Effect of interelectrode distance on sural nerve action potential parameters

Objective: To assess differences in amplitude, latency, and duration, using a 3-cm vs. 4-cm distance between the active and reference electrodes when performing sural nerve conduction studies (NCS). Current normative data in lower-limb studies are generally based on 3-cm interelectrode differences, although 4-cm differences have been reported to be optimal in the upper limb. Design: Prospective study comparing the onset latency, peak latency, duration, and amplitude for the sural sensory nerve action potential (SNAP) recording at two interelectrode distances in adult volunteers. Results: Forty-three sural nerves were studied in 22 normal subjects. Peak latencies recorded with a 4-cm interelectrode distance were significantly longer than those recorded with a 3-cm distance (mean difference 0.06 msecs [SD = 0.09, P = 0.0073]). Duration was significantly longer (mean difference = 0.03 msecs [SD = 0.07, P 0.0270]), conduction velocities were significantly slower (mean difference -0.7 msecs [SD 1.0, P = 0.0012]), and onset latency and amplitude were not found to differ significantly. Average differences in peak latencies, duration, velocity, onset latency, and amplitude were not correlated with gender, age, or BMI. Conclusions: In contrast to studies of upper-limb sensory NCS, sural SNAP parameters obtained with 3- and 4-cm interelectrode distances did not differ for onset latencies and amplitude. Peak latencies, duration, and conduction velocity differences, though statistically significant, were of insufficient magnitude to be clinically meaningful. By using a 4-cm instead of a 3-cm interelectrode difference for sural nerve studies, the small prolongation of 0.06 msecs in peak latency and tiny increment of 0.27 AV observed in our investigation is unlikely to influence the electrodiagnostician's interpretation of the study.