Spinal Muscular Atrophy (SMA)
Spinal Muscular Atrophy (SMA) is a neuromuscular disease characterized by degeneration of the spinal cord motor neurons and by atrophy of skeletal muscles. The incidence is 1 in 10.000 live births, representing the most common genetic cause of infant death. Four types are described: the most prevalent is type I, which is the most severe and affects half of the patients with SMA. Children with SMA I usually present with severe respiratory failure at birth or during the first months of life.
Children affected by SMA II (i.e. the intermediate form) are able to maintain a sitting position, though they are never able to stand and walk. These patients often suffer from respiratory issues and scoliosis. Most SMA II patients live well into adulthood.
SMA III and IV are less severe: the onset is often after 18 months of age (SMA III) up to the third decade of life (SMA IV). Patients are able to walk, though this ability may become gradually affected.
The disease is autosomal recessive caused by a genetic defect in the SMN1 and SMN2 genes, with insufficient production of SMN protein. Parents are usually asymptomatic carriers and may transmit the disease in 25% of siblings.