Clinical characteristics: Spinal muscular atrophy (SMA) is characterized by muscle weakness and atrophy resulting from progressive degeneration and irreversible loss of the anterior horn cells in the spinal cord (i.e., lower motor neurons) and the brain stem nuclei. The onset of weakness ranges from before birth to adulthood. The weakness is symmetric, proximal greater than distal, and progressive. Before the genetic basis of SMA was understood, it was classified into clinical subtypes based on maximum motor function achieved; however, it is now apparent that the phenotype of SMN1-associated SMA spans a continuum without clear delineation of subtypes. With supportive care only, poor weight gain with growth failure, restrictive lung disease, scoliosis, and joint contractures are common complications; however, newly available targeted treatment options are changing the natural history of the disease.

Diagnosis/testing: The diagnosis of SMA is established in a proband with a history of motor difficulties or regression, proximal muscle weakness, reduced/absent deep tendon reflexes, evidence of motor unit disease, and/or biallelic pathogenic variants in SMN1 identified by molecular genetic testing. Increases in SMN2 copy number often modify the phenotype.

Management: Targeted therapies: Therapies targeted to the underlying disease mechanism include risdiplam (Evrysdi^®; SMN2-directed RNA splicing modifier), nusinersen (Spinraza^®; antisense oligonucleotide), and onasemnogene abeparvovec-xioi (Zolgensma^®; gene replacement therapy) for the treatment of all types of SMA. Treatment with an SMA-specific disease-modifying treatment is most efficacious when initiated presymptomatically. The FDA has issued a black box warning about Zolgensma^®, noting the possibility of serious liver injury and acute liver failure; close monitoring of liver function prior to and in the months following infusion is indicated. These targeted treatments may prevent the development or slow the progression of some features of SMA. New phenotypes in treated individuals are arising, and long-term effects of these treatments are unknown.

Supportive care: Proactive supportive treatment by a multidisciplinary team is essential to reduce symptom severity, particularly in the most severe cases of SMA and/or in untreated individuals. When nutrition or dysphagia is a concern, placement of a gastrostomy tube early in the course of the disease is appropriate. Standard therapy for gastroesophageal reflux disease and chronic constipation is recommended. Formal consultation and frequent follow up with a pulmonologist familiar with SMA is necessary. As respiratory function deteriorates, tracheotomy or noninvasive respiratory support may be offered. Surgical repair for scoliosis should be considered based on progression of the curvature, pulmonary function, and bone maturity. Surgical intervention for hip dislocation for those with pain may be indicated.

Surveillance: Individuals with SMA require monitoring for the development of symptoms to determine appropriate timing to initiate supportive therapies. Surveillance recommendations for potential side effects and new phenotypes associated with the targeted treatments are emerging. Multidisciplinary evaluation every six months or more frequently for weaker children is indicated to assess nutritional state, respiratory function, motor function, and orthopedic status, and to determine appropriate interventions.

Agents/circumstances to avoid: Prolonged fasting, particularly in the acutely ill infant with SMA.

Evaluation of relatives at risk: It is appropriate to determine the genetic status of younger, apparently asymptomatic sibs of an affected individual in order to identify as early as possible those who would benefit from prompt initiation of disease-modifying treatments.

Pregnancy management: Women with SMA may have an increased rate of preterm birth and need for cesarean section compared to unaffected women. Women with SMA may also experience a persistent worsening of their general muscle weakness after delivery, particularly if disease-modifying therapies are discontinued due to pregnancy status. Due to the risk of respiratory failure, it is recommended that women with neuromuscular disorders, including those with SMA, obtain baseline pulmonary function prior to becoming pregnant, with frequent monitoring during pregnancy. There is limited to no data on the effects of disease-modifying treatments on the developing human fetus. However, based on animal models, risdiplam use should be avoided in pregnant women.

Genetic counseling: SMA is inherited in an autosomal recessive manner. Each pregnancy of a couple who have had a child with SMA has an approximately 25% chance of producing an affected child, an approximately 50% chance of producing an asymptomatic carrier, and an approximately 25% chance of producing an unaffected child who is not a carrier. These recurrence risks deviate slightly from the norm for autosomal recessive inheritance because about 2% of affected individuals have a de novo SMN1 pathogenic variant on one allele; in these instances, only one parent is a carrier of an SMN1 variant, and thus the sibs are not at increased risk for SMA. Ideally preconception (but also prenatal) carrier testing for all individuals in the general population and prenatal testing for pregnancies at increased risk are possible if the diagnosis of SMA has either been confirmed by molecular genetic testing in an affected family member and/or if both parents are found to be carriers of SMA on carrier screening testing.