The primary objective of this study is to examine the safety and tolerability of nusinersen (ISIS 396443) administered intrathecally to participants with Spinal Muscular Atrophy (SMA) who previously participated in ISIS 396443-CS2 (NCT01703988) or ISIS 396443-CS10 (NCT01780246). The secondary objective is to examine the plasma and cerebrospinal fluid (CSF) pharmacokinetic(s) (PK) of nusinersen administered intrathecally to participants with SMA who previously participated in ISIS 396443-CS2 or ISIS 396443-CS10.

This study was conducted and the protocol was registered by Ionis Pharmaceuticals, Inc.

In August 2016, sponsorship of the trial was transferred to Biogen.

Key Inclusion Criteria:

• Clinical signs attributable to Spinal Muscular Atrophy
• Satisfactory completion of dosing and all study visits in ISIS 396443-CS2 (NCT01703988) or ISIS 396443 CS10 (NCT01780246) with an acceptable safety profile, per Investigator judgement.
• Able to complete all study procedures, measurements and visits and parent/participant has adequately supportive psychosocial circumstances, in the opinion of the investigator
• Estimated life expectancy > 2 years from Screening
• Meets age-appropriate institutional criteria for use of anesthesia/sedation, if use is planned for study procedure

Key Exclusion Criteria:

• Have any new or worsening of existing condition which in the opinion of the Investigator would make the subject unsuitable for enrollment, or could interfere with the participant participating in or completing the study.
• Dosing in ISIS 396443-CS2 (NCT01703988) or ISIS 396443-CS10 (NCT01780246) within 180 days (6 months) of screening, or longer ago than 396 days (13 months) from screening
• Hospitalization for surgery (i.e. scoliosis surgery) or pulmonary event within 2 months of screening or planned during the duration of the study
• Presence of an untreated or inadequately treated active infection requiring systemic antiviral or antimicrobial therapy
• Clinically significant abnormalities in hematology or clinical chemistry parameters
• Treatment with another investigational drug, biological agent, or device within 1-month of Screening or 5 half-lives of study agent, whichever is longer. Any history of gene therapy or cell transplantation.

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