Ulnar neuropathy across the elbow (UNE) is a type of upper extremity mononeuropathy commonly referred for electrodiagnostic testing (EDX). The purpose of this descriptive normative study on healthy adults was to determine normal ulnar motot nerve conduction values across the elbow using the adductor pollicis (AP) muscle as a recording technique. The AP recording site was selected as the focus of this study based on frequent clinical observation of weakness of this muscle in suspected cases of UNE and the lack of information about recording from this muscle. Comparisons were also made to standard recording techniques using the abductor digiti minimi (ADM) and first dorsal interosseous (FDI) muscles. Methods: After signed consent 88 upper limbs in 44 healthy subjects were studied with a standardized battery of ulnar NCV tests performed when using surface stimulation and recording from the AP, FDI and ADM muscles. Conduction velocity was computed for the across elbow and forearm segments and amplitudes were recorded from three stimulation sites; wrist, below elbow and above elbow. A pain history and a battery of clinical tests were administered to measure hand strength and point tenderness. Results: The AP muscle recording site yielded compound muscle action potentials (CMAP) with consistent and reproducible amplitudes and waveforms. Normal values were computed for ulnar motor conduction velocity across the elbow and amplitude measurements for each of the three muscle sites. The null hypothesis was accepted as no statistical difference was found comparing elbow conduction velocity values recorded from the three muscles. Mean (SD) elbow conduction velocity for the AP, FDI and ADM recording sites was 60.9 (6.1), 61.1 (7.0) and 61.8 (7.0) m/s, respectively. Using the traditional mean – 2 SD method for computing the absolute lower limits of normal (LLN) determined cutoff values for elbow conduction velocity of 48,47 and 48 m/s using the AP, FDI and ADM, respectively. The 95th percentile method for the LLN produced higher cut off value for each muscle site at 51, 50 and 51 M/S for the AP, FDI and ADM, respectively. Higher CMAP amplitudes recorded on wrist compared to elbow stimulation correlated with the presence of an MGA innervation patter. This pattern appeared most often when recording from the AP muscle. Pain history and clinical tests of hand strength and elbow and hand tenderness revealed no correlation to elbow conduction velocity. Conclusion: Ulnar motor nerve conduction velocity across the elbow and forearm is the same when recording from the AP, FDI and ADM muscles. The normal values for ADM and FDI were comparable to those of prior reports. Conduction velocity and amplitude reference normals were determined for the AP muscle. Use of the alternate AP muscle site in clinically positive cases of UNE, particularly in the presence of AP muscle weakness, may help to identify conduction defects at the elbow in the case of selective ulnar motor fiber involvement and when traditional tests using the ADM muscle are normal. In addition to the AP muscle technique may enhance identification of the MGA anomalies with or without elbow conduction defects. A future study on a patient population with confirmed UNE is suggested to determine the sensitivity and specificity of the AP muscle recording technique and to assess its clinical utility in early identification of UNE pathology.
