Calf muscles initially enlarge -- the enlarged muscle tissue is eventually replaced by fat and connective tissue (pseudohypertrophy). Muscle contractures occur in the legs and heels, causing inability to use the muscles because of shortening of muscle fibers and fibrosis of connective tissue.

Symptoms usually appear in males 1 to 6 years old. 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 can be observed, but it is not inevitable and does not worsen as the disorder progresses.

The cause of this muscle impairment is related to an abnormal gene for dystrophin protein (a specific type of protein in the muscles). This gene is on the X chromosome. Duchenne muscular dystrophy is inherited in an X-linked recessive inheritance pattern. Boys with an abnormal gene have symptoms. Females with an abnormal gene are carriers (i.e., usually do not have symptoms). However, the sons of carrier females each have a 50% chance of having the disease, and the daughters each have a 50% chance of being carriers.

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.

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