A Study to Assess the Safety, Tolerability, Efficacy, Pharmacokinetics, and Immunogenicity of Intravenous Administration of ARGX-119 in Pediatric Participants Aged 5 to Less Than 18 Years With Spinal Muscular Atrophy

Inclusion Criteria: * Is aged ≥5 to \<18 years when completing the informed consent process, defined as providing informed assent according to local regulations and having a parent or guardian sign the ICF, and can comply with protocol * requirements. * Has documented historical genetic diagnosis of 5q-SMA. * Currently receiving a stable SMA treatment regimen (nusinersen or risdiplam) and/or have a history of onasemnogene abeparvovec treatment * Must be able to walk at least 50 meters without walking aids in the 6MWT at screening Exclusion Criteria: * Known medical condition that would interfere with an accurate assessment of SMA, confound the results of the study, or put the participant at undue risk, as assessed by the investigator * Recent major surgery, except spinal fusion, within 3 months of screening or intends to have major surgery during the study * Current or previous administration of antimyostatin therapies in the past 6 months * Severe scoliosis (defined as curvature \>40°) and/or contractures at screening. o History of spinal fusion within 6 months before screening or planned during the study * Respiratory insufficiency, defined by the medical necessity for invasive or noninvasive ventilation for daytime treatment while awake. Ventilation used overnight or during daytime naps is acceptable.