Latency and amplitude variability in serial median nerve SEP recordings

Intra-individual variability of SEP parameters was investigated by serial SEP recordings.

Median nerve SEP of 53 normal subjects (mean age 25.5+/-2.7 years) were evaluated. Recordings were repeated 1 week, 2 weeks, and 6 months after the initial recording.

Mean values of latencies from right median nerve stimulation were slightly longer (+0.1 ms) as compared to the left. The intra-individual (within stimulation side) variance did not depend on the time interval between recordings. The total within-stimulation-side variance of latencies was 0.056 ms2 for N20 (C3/4' vs. Fz), 0.070 ms2 for N13 (C7 vs. Fz), 0.048 ms2 for N10 (Erb's point vs. Fz), 0.111 ms2 for P9 (C3/4' vs. contralateral forearm), and 0.148 ms2 for P14 (C3/4' vs. contralateral forearm), and was about 3 times smaller than between-side variability with regard to cortical or spinal potentials. Upper limits for latency differences and lower limits for amplitude ratios in repeated recordings, as well as upper and lower limits for between-side latency differences and amplitude ratios were calculated, using F distributions with worst-case assumptions for degrees of freedom and error probability P = 0.05. Upper limits of within-stimulation-side latency differences were 0.55 ms for N20, 0.62 ms for N13, 0.51 ms for N10, 0.78 ms for P9, and 0.90 ms for P14, and corresponding lower limits of amplitude ratios were 0.69, 0.62, 0.66, 0.45, and 0.50, respectively.

Our results demonstrate a small systematic, and a larger random difference between right and left side stimulation, one arising from the peripheral, and the other one from the central pathway proximal to the source of N10.