Reference Electrode Position Affects Compound Muscle Action Potential Amplitude in Ulnar Nerve Conduction Studies Recorded From the First Dorsal Interosseous Muscle.

INTRODUCTION/AIMS: Nerve conduction studies (NCSs) of the ulnar nerve recording from the first dorsal interosseous muscle (FDI) are clinically relevant in assessing ulnar neuropathy and the split-hand index. Reference electrode (E2) position varies across normative datasets, with the base of the thumb favored to avoid an initial positive deflection in the compound muscle action potential (CMAP). While latency effects of E2 position have been described, consequences for CMAP amplitude and area have not been systematically examined in a within-subject comparison. This study aims to evaluate CMAP parameters in paired ulnar motor NCS recordings from FDI with E2 over the base of the index finger or thumb. METHODS: This retrospective study analyzed 183 ulnar motor NCS recordings from FDI in 155 patients referred for neurophysiological testing. CMAP amplitude, latency, negative peak duration, and area were recorded with E2 positioned at the base of the thumb and then the index finger. Within-subjects comparison was assessed using paired t-tests, with Bland-Altman analysis to assess agreement. RESULTS: CMAP amplitude was on average 2.8 mV lower with E2 at the thumb than at the index finger (95% CI: -3.1 to -2.4; p < 0.0001), with a corresponding reduction in CMAP area. There was no clinically meaningful effect on latency or peak duration. Bland-Altman analysis showed no proportional bias. In 40% of studies with abnormal CMAP amplitude at the thumb, repositioning E2 to the index finger normalized the amplitude. DISCUSSION: Reference electrode position substantially affects ulnar FDI CMAP amplitude, with implications for ulnar neuropathy diagnosis and split-hand index calculation.