A Study to Assess the Efficacy, Safety and Pharmacokinetics of Nusinersen (ISIS 396443) in Infants With Spinal Muscular Atrophy (SMA)

Key Inclusion Criteria:

   - Genetic documentation of 5q SMA (homozygous gene deletion or mutation)

   - Onset of clinical signs and symptoms consistent with SMA at ≥ 21 days and <6 months
   (180 days) of age

   - At study entry, receiving adequate nutrition and hydration (with or without
   gastrostomy), in the opinion of the Site Investigator

   - Body weight >5th percentile for age using Center of Disease Control and Prevention
   (CDC) guidelines

   - Medical care meets and is expected to continue to meet guidelines set out in the
   Consensus Statement for Standard of Care in SMA (Wang et al. 2007), in the opinion of
   the Site Investigator

   - Gestational age of 35 to 42 weeks and gestation body weight ≥2 kg

   - Reside within approximately 9 hours ground-travel distance from a participating study
   center for the duration of the study. Residence >2 hours ground-travel distance from a
   study center must obtain clearance from the Site Investigator and the study Medical
   Monitor

   - Able to complete all study procedures, measurements and visits and parent or
   guardian/participant has adequately supportive psychosocial circumstances, in the
   opinion of the Site Investigator

Exclusion Criteria:

   - Hypoxemia (O2 saturation awake <96% or O2 saturation asleep <96%, without ventilation
   support)

   - Presence of an untreated or inadequately treated active infection requiring systemic
   antiviral or antimicrobial therapy at any time during the screening period

   - History of brain or spinal cord disease that would interfere with the lumbar puncture
   (LP) procedures, CSF circulation, or safety assessments

   - Presence of an implanted shunt for the drainage of cerebrospinal fluid (CSF) or an
   implanted central nervous system (CNS) catheter

   - History of bacterial meningitis

   - Clinically significant abnormalities in hematology or clinical chemistry parameters,
   as assessed by the Site Investigator, at screening that would render the participant
   unsuitable for inclusion

   - Treatment with another investigational drug (e.g., albuterol, riluzole, carnitine,
   creatine, sodium phenylbutyrate, salbutamol, valproate, hydroxyurea etc), biological
   agent, or device within 90 days prior to enrollment or anytime during the study. Any
   history of gene therapy or cell transplantation

   - The participants parent(s) or legal guardian(s) is unable to understand the nature,
   scope, and possible consequences of the study, or does not agree to comply with the
   protocol defined schedule of assessments

   - Ongoing medical condition that according to the Site Investigator would interfere with
   the conduct and assessments of the study. Examples are medical disability other than
   SMA that would interfere with the assessment of safety or would compromise the ability
   of the participant to undergo study procedures

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