Assessment of motor unit loss in patients with spinal muscular atrophy

•Distal muscles in patients with spinal muscular atrophy (SMA) showed marked motor unit (MU) loss detected by the compound muscle action potential (CMAP) scan.•Severity of pathological MU changes differed between SMA types 2–4.•Pathological MU changes derived from the CMAP scan are potentially useful for follow-up in SMA. To assess motor unit (MU) changes in patients with spinal muscular atrophy (SMA) using compound muscle action potential (CMAP) scans. We performed CMAP scan recordings in median nerves of 24 treatment-naïve patients (median age 39; range 12–75 years) with SMA types 2–4. From each scan, we determined maximum CMAP amplitude (CMAPmax), a motor unit number estimate (MUNE), and D50 which quantifies the largest discontinuities within CMAP scans. Median CMAPmax was 8.1 mV (range 0.9–14.6 mV), MUNE was 29 (range 6–131), and D50 was 25 (range 2–57). We found a reduced D50 (