Spinal muscular atrophy (SMA) is a group of inherited disorders characterized by a loss of certain nerve cells called motor neurons. Motor neurons transmit nerve impulses from the brain or spinal cord (brainstem) to muscle or glandular tissue. The loss of motor neurons leads to progressive muscle weakness and muscle wasting (atrophy) in muscles closest to the trunk of the body (proximal muscles) such as the shoulders, hips and back. These muscles are necessary for crawling, walking, sitting up and head control. The more severe types of SMA can affect muscles involved in feeding, swallowing and breathing.
SMA is divided into subtypes based on age of onset and severity of symptoms. SMA types 0, I, II, III and IV are inherited as autosomal recessive genetic disorders and are associated with abnormalities (mutations) in the SMN1 and SMA2 genes. Finkel type SMA is inherited as an autosomal dominant genetic disorder and is associated with mutations in the VAPB gene.