Measuring sensory nerve action potential electrical power

Sensory nerve action potential (SNAP) amplitudes are frequently examined for evidence of axonal loss. Because digital nerves have parallel fibers, and parallel voltages do not add, SNAP amplitude may not accurately reflect axonal loss. In contrast, electrical power can be measured for digital nerves, and is additive. In this study we explore the ability of SNAP electrical power (SELP) to discriminate the increased number of axons in fingers with two median digital nerves versus fingers with only one median digital nerve. Antidromic SNAP amplitudes and SELPs were obtained for 15 fingers. Seventeen external resistors, R(ext), were sequentially connected from electrode to ground for SELP determination. The resulting bell-shaped power vs. R(ext) plot was regressed to the power transfer equation, which has a peak that defines SELP. SELPs of two-digital-nerve (median) fingers were 360-670 femtowatts (mean 525 fW). For one-digital-nerve fingers, SELP was 90-230 fW (mean 190 fW). Evaluation of one- vs. two-median-digital-nerve statistics showed the SNAP amplitude difference-of-means to be insignificant, whereas the SELP difference-of-means was 3.3 SD (P < 0.01). Using a criterion of 2 SDs, SNAP amplitude did not discriminate any one-median-digital-nerve fingers, whereas SELP discriminated all with no false positives. Because parallel voltages do not add, SNAP amplitudes may not reflect axonal loss. In contrast, electrical power is additive. We describe the SELP technique and demonstrate its ability to discriminate different numbers of axons as reflected by one digital nerve vs. two digital nerves.