Defining When Nusinersen Starts to Work: Time to Clinical Benefit in Patients with SMA Types 1–3 from a Real-World Cohort in China

Background: 5q spinal muscular atrophy (SMA) is a hereditary neuromuscular disorder characterized by progressive muscle weakness. Nusinersen, the first disease-modifying therapy for SMA, has demonstrated efficacy in both clinical trials and real-world studies. However, the precise timing of therapeutic onset following Nusinersen administration remains unclear. Methods: This retrospective study analyzed clinical data from patients with genetically confirmed 5q SMA who received Nusinersen treatment for at least six months at Peking University First Hospital. Motor function was assessed using standardized scales prior to each dose. Results: In total, 74 patients were screened, of whom 62 were enrolled, including 14 with type 1, 29 with type 2, and 19 with type 3 SMA. Thirty-two patients completed motor function assessments. After six months of treatment, 62.5% achieved a primary clinically meaningful response (an increase of ≥4 points in CHOP-INTEND or ≥3 points in HFMSE). Seven patients (21.9%) attained or regained motor milestones. Median improvements were 6 points in CHOP-INTEND (p = 0.001), 4 points in HFMSE (p = 0.003), and 1.5 points in RULM (p = 0.045). Further analysis indicated that the available median time to treatment response was approximately 2 months. In patients with severe scoliosis or prior spinal surgery, ultrasound-guided lumbar puncture demonstrated a high success rate (94.9%). Regarding safety, intrathecal injection-related adverse events occurred in eight patients (12.9%), and no adverse events led to treatment discontinuation. Conclusions: During the loading phase, Nusinersen provides clinical benefit for the majority of patients, with a median time to therapeutic response for monitoring of approximately 2 months. Ultrasound-guided intrathecal administration is the preferred approach for individuals with complicated spinal conditions. These findings may help guide clinical expectations for physicians, patients, and caregivers.