Optimal distance for segmental nerve conduction studies revisited.

In 1972, Maynard and Stolov showed that the experimental error in calculating nerve conduction velocity (NCV) depends on errors of latency and distance measurements. Their data suggested that a minimum distance of 10 cm should be used when calculating NCV because of an increase in error >/= 25% at shorter distances. The object of this study was to reestablish the minimum distance using current technology. Twenty physicians measured the proximal and distal onset latencies of the same stored ulnar compound muscle action potentials, as well as the forearm skin distance. The mean and standard deviation of the errors in conduction time and distance were determined. A spreadsheet was constructed, determining the error in NCV for a variety of distances and NCVs. The average conduction time between proximal and distal stimulation was 3.23 ms, with a standard deviation of 0.09 ms. The mean distance measurement was 212.6 +/- 2.1 mm. The errors in NCV were significantly less than previously reported. The experimental error increased as distance decreased, reaching 20% with distances less than 6 cm. The latency error accounted for 71% of the total experimental error, as opposed to 90% in the previous study. With advances in electrodiagnostic equipment, NCV can reasonably be calculated at distances less than 10 cm, perhaps as low as 5-6 cm.