What is Spinal Muscular Atrophy?

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

The childhood SMAs are all autosomal recessive diseases. This means that they run in families and more than one case is likely to occur in siblings or cousins of the same generation. Parents usually have no symptoms, but still carry the gene. The gene for SMA has 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, is evident before birth or within the first few months of life. There may be a reduction in fetal movement in the final months of pregnancy. Symptoms include floppiness of the limbs and trunk, feeble movements of the arms and legs, swallowing and feeding difficulties, and impaired breathing. Affected children never sit or stand and usually die before the age of 2.

Symptoms of SMA type II usually begin between 3 and 15 months of age. Children may have respiratory problems, floppy limbs, decreased or absent deep tendon reflexes, 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.

Symptoms of SMA type III (Kugelberg-Welander disease) appear between 2 and 17 years of age, and include abnormal manner of walking; difficulty running, climbing steps, or rising from a chair; and slight tremor of the fingers.

Kennedy syndrome or progressive spinobulbar muscular atrophy may occur between 15 and 60 years of age. Features of this type may include weakness of muscles in the tongue and face, difficulty swallowing, speech impairment, and excessive development of the mammary glands in males. The course of the disorder is usually slowly progressive. Kennedy syndrome is an X-linked recessive disorder, which means that women carry the gene, but the disorder only occurs in men.

Congenital SMA with arthrogryposis (persistent contracture of joints with fixed abnormal posture of the limb) is a rare disorder. Manifestations include severe contractures, curvature of the spine, chest deformity, respiratory problems, an unusually small jaw, and drooping upper eyelids.