Total Intramuscular Fat Fraction of Thigh Muscles as a Predictor of Nusinersen Efficacy in Pediatric SMA Type II and III

<b>Background/Objectives</b>: Nusinersen is a disease-modifying drug for spinal muscular atrophy (SMA) that improves motor function. However, its effects on the skeletal muscles remain unclear. This study aimed to assess the intramuscular fat fraction in patients with SMA types II and III using muscle magnetic resonance imaging (MRI) and to explore the relationship between muscle tissue, lipid metabolism, and motor function during nusinersen treatment. <b>Methods</b>: This study included seven pediatric patients with SMA types II and III who received nusinersen treatment. Muscle MRIs were performed at three time points. Images of the central thigh were used to measure the cross-sectional area (CSA) and muscle fat area, and the intramuscular fat fraction (IMFF) was calculated. The thigh muscles were categorized into three groups: quadriceps, adductor, and hamstrings. <b>Results</b>: The median (range) of total IMFF for SMA type II and III at T-0, T-2, and T-4 were 18.5 (12.6-48.4), 24.4 (10.1-61.4), and 39.0 (30.0-68.6) % and increased over time. In five patients whose motor function was evaluated, a moderate negative correlation was observed between the changes in the Hammersmith Functional Motor Score Expanded (HSFME) and IMFF (r = -0.51). No significant changes in serum triglyceride or total cholesterol levels were observed during treatment. <b>Conclusions</b>: An increase in IMFF was associated with a decline in motor function. The baseline IMFF score was related to improvements in motor function scores, suggesting that the IMFF of the thigh muscle may serve as a novel, objective, and quantitative skeletal muscle-related biomarker for predicting the effects of nusinersen on muscle tissue.