Electrophysiological Changes in Pediatric Spinal Muscular Atrophy: Results From an Observational Study

INTRODUCTION/AIMS: Previous studies of children with spinal muscular atrophy (SMA) have focused on the ulnar and median nerves, while lower-limb and proximal motor nerves remain insufficiently characterized. This study aimed to evaluate compound muscle action potential (CMAP) amplitudes in upper- and lower-limb motor nerves in children with SMA and changes after nusinersen treatment. METHODS: In this single-center retrospective study, CMAP amplitudes were collected from children with SMA and age-matched controls without neuromuscular disease. CMAP amplitudes of the tibial, peroneal, femoral, median, and ulnar nerves were assessed in children with SMA types 1-3. A cross-sectional analysis was conducted to assess CMAP amplitudes prior to treatment. Longitudinal changes after SMA disease-modifying therapies (nusinersen monotherapy or nusinersen plus risdiplam treatment) were evaluated. RESULTS: A total of 47 children with SMA were included. The baseline CMAP amplitudes of the peroneal, tibial, median, and ulnar nerves were the highest in type 3, followed by type 2, and lowest in type 1. Femoral nerve CMAP amplitudes were low in all SMA subtypes. At preliminary diagnosis, children with SMA had significantly reduced CMAP amplitudes for the five nerves compared with age-matched controls (n = 63, p < 0.05). After 18 months of nusinersen treatment, CMAP amplitudes showed significant increases from baseline in the peroneal, femoral, median, and ulnar nerves (p < 0.05). DISCUSSION: CMAP amplitudes can differentiate SMA disease severity and may increase after nusinersen treatment. Large-scale longitudinal studies are required to investigate CMAP amplitude as a biomarker of treatment response in patients with SMA.