Reference Electrode Placement in First Dorsal Interosseous (FDI) Muscle during Ulnar Nerve Conduction Studies: A Comparative Evaluation

Salim Hirani*

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. Recording from FDI muscles presents a challenge in placing the reference electrode due to its positive deflection. This study investigates the E2 recorded signal and its effect on CMAP measurements when E2 electrode is placed at different sites. 

The aim of this research is to determine the optimal position for placing the reference electrode in FDI muscles.

Method: A total of 46 hands were included in this study. Data collection adhered to the extensive and detailed descriptions provided in various research papers. The tests were conducted by a qualified clinical physiologist specializing in Neurophysiology, utilizing a Keypoint 9033A07 machine, in accordance with the Ulnar Nerve Screening Protocol (Protocol 1.1, 2020). All data were recorded numerically to ensure methodological reliability. The CMAP was recorded using the active electrode on the muscle belly and 5 different E2 electrodes placed at distal sites, including the tendon of the FDI muscles at the base of digit II, over the thumb, the tendon of ADM muscles at the base of digit V, Radial pathways at the wrist and tendon for FDI muscles in other hand.

Result: Out of 46 hands were tested for the Nerve Conduction Study (NCS) by placing reference electrodes in five different places while recording ulnar nerves from FDI muscles i.e. Tendon of the FDI muscles to the base of digit II shows positive deflection in all hands with amplitudes ranging from 6 to 15 mV, over the middle of the thumb shows the baseline slightly elevated, impacting distal motor latency calculations with amplitude between 5-8mV, tendon of ADM muscles at the base of digit V shows clear baseline for accurate distal motor latency with the higher amplitude rage 10-18mV, radial pathways of the wrist shows slightly elevated distal motor latency with the amplitude range between 5-10mV and to record from tendon of the FDI muscles in other hand placing the reference electrode shows no clear baseline distal latency with the amplitude range between 5-10 mV.

Conclusion: This study shows that recording the best and clearest response by placing the reference electrode at the tendon of ADM muscles at the base of digit V while recording from FDI muscles of the ulnar nerve is more reliable compared to other four areas to get the maximum amplitude. It also shows that distal motor latency in all placements is comparable.

Keywords:

Published on: Apr 18, 2025 Pages: 1-6

Full Text PDF Full Text HTML DOI: 10.17352/jnnsd.000064
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