Influence of reference electrode position on the compound muscle action potential

Abstract

Objective: When the compound muscle action potential (CMAP) is recorded in motor nerve conduction studies, the reference (E2) electrode can make a significant contribution to the CMAP. This study investigates the E2 recorded signal and its effect on CMAP measurements when E2 electrode is placed at different sites.

Methods: The CMAP was recorded using the active electrode on the muscle belly and 4 different E2 electrodes placed at distal and proximal sites. The signal recorded by each electrode was also measured using a reference electrode on the contralateral limb. Signals were recorded from the abductor pollicis brevis, abductor digiti minimi, tibialis anterior and biceps muscles.

Results: The E2 recorded a smaller signal when it was placed near or off the proximal tendon or muscle origin. This affected CMAP latency, duration for tibialis anterior. Contrary to expectation, initial upward deflection was noted for E2 signal.

Conclusion: A proximal E2 position records a lower volume conducted signal and yields a CMAP that is more representative of the muscle over which the E1 (active) electrode is placed.

Significance: The proposed 'Proximal E2' montage may be better suited to assess pathology.

Keywords: CMAP; Compound muscle action potential; Muscle origin; Nerve conduction studies; Reference electrode; Volume conduction.