What causes spinal muscular atrophy?
Spinal muscular atrophy (SMA) that is caused by a deletion of the SMN gene on chromosome 5 is an inherited progressive neuromuscular disorder characterized by degeneration of groups of nerve cells (motor nuclei) within the lowest region of the brain (lower brainstem) and certain motor neurons in the spinal cord (anterior horn cells). Motor neurons are nerve cells that transmit nerve impulses from the spinal cord or brain (central nervous system) to muscle or
glandular tissue. Typical symptoms are a slowly progressive muscle weakness and muscle wasting (atrophy). Affected individuals have poor muscle tone, muscle weakness on both sides of the body without, or with minimal, involvement of the face muscles, twitching tongue and a lack of deep tendon reflexes. SMA is divided into subtypes based on age of onset of symptoms and maximum function achieved.
Spinal muscular atrophy (SMA) is the second leading cause of neuromuscular disease. It is inherited as an autosomal recessive trait (a person needs to get the defective gene from both parents to be affected) and has an incidence of approximately 4 per 100,000 people. In its most severe form (SMA I), infants are born floppy with weak, thin muscles and feeding and breathing problems. Their lifespan seldom exceeds 2 to 3 years. Infants with SMA II have less severe symptoms during early infancy, but they become progressively weaker with time. Survival time with type II is longer, but the disease kills most of those affected while they are still children.
SMA III is the least severe form of the disease, and symptoms may not appear until the second year of life. Often, weakness is first noted in the shoulder muscles and proximal leg muscles. Weakness is progressive and will eventually become profound, but children with type III disease may survive into early adulthood. Rarely, SMA may begin in adulthood. This is usually a milder form of the disease. This form may be inherited in an autosomal dominant (only one copy of the gene is needed for the disease to occur) or autosomal recessive manner. Family history of spinal muscular atrophy is a risk factor for all types of the disorder.