Spinal Muscular Atrophy

What is Spinal Muscular Atrophy?

Spinal muscular atrophy (SMA) is a genetic disease characterized by progressive degeneration of motor neurons in the spinal cord. The disorder causes weakness and wasting of the voluntary muscles. This weakness is often more severe in the legs than in the arms.

Most of the childhood SMAs are inherited in an autosomal recessive manner. Parents usually have no symptoms but carry one copy of an SMA gene. The risk for each of their children to receive two copies of the SMA gene (onefrom each parent) and to have SMA is one-quarter.

Genes for SMA have been identified and accurate diagnostic tests exist. There are many types of SMA. Some of the more common types are described below.

SMA type I: Also called Werdnig-Hoffmann disease, SMA type I is evident before birth -- there may be a reduction in fetal movement during the final months of pregnancy -- or within the first few months of life. Symptoms include floppiness of the limbs and trunk, feeble movements of the arms and legs, swallowing and feeding difficulties, and impaired breathing. Children with SMA type I never sit or stand and usually die before the age of 2.

SMA type II: The disease usually becomes apparent between 3 and 15 months of age. Children with SMA type II may have respiratory problems, floppy limbs, decreased or absent deep tendon reflexes (with no kneejerk reflex), and twitching of arm, leg, or tongue muscles. These children may learn to sit but will never be able to stand or walk. Life expectancy varies.

SMA type III: Also called Kugelberg-Welander disease, SMA type III appears between 2 and 17 years of age with an abnormal way of walking; difficulty running, climbing steps, or rising from a chair; and slight tremor of the fingers.