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Spinal muscular atrophy

Other Names: Werdnig-Hoffman disease, Kugelberg-Welander disease

Spinal Muscular Atrophy is a group of conditions that affect the motor nerves of the spinal cord and brain stem. It is caused by mutations in the SMN1 gene which makes the survival motor neuron protein.

Characteristics of Spinal muscular atrophy
Spinal Muscular Atrophy (SMA) is a group of inherited conditions that affects the motor nerves and causes progressive muscle weakness. The most common types of SMA are SMA Type 1, SMA Type 2, and SMA Type 3.
Infants with SMA Type 1 usually are never able to sit by themselves, crawl or walk. Muscle weakness begins in the first few weeks to months of life. Infants with SMA Type 1 also have bulbar weakness or trouble swallowing, and trouble breathing because of muscle weakness in the chest. Infants with SMA Type 1 can develop a bell-shaped chest and contractures of the chest wall. Children with SMA Type 1 may die in early childhood because of respiratory complications.
Children with SMA Type 2 usually are able to sit independently, but are never able to walk independently. Symptoms appear between 6 and 18 months of age. Children with SMA Type 2 may have respiratory problems, but often live into adulthood.
Children with SMA Type 3 gain the ability to walk independently, but they may lose this ability later in life. Individuals with SMA Type 3 often have trouble going up or down stairs, running, and jumping.
Other features of SMA include joint contractures, scoliosis, and tremor. Individuals with SMA usually have normal intelligence.

Most individuals with SMA have a change or mutation in a gene called SMN1 that makes the Survival Motor Neuron (SMN) protein. Over 95% of individuals with SMA have a deletion (i.e. missing piece) in the same spot on both copies of their SMN1 gene. A second gene, called SMN2, is thought to act as a modifier for SMA. The SMN2 gene makes a shortened version of the SMN protein. This shortened protein cannot function and is broken down. However, approximately 10% of the time, there is a processing error of the SMN2 gene which allows for a full length SMN protein to be made. Individuals with SMA can have anywhere from 1 to greater than 5 copies of the SMN2 gene. In general, individuals with SMA who have more copies of SMN2 are stronger than individuals with SMA who have fewer copies.

Depending on the type of SMA and the level of weakness, management of SMA often involves respiratory, nutrition and orthopedic interventions. Most individuals with SMA receive physical and occupational therapies. There is currently no cure for SMA, but management is focused on maintaining current level of function and preventing muscle weakness from progressing. Surgical intervention may be needed to correct contractures or scoliosis. Standard of care guidelines are available for SMA. Clinical trials for different potential therapies are in progress.

Mode of inheritance
SMA is inherited in an autosomal recessive pattern. This means that an individual has to inherit two SMN1 mutations to be affected with SMA. Most of the time, an affected child inherits one mutation from each parent. Rarely, an affected child will inherit one mutation from one parent and the second mutation occurs brand new (de novo).

Risk to family members
If both parents are carriers, the risk to have a child with SMA is 25% (1 in 4) for each pregnancy. For a couple who has a child with SMA, the majority of the time, a second affected child will have the same type of SMA, however severity can vary with in a family. If a sibling of a child with SMA is unaffected, he/she has a 2 in 3 (66%) chance of being a carrier of SMA.

Special considerations

Families of SMA
International Spinal Muscular Atrophy Patient Registry
Genetics Home Reference: Spinal Muscular Atrophy
Medical Home Portal: Spinal Muscular Atrophy

Lefebvre, S. et al. (1997)."Correlation between severity and SMN protein level in spinal muscular atrophy." Nature Genetics 16(3): 265-269.
Prior TW, Russman BS. (Updated 27 January 2011). Spinal Muscular Atrophy. In: GeneReviews at GeneTests Medical Genetics Information Resource (database online). Copyright, University of Washington, Seattle. 1997-2013. Available at Accessed [04/02/2013].
Wang, CH. et al. (2007)."Consensus statement for standard of care in spinal muscular atrophy." Journal of Child Neurology 22(8): 1027-1049.



Created by:Tara Newcomb, MS, CGC

Edited by:Seema Jamal, MSc, LCGC

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