Assessment of motor unit loss in patients with spinal muscular atrophy

Abstract

Objective: To assess motor unit (MU) changes in patients with spinal muscular atrophy (SMA) using compound muscle action potential (CMAP) scans. Methods: 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. Results: 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 (<25) in patients with normal CMAPmax (n = 12), indicating MU loss and enlarged MUs due to reinnervation. Lower D50 values were associated with decreased MUNE (P < 0.001, r = 0.68, n = 43). CMAPmax, MUNE and D50 values differed between SMA types (P < 0.001). Lower motor function scores were related to patients with lower CMAPmax, MUNE and D50 values (P < 0.001). Conclusions: The CMAP scan is an easily applicable technique that is superior to routine assessment of CMAPmax in SMA. Significance: The detection of pathological MU changes across the spectrum of SMA may provide important biomarkers for evaluating disease course and monitoring treatment efficacy.