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Spinal Muscular Atrophy.


Prior TW1, Finanger E2.


GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2019.
2000 Feb 24 [updated 2016 Dec 22].

Author information

Department of Pathology, Ohio State University, Columbus, Ohio
Division of Neurology and Pediatrics, Oregon Health and Science University, Portland, Oregon



Spinal muscular atrophy (SMA) is characterized by muscle weakness and atrophy resulting from progressive degeneration and 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 adolescence or young adulthood. The weakness is symmetric, proximal > distal, and progressive. Before the genetic basis of SMA was understood, it was classified into clinical subtypes; however, it is now apparent that the phenotype of SMN1-associated SMA spans a continuum without clear delineation of subtypes. Poor weight gain with growth failure, restrictive lung disease, scoliosis, joint contractures, and sleep difficulties are common complications.


The diagnosis of SMA is established in a proband with a history of motor difficulties, evidence of motor unit disease on physical examination, and identification of biallelic pathogenic variants in SMN1 on molecular genetic testing. Increases in SMN2 copy number often modify the phenotype.


Treatment of manifestations: When nutrition is a concern in SMA, placement of a gastrostomy tube early in the course of the disease is appropriate. Formal consultation with a pulmonologist familiar with SMA is indicated. As respiratory function deteriorates, tracheotomy or noninvasive respiratory support is offered. Surgical repair for scoliosis should be considered based on progression of the curvature, pulmonary function, and bone maturity. Surveillance: Evaluation every six months or more frequently for weaker children to assess nutritional state, respiratory function, and orthopedic status.


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.

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