This clinical trial is focused on studying the long-term safety and effectiveness of a treatment called apitegromab in patients with Spinal Muscular Atrophy (SMA). SMA is a genetic condition that leads to muscle weakness and shrinking. The study specifically involves patients with Type 2 and Type 3 SMA who have already participated in previous trials of apitegromab. Apitegromab is administered as an infusion, which means it is given directly into the bloodstream through a vein.

The purpose of this study is to evaluate how well patients tolerate apitegromab over an extended period. Participants will receive regular infusions of the medication and will be monitored for any side effects or changes in their condition. The study will track various health measures to understand the impact of the treatment on muscle function and overall health. Some participants may receive a placebo, which is a substance with no active medication, to compare the effects of apitegromab.

Throughout the study, participants will have regular check-ups and assessments to ensure their safety and to gather information on how the treatment is working. The study aims to provide valuable insights into the long-term use of apitegromab for managing SMA, helping to improve future treatment options for individuals with this condition.