A Study to Learn About the Effect of Nusinersen (BIIB058) Given as Injections to Children With Spinal Muscular Atrophy (SMA) Who Were Previously Treated With Onasemnogene Abeparvovec (RESPOND)

Protocol Summary

In this study, researchers will learn more about the use of nusinersen (BIIB058) in participants with spinal muscular atrophy (SMA). This study will focus on children under the age of 3 who were previously treated with the gene therapy onasemnogene abeparvovec but are still facing health challenges related to their disease. The main goal of the study is to learn about the effect nusinersen has on muscle and movement ability (motor function). The main question researchers want to answer is: - What score do participants have on the HINE Section 2 Motor Milestones test after treatment? The Hammersmith Infant Neurological Examination (HINE) Section 2 Motor Milestones is an assessment that tests movements in different positions. This includes grasping, kicking, head control, rolling, sitting, crawling, standing, and walking. Researchers will use a group of tests to study body movements, reflexes, balance, and coordination. They will also record if participants need help with breathing. Researchers will also learn more about the safety of nusinersen. They will check participants for adverse events and changes in vital signs, heart tests, and laboratory tests including blood and urine tests. The study will be done as follows: * Participants will be screened to check if they can join the study. * Each participant will receive 4 initial doses of 12 mg of nusinersen on Days 1, 15, 29, and 64 of the Treatment Period. Then, they will receive 12 mg doses once every 4 months. * The total number of doses of nusinersen will be 9. * Nusinersen will be given through a lumbar puncture, which involves injecting the drug into the fluid around the spinal cord in the lower back. * The treatment period will last for up to 95 weeks (close to 2 years). * There will be a follow-up safety period that lasts about 4 months. * In total, participants will have up to 14 study visits. Participants will stay in the study for up to 115 weeks.