Amplitude, area and duration of the compound muscle action potential change in different ways over the length of the ulnar nerve

Objective To study physiological changes of the compound muscle action potential (CMAP) obtained from stimulation at different sites over the full length of a motor nerve and to study possible effects of anthropometrical factors. Methods Multicentre study of ulnar motor nerve conduction in five segments to Erb’s point performed bilaterally on 100 healthy subjects aged 17–83 years. Results CMAP amplitude decreased linearly with conduction distance (0.31%/cm) from wrist to Erb’s point. CMAP area decreased with the square of conduction distance. Decrease in area was smaller than decrease in amplitude especially distally. CMAP duration increased linearly (0.17%/cm). Amplitude decay correlated with age, height and BMI and dispersion correlated with age and height. There were no correlations between area decay and anthropometrical factors. There was no significant inter-examiner variation. Conclusions Area decay may be preferred to amplitude decay in the evaluation of conduction block over short segments due to smaller physiological changes and independence of anthropometrical factors. The absence of inter-examiner variation indicates that the results are robust and may be used by other laboratories. Significance This study provides knowledge of physiological changes of CMAP parameters that may be of importance in the evaluation of nerve pathology, in particular conduction block.