Influence of reference electrode position on the compound muscle action potential

Area of Science:

• Neurophysiology
• Electromyography
• Neuromuscular Studies

Background:

• The reference electrode (E2) placement significantly influences compound muscle action potential (CMAP) recordings in motor nerve conduction studies.
• Understanding the E2 signal's contribution is crucial for accurate CMAP interpretation.

Purpose of the Study:

• To investigate the impact of different E2 electrode placements on CMAP measurements.
• To evaluate the characteristics of the E2 recorded signal across various electrode positions.

Main Methods:

• CMAPs were recorded with the active electrode on the muscle belly and four different E2 electrode positions (distal and proximal).
• Signals were also measured using a reference electrode on the contralateral limb.
• Recordings were performed on abductor pollicis brevis, abductor digiti minimi, tibialis anterior, and biceps muscles.

Main Results:

• A proximal E2 electrode placement resulted in a smaller recorded signal.
• This proximal placement affected CMAP latency and duration, particularly for the tibialis anterior muscle.
• An unexpected initial upward deflection was observed in the E2 signal.

Conclusions:

• A proximal E2 electrode position records a lower volume-conducted signal.
• This positioning yields a CMAP more representative of the muscle under the active electrode.
• The proposed 'Proximal E2' montage may enhance the assessment of neuromuscular pathology.