Optimal Site of the Reference Electrode for Recording the Compound Muscle Action Potential of the Deltoid Muscle

For recording the compound muscle action potential (CMAP) of the deltoid muscle, the reference electrode over the acromion (Ac) has been used to avoid contamination of responses from other arm muscles to the distal tendon (DT). A recent article recommended the sternum (St) as the reference electrode. In this study, we aimed to find the appropriate reference site for the deltoid CMAP.

Subjects were 12 healthy volunteers. The deltoid CMAP was recorded using Ac, St and DT references. CMAPs of the biceps brachii (BB) and triceps brachii (TB) were also recorded. In addition to stimulation at Erb's point, selective stimulations of the axillary, musculocutaneous, or radial nerve were attempted at the axilla.

The deltoid CMAPs with Ac reference had similar shapes following Erb's point (proximal) stimulation and axillary plus radial (distal) stimulation at the axilla. In contrast, CMAP using St reference was considerably smaller following proximal than distal stimulation. This difference was derived from the Ac-St lead, to which proximal muscles such as pectoralis major are supposed to contribute only following the proximal stimulation. Such a contribution can be explained by the far-field potential (FFP) theory, which suggests that the Ac-St lead can record FFPs from muscles situated between the Ac and St electrodes.

Consistency between proximal and distal stimulations is preferred for motor nerve conduction studies. We propose that Ac reference that enables selective recording from the deltoid muscle is the most appropriate way to record deltoid CMAP to date.