A Study to Learn About the Effect of Higher Doses of Nusinersen (BIIB058) Given as Injections to Participants With Spinal Muscular Atrophy (SMA) Who Were Previously Treated With Risdiplam (ASCEND)

In this study, researchers will learn more about the use of a higher dose of nusinersen (BIIB058) in participants with spinal muscular atrophy (SMA). This study will focus on teenagers and adults who are unable to walk on their own and who have previously taken another drug for SMA called risdiplam. The main goal of this study is to learn about the effect of high dose (HD) nusinersen on muscle and movement ability (motor function) in SMA. The main question that researchers want to answer is: \- How do the scores of a movement test called the Revised Upper Limb Module change from the start of treatment? The Revised Upper Limb Module is a test used to measure a participant's ability to do specific tasks that involve their shoulders, arms, wrist, elbows, and hands. It measures the changes in their abilities over time. Researchers will also learn more about the safety of HD 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. * After screening, participants will enter the Core Treatment period. * At the start of the Core Treatment period, they will receive 2 "loading" doses of nusinersen. These are 50 mg doses of nusinersen given 2 weeks apart. * Afterwards, they will continue to receive "maintenance" doses of nusinersen once every 4 months. These doses will be 28 mg. * The Core Treatment period will last about 2 years, with a follow-up visit 4 months after the last dose. * Participants who complete the Core Treatment period will have the option to continue receiving 28 mg of nusinersen in the Long-Term Extension (LTE) period for about 2 years. There will also be a follow-up visit 4 months after the last dose. * Nusinersen will be given through a lumbar puncture, which involves injecting the drug into the fluid around the spinal cord in the lower back. * In tota