerve simulation is recommended to avoid false results due to admixture of dermatomal to sensory nerve SEP.