Duchenne muscular dystrophy

Definition

Duchenne muscular dystrophy is an inherited disorder, characterized by rapidly-worsening muscle weakness that starts in the legs and pelvis, and later affects the whole body.

Alternative Names

Causes

Duchenne muscular dystrophy is a rapidly-worsening form of muscular dystrophy. It is caused by a defective gene for dystrophin (a protein in the muscles). However, it often occurs in people without a known family history of the condition. This disorder is marked by worsening loss of muscle function, which begins in the lower limbs.

Duchenne muscular dystrophy is inherited in what is known as an X-linked recessive pattern. The defective gene is found on the X chromosome. Because women have two X chromosomes, if one contains a normal copy of the gene, that gene will make enough of the protein to prevent symptoms. But boys have an X chromosome from their mother and a Y from father, so if the X chromosome is defective, there is no second X to make up for it and they will develop the disease.

The sons of carrier females (women with one defective chromosome but no symptoms themselves) each have a 50% chance of having the disease, and the daughters each have a 50% chance of being carriers.

Symptoms usually appear before age 6 and may appear as early as infancy. There is progressive muscle weakness of the legs and pelvis, which is associated with a loss of muscle mass (wasting). Muscle weakness also occurs in the arms, neck, and other areas, but not as severely or as early as in the lower half of the body.

Calf muscles initially grow larger -- the enlarged muscle tissue is eventually replaced by fat and connective tissue (a condition called pseudohypertrophy). Muscle contractures occur in the legs. Thus, the muscles are unusable because the muscle fibers shorten and fibrosis (scarring) occurs in connective tissue.

By age 10, braces may be required for walking, and by age 12, most patients are confined to a wheelchair. Bones develop abnormally, causing skeletal deformities of the spine and other areas.

Muscular weakness and skeletal deformities contribute to frequent breathing disorders. Cardiomyopathy occurs in almost all cases. Intellectual impairment may occur, but it is not inevitable and does not worsen as the disorder progresses.

Duchenne muscular dystrophy occurs in approximately 2 out of 10,000 people. Because this is an inherited disorder, risks include a family history of Duchenne muscular dystrophy. In contrast, Becker muscular dystrophy is a form that progresses (gets worse) much more slowly.