Motor Unit Patterns Correlate With Severity in Symptomatic Patients With Spinal Muscular Atrophy

Abstract

BACKGROUND: We systematically evaluated the different components of the motor unit using an integrated set of noninvasive electrophysiological techniques across a broad spectrum of disease severity in symptomatic adolescents and adults with spinal muscular atrophy types 1-4. METHODS: We performed detailed electrophysiological mapping of the median nerve, including the compound muscle action potential scan, and repetitive nerve stimulation, in 104 genetically confirmed patients with SMA (aged ≥ 12 years, types 1c-4) before the start of DMT. We compared data to a reference group of 65 healthy controls. RESULTS: Motor unit patterns were significantly altered in patients with SMA, showing severe motor unit loss and enlarged motor units. Distinct patterns reflected disease severity, independent of age or disease duration. Patterns were characterized by varying proportions of enlarged motor units relative to motor unit number, with significantly reduced motor unit number and high contributions of enlarged units in advanced disease stages. Neuromuscular junction (NMJ) dysfunction (≥ 10% decrement) was present in 13%-38% of patients, irrespective of SMA severity. In line with these findings, clinical motor function scores correlated with greater motor unit loss and higher contributions of enlarged motor units. CONCLUSIONS: We identified altered motor unit patterns and NMJ function in patients with SMA, with distinct patterns across SMA severity independent of age or disease duration. These measures may serve as complementary biomarkers for disease severity in patients with SMA. TRIAL REGISTRATION: Dutch registry for clinical studies and trials (www.toetsingonline.nl): NL72562.041.20 (March 26, 2020).