Subject-based P1 latency inter-root comparison, a method to evaluate P1 latency in scalp recorded somatosensory...

The purpose of the present study was to establish a method that allows the general use of subject-based criteria to evaluate P1 latency in scalp recorded somatosensory evoked potentials obtained with stimulation of the sural (S1), superficial peroneal (L5) and saphenous (L4) nerves bilaterally. The nerves were stimulated at the same distance from the registration electrode. Two groups of normal nerve roots were studied: (1) nerve roots on both sides in 20 asymptomatic volunteers, and (2) neuroradiologically normal nerve roots on the asymptomatic side in 22 patients with unilateral sciatica. The results presented show that the P1 latencies after stimulation of the 6 different nerves in the same person can be regarded as equal. On this basis 2 criteria to evaluate P1 latency by within-subject P1 latency inter-root comparison were defined. They were the difference between P1 latency of 1 registration and (1) that of any one of the other 5 registrations and (2) the mean P1 latency of the other registrations. The variability of these subject-based criteria and the width of their reference limits were compared to those of the population-based criteria of height- and height-age-corrected P1 latency. This comparison showed that the use of within-subject P1 latency inter-root comparison should enhance the ability to demonstrate small bilateral P1 latency prolongations at the same segmental level.