Spinal muscular atrophy (SMA) is a rare neuromuscular disorder that results in the loss of motor neurons and progressive muscle wasting. It is usually diagnosed in infancy or early childhood and if left untreated it is the most common genetic cause of infant death. It may also appear later in life and then have a milder course of the disease. The common feature is the progressive weakness of voluntary muscles, with the arm, leg, and respiratory muscles being affected first. Associated problems may include poor head control, difficulties swallowing, scoliosis, and joint contractures.
The age of onset and the severity of symptoms form the basis of the traditional classification of spinal muscular atrophy into several types.