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Genetic and Rare Diseases Information Center (GARD)

Spinal muscular atrophy


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What are the signs and symptoms of spinal muscular atrophy?

Spinal muscular atrophy (SMA) is chiefly characterized by progressive muscle weakness. Depending on the type, onset may range from before birth to adolescence or young adulthood.[1]

SMA type 0 (the prenatal form) is the most severe form and begins before birth. Usually, the first symptom of type 0 is reduced movement of the fetus that is first seen between 30 and 36 weeks of the pregnancy. After birth, these newborns have little movement and have difficulties with swallowing and breathing.[2] Life span is approximately 2-6 months.[1] 

There are 4 types of SMA that tend to affect children before the age of one (SMA type I, SMA type II, X-linked SMA and distal SMA type I). SMA I is a severe form that may be apparent at birth or the first few months of life. Features may include difficulty swallowing or breathing and inability to sit without support.[3] The life span is usually less than 2 years.[1] SMA II typically becomes apparent between 6 and 12 months of age; affected children may sit without support, although they cannot stand or walk unaided.[3] About 70% of individuals with this type live to be at least 25 years of age.[1] X-linked infantile SMA is similar to SMA I; additional features may include joint deformities (contractures) or being are born with broken bones in very severe cases. Signs and symptoms of distal SMA type I typically appear between 6 weeks and 6 months of age, although in rare cases affected individuals may not show symptoms until as late as adolescence. This type is characterized by progressive muscle weakness in the hands and feet that eventually spreads to the limbs, as well as paralysis of the diaphragm that leads to respiratory failure.[3]

Three other types of SMA can affect people in early childhood and adulthood.[3] SMA type III (called Kugelberg-Welander disease or juvenile type) is a milder form of spinal muscular atrophy than types 0, I or II. Symptoms appear between early childhood (older than age 1 year) and early adulthood. Individuals with type III are able to stand and walk without help. They usually lose their ability to stand and walk later in life.[2] SMA type IV and Finkel type occur in adulthood, usually after age 30. Symptoms of adult-onset SMA are usually mild to moderate and include muscle weakness, tremor and twitching.[2]
Last updated: 11/28/2011

  1. Thomas W Prior, Barry S Russman. Spinal Muscular Atrophy. GeneReviews. January 27, 2011; Accessed 11/28/2011.
  2. Learning About Spinal Muscular Atrophy. National Human Genome Research Institute (NHGRI). October 19, 2009; Accessed 4/21/2010.
  3. Spinal muscular atrophy. Genetics Home Reference. December 2009; Accessed 11/28/2011.
  4. Spinal muscular atrophy. Genetics Home Reference (GHR). December 2009; Accessed 5/13/2011.

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