Inter- and intraindividual variability of posterior tibial nerve somatosensory evoked potentials in comatose patients

Thirty-two sequential posterior tibial nerve somatosensory evoked potentials (PTN-SEP) were recorded in 30 neurologically impaired, ventilated, comatose patients. To establish the time invariance of PTN-SEP in this population, Spearman rank correlations of latencies, interpeak latency, amplitudes, and mean absolute amplitude with time were computed. The results revealed no significant time dependency. The mean, standard deviation, and 5th and 95th percentiles for the inter- and intraindividual distribution of PTN-SEP parameters, pairwise PTN-SEP parameter differences, and direct cross-correlation of PTN-SEP were estimated using bootstrap procedures. The standard deviations of the interindividual distribution of PTN-SEP parameters are two to three times higher than the standard deviations of the intraindividual distributions. The coefficients of variation, that is, standard deviations divided by means, for the intraindividual distribution of latencies ranged from 0.012 to 0.042, of amplitudes from 0.146 to 0.230. The mean maximal cross-correlation coefficient of two randomly chosen PTN-SEP across patients equaled 0.65, and within patients 0.91. These data demonstrate the interindividual variability and intraindividual stability of PTN-SEP. The normal limits of intraindividual variability (1.96 X standard deviation) are 3.76, 1.33, 2.92, 6.00, and 3.04 ms for latencies N1, P1, N2, P2, and interpeak latency P1-N2, respectively. The intraindividual differences of amplitudes N1/P1, P1/N2, N2/P2, and the mean absolute amplitude should not exceed 0.67, 0.67, 0.90, and 0.27 mu V or, expressed as quotients, 61, 52, 41, and 61%, respectively. The intraindividual maximal cross-correlation coefficient should not be lower than 0.74 with a lag of <2.00 ms. The results of this study are of use for discrete and continuous PTN-SEP monitoring on intensive care units and during neuroradiological interventions and neurosurgery.