SUBJECT-BASED P1 LATENCY INTERROOT COMPARISON, A METHOD TO EVALUATE P1 LATENCY IN SCALP RECORDED SOMATOSENSORY-EVOKED POTENTIALS OBTAINED WITH SENSORY NERVE-STIMULATION IN THE LOWER-EXTREMITIES

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 s calp recorded somatosensory evoked potentials obtained with stimulatio n of the sural (S1), superficial peroneal (L5) and saphenous (L4) nerv es bilaterally. The nerves were stimulated at the same distance from t he registration electrode. Two groups of normal nerve roots were studi ed: (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 sa me person can be regarded as equal. On this basis 2 criteria to evalua te P1 latency by within-subject pi 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 subje ct-based criteria and the width of their reference limits were compare d to those of the population-based criteria of height- and height-age- corrected PI latency. This comparison showed that the use of within-su bject P1 latency inter-root comparison should enhance the ability to d emonstrate small bilateral P1 latency prolongations at the same segmen tal level.