Acceptable differences in sensory and motor latencies between the median and ulnar nerves

The median and ulnar nerves are often studied during the same electrodiagnostic examination. The sensory and motor latencies of these nerves have been compared to detect a common electrodiagnostic entity: median neuropathy at the wrist. However, this comparison could also be used to diagnose less common ulnar pathology. For this reason, it is important to establish normal values for comparing median and ulnar sensory and motor latencies. Previous research deriving these differences in latency has had some limitations. The purpose of this study was to derive an improved normative database for the acceptable differences in latency between the median and ulnar sensory and motor nerves of the same limb. Median and ulnar sensory and motor latencies were obtained from 219 and 238 asymptomatic risk-factor-free subjects, respectively. An analysis of variance was performed to determine whether physical characteristics, specifically age, race, gender, height, or body mass index (as an indicator of obesity), correlated with differences in latency. Differences in sensory latencies were unaffected by physical characteristics. The upper limit of normal difference between median and ulnar (median longer than ulnar) onset latency was 0.5 ms (97th percentile), whereas the peak latency value was 0.4 ms (97th percentile). The upper limit of normal difference between ulnar-versus-median (ulnar longer than median) onset latency was 0.3 ms (97th percentile), whereas the peak-latency value was 0.5 ms (97th percentile). The mean difference in motor latencies correlated with age, with older subjects having a greater variability. In subjects aged 50 and over, the mean difference in median-versus-ulnar latency was 0.9 ms +/- 0.4 ms. The upper limit of normal difference (median longer than ulnar) was 1.7 ms (97th percentile). The upper limit of normal ulnar motor latency is attained if the ulnar latency comes within 0.3 ms of the median latency. In individuals less than 50 years of age, the mean difference in latency was 0.6 ms +/- 0.4 ms, with the median latency usually being greater than the ulnar. The upper limit of normal difference (median longer than ulnar) was 1.4 ms (97th percentile), whereas the upper limit of ulnar latency relative to median latency was attained if the ulnar latency was equal to median latency.