Establishment of normal values for tibial somatosensory evoked potentials

Normal values for tibial somatosensory evoked potentials (SEPs) have been reported by several authors, although subject factors, such as height, age or gender, have not been fully considered. We aimed to establish standard values fully considering subject factors using multivariate analysis for tibial SEPs, including the P15 component which we reported previously as a new stable component generated at the greater sciatic foramen. We examined SEPs following left tibial nerve stimulation in 65 normal subjects of various ages, and performed multiple regression analysis using height, age (age 20)2 and gender as predictor variables. We objectively selected the latency or interval parameters with less intersubject variability as the standard parameters. As a result, P15 was clearly identified in all of the subjects and is promising as a new parameter to evaluate the proximal segment of the tibial nerve. Among three cortical bipolar derivations investigated, the Cz′-Cc lead gave the most constant and stable P38 component. The latencies of the N8o (N8 onset), P15 (P15 peak), N21, N30 and P38o/P38 in the Cz′-Cc lead, as well as the intervals between them were selected as standard parameters. We established nomograms to find the normal limit values of each parameter.