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, 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.

Growth parameters will be assessed by measuring body length or height (if feasible and appropriate), ulnar length (all participants), and head circumference, chest circumference, and arm circumference (all participants 3 years of age and younger) in centimeters.

For participants 2 years of age and younger, the Hammersmith Infant Neurological Exam (HINE) Sections 1 and 3 will be conducted. This standard examination (developed by \[Dubowitz and Dubowitz 1981\]) is a quantitative scorable method for assessing the neurological development of infants between 2 and 24 months of age. The examination includes assessment of cranial nerve functions, posture, movements, tone, and reflexes. The HINE Section 1 form utilized in ONWARD contains 26 items and the Section 3 form utilized contains 3 items. For HINE Section 1 items, each item is scored 0-3. For HINE Section 3 items, scoring is variable (1-4, 1-5, or 1-6). Higher scores indicate better neurological function.

For all participants \>2 years of age, standard neurological examinations, which include assessments of mental status, level of consciousness, sensory function, motor function, cranial nerve function, and reflexes, will be conducted.

The CHOP-INTEND test was designed to evaluate the motor skills of infants with significant motor weakness. It included 16 items (capturing neck, trunk, and proximal and distal limb strength), nine of which were scored 0, 1, 2, 3, or 4, five were scored as 0, 2, or 4, one was scored as 0, 1, 2, or 4, and one as 0, 2, 3, or 4 with higher scores indicating greater muscle strength and function. Total score was calculated as the sum of scores for each item. Total score ranged from 0 (worst possible score) and 64 (best possible score). The change from baseline to Day 183 in the CHOP-INTEND total score was compared to CS3B study (NCT02193074) sham control group using the joint-rank methodology to account for mortality.

An adverse event (AE) was any unfavorable \& unintended sign (including an abnormal assessment such as an abnormal laboratory value), symptom, or disease temporally associated with use of an investigational product, whether or not related to investigational product. An SAE was any untoward medical occurrence that at any dose resulted in death, in the view of Investigator, placed participant at immediate risk of death, required inpatient hospitalization or prolongation of existing hospitalization, resulted in persistent or significant disability/incapacity, resulted in a birth defect. AE and SAEs were regarded as treatment-emergent if it was present prior to receiving first dose of nusinersen in this current study and subsequently worsened in severity or was not present prior to receiving first dose of nusinersen and subsequently appeared.

Blood chemistry parameters included protein, albumin, creatinine, blood urea nitrogen, bilirubin (total and direct), alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, gamma-glutamyl transferase, glucose, calcium, phosphorus, bicarbonate, chloride, sodium, potassium, cystatin C, and creatine kinase. These parameters were flagged as low, normal, or high relative to parameter's normal range or as unknown if no result was available, by the Investigator. Here, shift to low indicates values that were normal, high or unknown at baseline and shifted to low values postbaseline. Shift to high indicates values that were normal, low or unknown at baseline and shifted to high values postbaseline. The categories with at least one participant with shift from baseline in these parameters are reported.

Hematology parameters included complete blood cell count, with differential and platelet count, and absolute neutrophil count. These parameters were flagged as low, normal, or high relative to parameter's normal range or as unknown if no result was available, by the Investigator. Here, shift to low indicates values that were normal, high or unknown at baseline and shifted to low values postbaseline. Shift to high indicates values that were normal, low or unknown at baseline and shifted to high postbaseline values. The categories with at least one participant with shift from baseline in these parameters are reported.

Urinalysis included assessments of urine total protein, specific gravity, pH, protein, glucose, ketones, bilirubin, blood, red blood cells (RBC), white blood cells (WBC), epithelial cells, bacteria, casts and crystals. These parameters were flagged as low, normal, or high relative to parameter's normal range or as unknown if no result was available, by the Investigator. Here, shift to low indicates values that were normal, high or unknown at baseline and shifted to low values postbaseline. Shift to high indicates values that were normal, low or unknown at baseline and shifted to high postbaseline. The categories with at least one participant with shift from baseline in these parameters are reported.

CSF parameters included cell count, total protein, and glucose. These parameters were flagged as low, normal, or high relative to parameter's normal range or as unknown if no result was available, by the Investigator. Here, shift to low indicates values that were normal, high or unknown at baseline and shifted to low values postbaseline. Shift to high indicates values that were normal, low or unknown at baseline and shifted to high values postbaseline. The categories with at least one participant with shift from baseline in these parameters are reported.

The ECGs were assessed by the investigator to be normal, abnormal and abnormal AE. The number of participants with ECG shifts from normal to each of the categorical values denoting an abnormal scan (abnormal not AE, abnormal AE) was assessed. Shift from baseline to worst post-baseline values were reported. The categories with at least one participant with shift from baseline in ECG are reported.

Vital sign assessment included temperature, pulse rate, systolic blood pressure, diastolic blood pressure, and respiratory rate. As pre-specified in protocol, the criteria for determining potentially clinically relevant abnormalities in vital signs included: temperature \< 36.0 and \> 38.0 degrees Celsius (C), pulse rate \< 60 and \> 100 beats per minute (bpm), systolic blood pressure \[\< 90, \> 140 and \> 160 millimeters of mercury (mmHg)\], diastolic blood pressure \< 50, \> 90 and \> 100 mmHg and respiratory rate \< 12 and \> 20 breaths per minute. The categories with at least one participant with clinically relevant vital sign abnormalities are reported.

As pre-specified in the protocol, head circumference was measured for participants with infantile-onset SMA only.

As pre-specified in protocol, chest circumference was measured for participants with infantile-onset SMA only.

As pre-specified in the protocol, arm circumference was measured for participants with infantile-onset SMA. Here, negative change from baseline indicated reduction in arm circumference.

As pre-specified in the protocol, ulnar length was measured for participants with later-onset SMA.

World Health Organization (WHO) child growth standards (2006) was used to calculate the weight for age percentile in the infantile-onset participants while the 2000 Centers for Disease Control and Prevention (CDC) Growth Charts was used to calculate the weight for age percentile for later-onset participants. Negative change from baseline indicates low weight for age percentile.

As pre-specified in the protocol, weight for length ratio was assessed only for the participants with infantile-onset SMA.

As pre-specified in the protocol, head to chest circumference ratio was assessed only for the participants with infantile-onset SMA.

Activated partial thromboplastin time was evaluated to assess safety. "Shift to low" measured change in normal, high and unknown values of aPTT at baseline to low values postbaseline. "Shift to high" measured change in normal, low and unknown values of aPTT at baseline to high values postbaseline.

Prothrombin time was evaluated to assess safety. "Shift to low" measured change in normal, high and unknown values of PT at baseline to low values postbaseline. "Shift to high" measured change in normal, low and unknown values of PT at baseline to high values postbaseline.

INR was evaluated to assess safety. "Shift to low" measured change in normal, high and unknown values of INR at baseline to low values postbaseline. "Shift to high" measured change in normal, low and unknown values of INR at baseline to high values postbaseline. The category with at least one participant with shift from baseline in INR ratio is reported.

Participants with abnormalities in neurological examinations recorded as AEs were reported.