INTERINDIVIDUAL AND INTRAINDIVIDUAL VARIABILITY OF POSTERIOR TIBIAL NERVE SOMATOSENSORY-EVOKED POTENTIALS IN COMATOSE PATIENTS

Thirty-two sequential posterior tibial nerve somatosensory evoked pote ntials (PTN-SEP) were recorded in 30 neurologically impaired, ventilat ed, comatose patients. To establish the time invariance of PTN-SEP in this population, Spearman rank correlations of latencies, interpeak la tency, amplitudes, and mean absolute amplitude with time were computed . The results revealed no significant time dependency. The mean, stand ard deviation, and 5th and 95th percentiles for the inter- and intrain dividual distribution of PTN-SEP parameters, pairwise PTN-SEP paramete r differences, and direct cross-correlation of PTN-SEP were estimated using bootstrap procedures. The standard deviations of the interindivi dual 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 me ans, for the intraindividual distribution of latencies ranged from 0.0 12 to 0.042, of amplitudes from 0.146 to 0.230. The mean maximal cross -correlation coefficient of two randomly chosen PTN-SEP across patient s equaled 0.65, and within patients 0.91. These data demonstrate the i nterindividual variability and intraindividual stability of PTN-SEP. T he normal limits of intraindividual variability (1.96 X standard devia tion) are 3.76, 1.33, 2.99, 6.00, and 3.04 ms for latencies N1, P1, N2 , P2, and interpeak latency P1-N2, respectively. The intraindividual d ifferences of amplitudes N1/P1, P1/N2, N2/P2, and the mean absolute am plitude should not exceed 0.67, 0.67, 0.90, and 0.27 mu V or, expresse d as quotients, 61, 52, 41, and 61%, respectively. The intraindividual maximal cross-correlation coefficient should not be lower than 0.74 w ith a lag of <2.00 ms. The results of this study are of use for discre te and continuous PTN-SEP monitoring on intensive care units and durin g neuroradiological interventions and neurosurgery.