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The objective of this study was to investigate the usefulness of muscle ultrasound in evaluating dissociated small hand muscle atrophy, termed ‘split hand’, and its feasibility in the diagnosis of amyotrophic lateral sclerosis (ALS).Forty-four patients with ALS, 18 normal subjects and 9 patients with other neuromuscular disorders were included in this study. The hand muscles were divided into three regions, the median-innervated lateral hand muscle group (ML), the ulnar-innervated lateral hand muscle (UL) and the ulnar-innervated medial hand muscle (UM), and the muscle echo intensity (EI) and compound muscle action potential (CMAP) were measured. We calculated the split hand index (SHI) using muscle EI (SHImEI) and CMAP (SHICMAP) for comparison among groups. The SHI was derived by dividing muscle EI (or CMAP) measured at the ML and UL by that measured at the UM.The SHImEI was significantly higher in patients with ALS (51.7±28.3) than in normal controls (29.7±9.9) and disease controls with other neuromuscular disorders (36.5±7.3; P<0.001), particularly in upper limb-onset ALS (66.5±34.0; P<0.001). Receiver operating characteristic curve analysis indicated that the SHImEI had significantly better diagnostic accuracy than the SHICMAP.The SHImEI was more sensitive in evaluating dissociated small hand muscle atrophy compared with the SHICMAP and may be a reliable diagnostic marker for differentiating ALS from other neuromuscular disorders and healthy controls.