Spinal muscular atrophy (SMA) is a collection of different muscle diseases. Grouped together, it is the second leading cause of hereditary neuromuscular disease, after Duchenne muscular dystrophy.
Most of the time, a person must get the defective gene from both parents to be affected. About 4 out of every 100,000 people have the condition.
The most severe form is SMA type I, also called Werdnig-Hoffman disease. Infants with SMA type II have less severe symptoms during early infancy, but they become weaker with time. SMA type III is the least severe form of the disease.
Rarely, SMA may begin in adulthood. This is usually a milder form of the disease.
A family history of spinal muscular atrophy is a risk factor for all types of the disorder.