A Study to Learn About the Effect of Nusinersen (BIIB058) Given as Injections to Children With Spinal Muscular Atrophy (SMA) Who Were Previously Treated With Onasemnogene Abeparvovec (RESPOND)

Number of Participants with Adverse Events (AEs) and Serious Adverse Events (SAEs)

An AE is any untoward medical occurrence in a participant administered a pharmaceutical product and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (including an abnormal assessment such as an abnormal laboratory value), symptom, or disease temporally associated with the use of a medicinal (investigational) product, whether or not related to the medicinal (investigational) product. An SAE is any untoward medical occurrence that at any dose results in death, in the view of the Investigator, places the participant at immediate risk of death, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, results in a birth defect or is a medically important event.

Time frame: Up to Day 778

Number of Participants with Change from Baseline in Clinical Laboratory Parameters

Time frame: Up to Day 778

Number of Participants with Change from Baseline in Electrocardiograms (ECGs)

Time frame: Up to Day 778

Number of Participants with Change from Baseline in Vital Signs

Time frame: Up to Day 778

Number of Participants who Achieved Motor Milestones as Assessed by World Health Organization (WHO) Criteria

The motor milestones as defined by WHO criteria includes the following six test items: sitting without support, hands-and-knees crawling, standing with assistance, walking with assistance, standing alone, and walking alone.

Time frame: Up to Day 778

Change from Baseline in Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP INTEND) Score

The CHOP INTEND test is designed to evaluate the motor skills of infants with significant motor weakness. It includes 16 items (capturing neck, trunk, and proximal and distal limb strength) structured to move from easiest to hardest with the grading including gravity eliminated (lower scores) to antigravity movements (higher scores). All item scores range from 0-4. The total score ranges from 0-64, with higher scores depicting better response.

Time frame: Up to Day 778

Change from Baseline in Hammersmith Functional Motor Scale - Expanded (HFMSE) Score

The HFMSE is a tool used to assess motor function in children with SMA. The original 20 item Hammersmith Functional Motor Scale (HFMS) was expanded to include 13 additional items to improve sensitivity for the higher functioning ambulant population. Participants will be asked to complete a specific movement and are then graded on the quality and execution of that movement. Higher scores indicate higher levels of motor ability. The overall score is the sum of the scores for all activities, with a maximum score of 66 with higher scores depicting better ability to perform activities.

Time frame: Up to Day 778

Change from Baseline in Revised Upper Limb Module (RULM) Score

The RULM is developed to assess upper limb functional abilities participants with SMA. This test consists of upper limb performance items that are reflective of activities of daily living. The RULM is scored from 0 to 37 points, with higher scores indicating better function.

Time frame: Up to Day 778

Time to Death or Permanent Ventilation

Permanent ventilation is defined as tracheostomy or ≥16 hours ventilation/day continuously for \>21 days in the absence of an acute reversible event.

Time frame: Up to Day 778

Change From Baseline in Cerebrospinal Fluid (CSF) Levels of Neurofilament Light Subunit (NF-L)

Time frame: Up to Day 659

Change From Baseline in Plasma Levels of NF-L

Time frame: Up to Day 778