This study is for people with Spinal Muscular Atrophy (SMA), a condition that makes muscles weak. It will test a higher dose of a medicine called nusinersen in people who have taken another SMA drug, risdiplam. Nusinersen helps improve motor function, which is how well you can move your arms and hands. The goal is to see if higher doses help more.

Participants will get two initial "loading" doses of 50 mg each and then 28 mg doses every four months for about two years. If you continue, there’s an option to get more doses for another two years. Nusinersen is given through a lumbar puncture, an injection into the lower back. There will be up to 18 study visits over 4.5 years.

• Study length: about 4.5 years if all phases are completed.
• Frequent visits: up to 18 visits during the study.
• Monitoring safety: includes heart, blood, and urine tests.

Participants must have SMA, be aged 15-50, and have used risdiplam before. They must stop risdiplam to join. The study checks for side effects and how nusinersen affects muscle movement.