Reverse Split Hand as a Neurophysiological Hallmark of Spinal Muscular Atrophy

<b>Objective:</b> Motor unit number estimation (MUNE) methods are crucial for estimating lower motor neuron loss in motor neuron diseases. The MScanFit MUNE (MScanFit) is a novel method that estimates MUNE values from compound motor action potential (CMAP) scans, demonstrating high sensitivity and reproducibility in detecting motor unit loss in amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA). In this study, we aimed to characterize the pattern of motor unit loss in the hand intrinsic muscles of SMA patients compared to ALS patients and healthy controls (HC) using MScanFit MUNE. <b>Methods:</b> Patients diagnosed with ALS, adult SMA patients, and HC were prospectively enrolled. MScanFit examinations were performed on the abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles. To focus on the different patterns of motor neuron degeneration in the intrinsic hand muscles, the ratio of CMAP amplitude of APB to ADM (CMAP ratio) and the ratio of MUNE values of APB to those of the ADM muscle (MUNE ratio) were calculated. <b>Results:</b> The study included 46 ALS patients, 16 SMA patients, and 23 HC. MScanFit MUNE revealed distinct patterns of motor unit degeneration in SMA patients, notably more severe in the ADM than in the APB muscle, indicating a "reverse" split-hand phenomenon. Both CMAP and MUNE ratios demonstrated high diagnostic accuracy in distinguishing ALS from SMA, with the MUNE ratio performing better. <b>Conclusions:</b> MScanFit MUNE is a valuable tool for exploring distinct patterns of motor neuron degeneration in patients with different types of motor neuron diseases.