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.