The Motor Unit Number Index of Subclinical Abnormality in Amyotrophic Lateral Sclerosis

Purpose: Diagnosis of amyotrophic lateral sclerosis (ALS) at an early stage is challenging, thus making the enrollment of these patients in clinical trials infeasible. In this study, we investigated the potential usability of motor unit number index (MUNIX) to detect denervation of clinically intact muscles of ALS patients. Methods: Thirty-two first dorsal interosseous muscles of 26 ALS patients were evaluated with both MUNIX and needle electromyography. Results: The mean MUNIX value of first dorsal interosseous muscles was 131 in the control group, whereas it was 48, 34, 15, and 8 for Medical Research Council scales of 5, 4, 3, and 2, respectively, in the ALS patients. The optimal cutoff point gave a sensitivity of 0.89 and a specificity of 1.0. Among 9 intact first dorsal interosseous muscles of the ALS patients, 8 showed MUNIX values below the cutoff point, whereas only 2 first dorsal interosseous muscles showed denervation on needle electromyography. Conclusions: MUNIX could serve as a sensitive technique to detect denervation of clinically intact muscles of ALS patients.