OBJECTIVES: To increase the specificity of motor unit potential (MUPs) detection by using ultrasound guided electromyography (USG-EMG) in patients with muscle plegia due to traumatic nerve lesions.METHODS: Forty-six patients with recent nerve trauma underwent baseline standard EMG (ST-EMG) evaluation with evidence of absent MUPs. In 41 of them, ST-EMG was repeated after 2-3 months (T1) and the patients were accordingly divided in two groups: ST-EMG+ (if MUPs were detected) or ST-EMG- (MUPs not detected). Then, ST-EMG- patients underwent muscle ultrasound evaluation (M-US) and, if isles of muscular contractility were found, they also had USG-EMG. The same protocol was repeated 4-6 months after baseline (T2).RESULTS: At T1, 22/41 patients were ST-EMG+. While 19/41 were ST-EMG-; 9 of these patients had M-US consistent with residual muscular activity, for that reasons underwent USG-EMG with 7 of 9 demonstrating MUPs (at T2 all of these 7 patients resulted ST-EMG). In the other 2 patients, we found no MUPs at T1 but they became ST-EMG+ or USG-EMG positive at T2. The remaining 10 ST-EMG- patients had no EMG or US evidence of muscle contraction at T1, but at T2 2 of 10 became ST-EMG+ and 2 had USG-EMG showing MUPs. In the remaining 6 patients still M-US negative at T2, complete denervation was diagnosed. Concerning the 22 patients who were ST-EMG+ at T1, all but one showed increase of MUPs at T2.CONCLUSIONS: In this study, we demonstrated the utility of US guidance when performing EMG evaluation in locating isles of muscular contractility in patients who have no detectable MUPs on EMG after nerve trauma.SIGNIFICANCE: USG-EMG significantly increases the specificity of needle EMG allowing earlier detection of MUPs.