Spinal muscular atrophy, type I
What is spinal muscular atrophy?
A spinal muscular atrophy (SMA) is one of a group of genetic conditions that affect the nerves, brain, and muscles in the body. In SMA, the nerves cells (motor nuclei) in the lowest region of the brain (lower brainstem) and nerve cells in the spinal cord (motor neurons) become more and more damaged and are unable to work correctly. The first symptoms of the nerve damage are usually "floppy" muscles that mean people can no longer sit up or use their arms and legs how they would like to. Muscles will continue to become weaker and weaker, slowly breaking down. This is called atrophy. As the disease gets worse, people will have increasing problems eating, swallowing, and breathing.
The main subtypes, SMA 0-4 are based on the age of onset of symptoms and the course and progression of the disease. SMA represents a continuum or spectrum of disease with a mild end and a severe end. This spectrum will range from early death in infancy to people who live normal adult lives with mild weakness. SMA0 patients are extremely weak at birth, require immediate artificial ventilation and will never breathe independently. Werdnig-Hoffmann disease, which is also known as proximal spinal muscular atrophy type 1 (SMA1), refers to infants with onset before 6 months of age. These infants have muscle weakness and breathing problems. SMA II patients will show symptoms prior to age 1 year, will sit but never stand or walk on their own. SMA III patients (Kugelberg-Welander disease) will show symptoms after age 1; the amount of muscle weakness in these children will vary. SMA IV patients will not develop symptoms much before age 10 years.
These disorders are sometimes called proximal SMAs. Proximal refers to the muscles that are closest to the body such as the upper arms or upper legs. These muscles are usually affected first in SMA. The proximal forms of SMA are inherited as an autosomal recessive trait and are cause alterations (mutations) in the SMN1 (survival motor neuron 1) gene on chromosome 5.
- Spinal Muscular Atrophy. Genetics Home Reference website. Accessed August 28, 2016. https://ghr.nlm.nih.gov/condition/spinal-muscular-atrophy
- Prior TW, Russman BS. Spinal Muscular Atrophy. GeneReviews website. Accessed August 28, 2016. http://www.ncbi.nlm.nih.gov/books/NBK1352/
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Are there other names for spinal muscular atrophy?
Spinal muscular atrophy as a group of diseases is not known by any other names. However, the individual subtypes may be known by different names. This is as follows:
- Spinal muscular type I, or SMAI, or Werdnig-Hoffman syndrome. This form is sometimes called the infantile form.
- Spinal muscular type II, or SMAII. This form is sometimes called the intermediate form.
- Spinal muscular type III, or SMAIII, or Kugelberg-Welander syndrome. This form is sometimes called the juvenile form.
- Spinal muscular type IV, or SMAIV. This form is sometimes called the adult form.
Sometimes SMAI is written as SMA1, with Arabic numerals instead of Roman numerals.
How common is spinal muscular atrophy?
Spinal muscular atrophy (SMA) is a rare disorder. Rare disorders often go unrecognized or misdiagnosed and the incidence and prevalence of individual rare disorders is often unclear or unknown. Prevalence is the measurement of all individuals affected by the disease at a particular time. Incidence is the number of new people with a disorder or newly-diagnosed. SMA is estimated to affected about 1 in 4,000 to 7,000 people. The most common form of the disorder is SMA I, or Werdnig-Hoffman syndrome. It accounts for more than 90% of people with SMA and is estimated to affected about 1 in 6,000 to 10,000 infants. As many as 1 in 40 people may be a carrier for the disorder. SMA is second most common autosomal recessive disorder after cystic fibrosis.
- Kolb SJ, Kissel JT. Spinal muscular atrophy: a timely review. Arch Neurol. 2011;68(8):979-84.
- Spinal Muscular Atrophy. The National Organization for Rare Disorders website. Accessed August 29, 2016. http://rarediseases.org/rare-diseases/spinal-muscular-atrophy/