A Study for Participants With Spinal Muscular Atrophy (SMA) Who Previously Participated in Nusinersen (ISIS 396443) Investigational Studies

Key Inclusion Criteria:

* Signed informed consent of parent or guardian and signed informed assent of participant, if indicated per participant's age and institutional guidelines.
* Completion of the index study in accordance with the study protocol or as a result of Sponsor decision (e.g., early termination of the index study) within the preceding 16 weeks

Key Exclusion Criteria:

* Have any condition or worsening condition which in the opinion of the Investigator would make the participant unsuitable for enrollment, or could interfere with the participant participating in or completing the study
* Clinically significant abnormalities in hematology or clinical chemistry parameters or electrocardiogram (ECG), as assessed by the Site Investigator, at the Screening visit that would render the participant unsuitable for participation in the study
* Participant's parent or legal guardian is not willing or able to meet standard of care guidelines (including vaccinations and respiratory syncytial virus prophylaxis if available), nor provide nutritional and respiratory support throughout the study
* Treatment with another investigational agent, biological agent, or device within one month of Screening, or 5 half-lives of study agent, whichever is longer

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.