Effect of skin thickness on sensory nerve action potential amplitude

Objective It has been shown that finger circumference negatively correlates with sensory nerve action potential amplitude (SNAP-A). Also fat people have lower sensory nerve amplitudes. Factors that cause electrodes displaced further away from underlying nerves, such as increased cutaneous and subcutaneous tissue, will lower SNAP-A. This study was designed to evaluate correlation between skin thickness and SNAP amplitude. Methods Thirty-seven healthy 22–40-year-old subjects were selected. Nineteen (51.4%) were males and 18 (48.6%) were females, without significant difference between males and females regarding their ages. For all subjects, height and weight were measured. Anteroposterior and mediolateral diameters of the proximal phalanges of the index and little fingers and also finger circumferences were measured. Palmar digital skin thickness was measured in two ways: first with sonography machine, and second with skin fold caliper. Median and ulnar nerve sensory and motor conduction studies were performed. Results In bivariate analysis, SNAP-A correlated negatively with female sex, height, anteroposterior diameter of the fingers, finger circumference and skin thickness measured by sonography, but in multiple regression analysis only skin thickness measured by sonography could predict SNAP-A. Conclusions This study demonstrates that among physiological factors of sex, height, BMI and also finger size measures, skin thickness is the best predictor of SNAP-A. Significance In clinical practice, this effect must be taken into account when making determination of abnormality based on sensory nerve amplitude.