Compound motor action potential interexaminer variability in photoguided placement of the recording electrodes

Interpretation of neurographic data in follow-up studies of patients with neuromuscular disease is sometimes challenging because of the expected variability in repeated tests. In this study, we explored whether the interexaminer variability of the compound motor action potential (CMAP) amplitude is reduced if the examiner is guided by the previously taken photographs of the position of the recording electrodes. The CMAPs were recorded from 20 subjects by unilateral distal stimulation of the median, ulnar, peroneal, and tibial nerve by 4 different technicians. All the subjects were examined on 2 occasions (∼1 week apart). On the first occasion, the technicians recorded the CMAP values according to routine protocols. On the second occasion, the technicians had additional guidance from photographs displaying the recording electrode positions from the first study. The CMAP coefficient of variation (CoV) was calculated for each nerve examined by the four technicians. Differences in CoV between the two types of tests (i.e., with or without photographs) were evaluated. When the examiners were guided by the photographs during electrode application, the CMAP CoV for the tibial innervated abductor hallucis (AH) muscle was reduced (P = 0.02) from 18.5% to 13%. There were, however, no significant reductions in CoV for the abductor pollicis brevis (APB, P = 0.23, median nerve), abductor digiti minimi (P = 0.37, ulnar nerve), or extensor digitorum brevis (EDB, P = 0.15, peroneal nerve) muscles. Photographic documentation of the CMAP recording electrodes seems to have a limited overall effect on interexaminer variability in a subsequent study.