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.

Kennedy syndrome: Also known as progressive spinobulbar muscular atrophy, Kennedy syndrome has its clinical onset between 15 and 60 years of age. It is inherited in an X-linked recessive manner. Women carry the gene on one of their two X chromosomes, but the disorder only occurs in their sons. The risk to each son of a carrier mother is one-half to receive the gene and manifest the disease. Features may include weakness of muscles in the tongue and face, difficulty swallowing, speech impairment, and excessive development of the mammary glands in males. The disorder is slowly progressive.

Congenital SMA with Arthrogryposis: This is a rare disorder characterized by persistent contracture of joints (arthrogryposis) evident at birth. Features include the severe contractures, curvature of the spine, chest deformity, respiratory problems, an unusually small jaw, and drooping upper eyelids.

Adult SMA: This disorder may begin between 40 and 60 years of age and progresses rapidly, with an average life expectancy of about 5 years from the onset of symptoms. Most cases prove to be variants of amyotrophic lateral sclerosis (ALS, commonly called Lou Gehrig's disease). Symptoms include progressive limb weakness and weakening of the muscles, difficulty speaking and swallowing, and respiratory problems.

Is there any treatment for SMA?

Treatment of all forms of SMA is symptomatic and supportive and includes treating pneumonia, curvature of the spine, and respiratory infections, if present. Also, physical therapy, orthotic supports,and rehabilitation are useful. Genetic counseling is imperative.

What is the prognosis for SMA?

The prognosis for individuals with SMA varies depending on the type of SMA and the degree of respiratory function. The patient's condition tends to deteriorate over time.

What research is being done on SMA?

Researchers have found specific genes that, when mutated, cause SMA. Several animal models of the disease have been developed as well as tests that can determine SMA gene function. This allows scientists to screen drugs that may be useful in treating SMA.

Source: Based in part on information from the National Institutes of Health (NIH).

Last Editorial Review: 7/7/2004