Spinal Muscular Atrophy

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 > 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 this 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 by the identification of biallelic pathogenic variants in SMN1 on molecular genetic testing. Increases in SMN2 copy number often modify the phenotype.

Management: Treatment of manifestations: Therapies targeted to the underlying disease mechanism include nusinersen (Spinraza^®; an antisense oligonucleotide) for the treatment of all types of SMA and onasemnogene abeparvovec-xioi (Zolgensma^®; gene replacement therapy) for the treatment of type I SMA. These targeted treatments may prevent the development or slow the progression of some features of SMA; efficacy is improved when treatment is initiated before symptom onset. It is unclear what the long-term effect of these treatments will be or if new phenotypes will arise in treated individuals.

Proactive supportive treatment by a multidisciplinary team is essential to reduce symptom severity, particularly in the most severe cases of SMA. 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. 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.

Surveillance: Presymptomatic individuals require monitoring for the development of symptoms to determine appropriate timing to initiate targeted and/or supportive therapies. 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 targeted treatment.

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 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. Carrier testing for at-risk relatives and prenatal testing for pregnancies at increased risk are possible if the diagnosis of SMA has been confirmed by molecular genetic testing in an affected family member.