Effect of reference electrode position on the compound muscle action potential (CMAP) onset latency

Area of Science:

• Neuroscience
• Clinical Electrophysiology

Background:

• Compound muscle action potential (CMAP) onset latency is a key metric in motor nerve conduction studies (NCS).
• It is traditionally interpreted as the arrival time of nerve impulses at the muscle, reflecting the fastest motor nerve fiber conduction velocity.
• The influence of electrode placement, particularly the reference electrode (E2), on CMAP latency is not fully elucidated.

Purpose of the Study:

• To investigate the effect of reference electrode (E2) position on CMAP onset latency.
• To determine the variability in CMAP onset latency introduced by altering E2 placement during NCS.
• To assess the implications of these findings for NCS interpretation and reference values.

Main Methods:

• Motor nerve conduction studies (NCS) were performed on 20 normal hands.
• Median, ulnar, and deep ulnar motor (DUM) nerves were tested.
• Both traditional bipolar and experimental monopolar montages (E2 referenced to the contralateral hand) were utilized, with systematic variation of E2 position.

Main Results:

• Alterations in E2 position caused measurable variations in CMAP onset latency across all tested nerves.
• Specific latency variations observed: 0.1-0.5 ms (median nerve), 0.1-0.3 ms (ulnar nerve), and 0.1-1.5 ms (DUM nerve).
• These results demonstrate that E2-recorded potentials are significant and electrode positioning directly affects motor onset latency.

Conclusions:

• The position of the reference electrode (E2) significantly influences CMAP onset latency.
• Current interpretations of CMAP latency may need revision due to the impact of E2 placement.
• Findings necessitate a re-evaluation of established reference values and the physiological interpretation of NCS waveforms.