A Study Evaluating the Effectiveness and Safety of Risdiplam Administered in Pediatric Patients With Spinal Muscular Atrophy Who Experienced a Plateau or Decline in Function After Gene Therapy

Inclusion Criteria: * \<2 years of age at the time of informed consent * Confirmed diagnosis of 5q-autosomal recessive SMA, including genetic confirmation of homozygous deletion or compound heterozygosity predictive of loss of function of the Survival of Motor Neuron 1 (SMN1) gene * Confirmed presence of two SMN2 gene copies as documented through laboratory testing * Administration of onasemnogene abeparvovec pre-symptomatically or post-symptomatically * Has received onasemnogene abeparvovec for SMA no less than 13 weeks prior to enrollment * If treated with risdiplam prior to onasemnogene abeparvovec, risdiplam treatment must not have exceeded 3 weeks and must be discontinued 1 day prior to onasemnogene abeparvovec administration. * In the opinion of the investigator, has demonstrated a plateau or decline in function post-gene therapy (with a duration of 26 weeks or less) documented by 2 individual time points in the functions as follows: swallowing AND one additional function/ability (respiratory, motor function, other) per appropriate expectation. Exclusion Criteria: * Previous or current enrolment in investigational study prior to initiation of study treatment * Any unresolved standard-of-care laboratory abnormalities per the onasemnogene abeparvovec prescribing information * Concomitant or previous administration of an SMN2-targeting antisense oligonucleotide * Concomitant or previous use of an anti-myostatin agent * Participants requiring invasive ventilation or tracheostomy * Presence of feeding tube and an OrSAT score of 0 * Hospitalization for pulmonary event within the last 2 months, or any planned hospitalization at the time of screening * Any major illness requiring hospitalization within 1 month before the screening examination or any febrile illness within 1 week prior to screening and up to first dose administration.