Assessment of motor unit loss in patients with spinal muscular atrophy.

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 (CMAP), a motor unit number estimate (MUNE), and D50 which quantifies the largest discontinuities within CMAP scans.

Results: Median CMAP 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 CMAP (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). CMAP, MUNE and D50 values differed between SMA types (P < 0.001). Lower motor function scores were related to patients with lower CMAP, MUNE and D50 values (P < 0.001).

Conclusions: The CMAP scan is an easily applicable technique that is superior to routine assessment of CMAP 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.