A Study to Learn About the Long-Term Safety of Higher Doses of Nusinersen (BIIB058) Given as Injections to Participants With Spinal Muscular Atrophy (SMA) Who Took Part in an Earlier Nusinersen Trial (ONWARD)

In this study, researchers will learn more about the use of nusinersen (BIIB058) in participants with spinal muscular atrophy (SMA). This study is an extension study and will enroll only those participants who have completed treatment in the parent study, 232SM203. The main goal of the study is to learn about the long-term safety of nusinersen. The main questions researchers want to answer are: * How many participants have adverse events and serious adverse events during the study? * How do the results of electrocardiograms (ECGs), vital signs, and laboratory tests including blood and urine tests change after treatment? * How many participants have a low platelet count after treatment? * How many participants had a change in the time it took for their heart to recharge between beats after treatment? * How does each participant's height and other measures of growth change after treatment? * How much do the results of neurological exams that check movement, reflexes, and brain function change after treatment? Researchers will also learn about the effect of nusinersen on mobility using various tests. They will study body movements, reflexes, balance, and coordination. They will also record if participants need help with breathing. The 232SM302 study will be done as follows: * Participants will be screened to check if they can join the study. * Participants will receive their 1st dose of nusinersen in this study about 4 months after their final dose in the parent study. * Each participant will receive nusinersen once every 4 months during the treatment period. * 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 64 months (1921 days). * There will be a follow-up safety period that lasts from 4 to 8 weeks. * In total, participants will have up to 19 study visits. Participants will stay in the study for close to 6 years.