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 (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.