The influence of wrist thickness on amplitudes of sensory nerve action potentials

Normal values of sensory nerve action potential (SNAP) amplitudes vary widely. There are many factors which can influence the amplitude of the SNAP, such as the type of recording techniques, temperature, distance between recording electrodes, skin impedance, etc. In this study, we investigated the correlation between wrist thickness (palmar to dorsum diameter) and amplitudes of the SNAP of median and ulnar nerves of 20 healthy subjects. Ring electrodes were placed on the second and the fifth fingers and a surface block electrode was placed at the wrist between the flexor carpi radialis and the palmaris longus tendon and just radial to the flexor carpi ulnaris tendon, for testing the median and ulnar nerves, respectively. Both orthodromic and antidromic techniques were used. Depending on the technique, either ring or surface block electrodes could be the stimulating or the recording electrodes. By supramaximal intensity stimulation, the amplitude of the SNAP was measured from peak to peak. The results revealed that SNAP amplitudes and wrist thickness are reversely correlated, with statistical significance (p ≤ 0.005) by using the antidromic technique. The influence of wrist thickness on SNAP amplitudes should be recognized when performing an electrodiagnostic study to determine axonal degeneration by using the SNAP amplitude parameter.