Spinal Muscular Atrophy: Explained
Spinal muscular atrophy is a genetic disorder very similar to ALS (Lou Gehrigs disease) SMA affects the control of muscle movement. It is caused by a loss of nerve cells (called motor neurons) in the spinal cord and the part of the brain that is connected to the spinal cord. The loss of motor neurons leads to weakness and wasting of muscles used for activities such as crawling, walking, sitting up, head movement, breathing and swallowing. There are 3 types of spinal muscular atrophy, which are distinguished by the pattern of features, severity of muscle weakness, and age when the muscle problems begin.
Type I SMA is a severe form of the disorder that is evident at birth or within the first few months of life. Affected infants are developmentally delayed; most are unable to support their head or sit unassisted. Children with this type have breathing and swallowing problems that may lead to choking or gagging.
Type II SMA is characterized by muscle weakness that develops in children between ages 6 and 12 months. Children with type II can sit without support, although they may need help getting to a seated position. Individuals with this type of spinal muscular atrophy cannot stand or walk unaided.
Type III SMA typically develops between early childhood and adolescence. Individuals with type III spinal muscular atrophy can stand and walk unaided, but as the disease progresses, many affected individuals will require wheelchair assistance.
The signs and symptoms of type IV SMA often occur after age 30. Affected individuals usually experience mild to moderate muscle weakness, tremor, twitching, or mild breathing problems. Typically, only muscles close to the center of the body such as the upper arms and legs, are affected in type IV SMA.
The features of X-linked SMAappear in infancy and include severe muscle weakness and difficulty breathing. Children with this type often have joint deformities or contractures that impair movement. In severe cases, affected infants are born with broken bones. Poor muscle tone before birth may contribute to the contractures and broken bones seen in these children.