The consequences of stimulus intensity on sensory nerve action potentials

Although clinical references recommend evoking sensory nerve action potentials (SNAPs) using supramaximal stimulation, many patients and research subjects report that the maximal and supramaximal stimulation is uncomfortable. The purpose of this study was to determine whether using stimulation intensities at maximal and submaximal levels results in differences in SNAP latency or amplitude outcomes relative to SNAPs recorded using supramaximal stimulation. Twenty healthy male and female subjects participated. Antidromic SNAPs were evoked at two separate sites along the course of the median nerve: (1) just proximal to the wrist and (2) just proximal to the antecubital fossa in each subject. All evoked potentials were recorded from the second digit. The stimulation current that resulted in a maximal amplitude SNAP was determined and 10 SNAPs were ensemble averaged using this stimulation level. Ten SNAPs were then evoked at each of four other stimulation intensities (20% and 10% both above and below that which generated a maximal amplitude SNAP) and were ensemble averaged. For each stimulation intensity and stimulation site, SNAP amplitude and latency determined for each ensemble average. SNAP amplitudes and latencies were compared among stimulus levels and between sexes using a repeated measures analyses of variance. The results of this study suggest that stimulus intensities used to generate SNAPs at the median nerve do not need to be supramaximal in order to generate valid and reliable SNAP data, with stimulation intensities as low as 20% below maximal proving to be sufficient, particularly for the measurement of SNAP latency.