Spinal muscular atrophy, type I

Overview

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.

SOURCE: Emory University - Department of Human Genetics in collaboration with ThinkGenetic • https://www.thinkgenetic.com/diseases/spinal-muscular-atrophy-type-i/overview/43117 • DATE UPDATED: 2016-08-31

References

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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