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Spinal muscular atrophy

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What is spinal muscular atrophy?

What are the signs and symptoms of spinal muscular atrophy?

How is spinal muscular atrophy inherited?

How might spinal muscular atrophy be treated?

What is spinal muscular atrophy?

Spinal muscular atrophy is a group of inherited disorders that cause progressive muscle degeneration and weakness.[1][2] It is caused by a loss of specialized nerve cells, called motor neurons, in the spinal cord and the part of the brain that is connected to the spinal cord (the brainstem). The loss of motor neurons leads to weakness and wasting (atrophy) of muscles used for activities such as crawling, walking, sitting up, and controlling head movement. In severe cases, the muscles used for breathing and swallowing are affected.[3][4] Spinal muscular atrophy is divided into subtypes based on the severity of the disease and the age when symptoms first appear.[1][3] It is usually inherited as an autosomal recessive trait.[1]

Last updated: 4/21/2010

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.[5]

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.[1] Life span is approximately 2-6 months.[5] 

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.[6] The life span is usually less than 2 years.[5] 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.[6] About 70% of individuals with this type live to be at least 25 years of age.[5] 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.[6]

Three other types of SMA can affect people in early childhood and adulthood.[6] 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.[1] 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.[1]
Last updated: 11/28/2011

How is spinal muscular atrophy inherited?

Spinal muscular atrophy types I, II, III, IV and distal spinal muscular atrophy type 1 are inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.[3]

Finkel type spinal muscular atrophy is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder.[3]

X-linked infantile spinal muscular atrophy is inherited in an X-linked pattern. The gene associated with this condition is located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked disorders much more frequently than females. A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.[3]

Last updated: 11/28/2011

How might spinal muscular atrophy be treated?

There is currently no specific cure for spinal muscular atrophy. Infants who have a severe form of the disease frequently die of respiratory failure due to weakness of the muscles that help with breathing. Children who have milder forms will live much longer but may need extensive medical support.[1]

The current treatment for spinal muscular atrophy involves prevention and management of the secondary effects of muscle weakness and loss. Respiratory, nutritional and rehabilitation care are available. In addition, several drugs have been identified in laboratory experiments that may help some patients. Some of the drugs that are currently being investigated include: Butyrates, valproic acid, hydroxyurea, and riluzole.[1]
Last updated: 11/28/2011

  1. Learning About Spinal Muscular Atrophy. National Human Genome Research Institute (NHGRI). October 19, 2009; Accessed 4/21/2010.
  2. Kaneshiro NK, Hoch DB. Spinal muscular atrophy. MedlinePlus. December 17, 2008; Accessed 4/21/2010.
  3. Spinal muscular atrophy. Genetics Home Reference (GHR). December 2009; Accessed 5/13/2011.
  4. Russman B. Spinal Muscular Atrophy. National Organization for Rare Disorders (NORD). 2008; Accessed 4/21/2010.
  5. Thomas W Prior, Barry S Russman. Spinal Muscular Atrophy. GeneReviews. January 27, 2011; Accessed 11/28/2011.
  6. Spinal muscular atrophy. Genetics Home Reference. December 2009; Accessed 11/28/2011.

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