muscular dystrophy

The Columbia Encyclopedia, 6th ed.

muscular dystrophy

muscular dystrophy (dĬs´trōfē), any of several inherited diseases characterized by progressive wasting of the skeletal muscles. There are five main forms of the disease. They are classified according to the age at onset of symptoms, the pattern of inheritance, and the part of the body primarily affected.

Types of Muscular Dystrophy

The most common form of muscular dystrophy, Duchenne, was first described by the French physician Guillaume Benjamin Amand Duchenne in 1861. Most cases are caused by a recessive sex-linked gene located on the X chromosome and carried only by females. Each son of a carrier has a 50% chance of inheriting the gene and developing the disease. Each daughter has a 50% chance of inheriting the gene and becoming a carrier. In small number of "sporadic" cases there is no family history. The disease begins with leg weakness before age 3 and progresses rapidly, with death often occurring before age 30, often because of involvement of lung or heart muscle. Research has shown that the abnormal gene fails to produce an essential skeletal muscle protein called dystrophin. Becker's muscular dystrophy is similar to the Duchenne form, but appears somewhat later in life and progresses more slowly.

Fascioscapulohumeral muscular dystrophy primarily involves facial and shoulder muscles and affects both sexes. Symptoms can begin from adolescence to around age 40. It is caused by an autosomal dominant trait (at least one parent will have the disease). Progression is usually slow and severe disability is unusual.

Myotonic muscular dystrophy is another autosomally dominant disease affecting both sexes. It appears to be caused by the repetition of a section of DNA on chromosome 4. In a surprising development, researchers found that the number of repetitions on the chromosome increase and the disease becomes more severe with each generation. It is characterized by an inability of the muscles to relax properly after contraction and primarily affects the muscles of the hands and feet. It usually begins in adulthood and is often accompanied by cataracts, baldness, and abnormal endocrine function.

The limb-girdle form of the disease first affects the muscles of the hip and shoulder areas. Symptoms usually become apparent in late adolescence or early adulthood. Caused by an autosomal recessive trait (carried by a gene passed on by both asymptomatic parents), it can affect males and females alike. This form usually progresses slowly.

Treatment

There is no known cure for muscular dystrophy. Corticosteriods may slow the destruction of muscle tissue in persons with Duchenne muscular dystrophy, and phenytoin, procainamide, or quinine is used to treat delayed muscle relaxation in myotonic muscular dystrophy. Scientists have identified genetic abnormalities responsible for multiple dystrophy, and some treatments focusing on those mutations are being explored. Exon-skipping drugs, which use RNA-splicing to allow muscle cells to produce a form of dystrophin that can function, are being studied for Duchenne muscular dystrophy, and one such drug has been approved for patients with one genetic mutation. Supportive measures and exercises can improve the quality of life and preserve mobility for as long as possible. Genetic screening is recommended for all family members who might be carriers. Prenatal tests such as chorionic villus sampling and amniocentesis can detect some forms of muscular dystrophy early in a pregnancy.

Bibliography

See A. E. H. Emery, Muscular Dystrophy: The Facts (1994).

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