Becker's muscular dystrophy is very similar to Duchenne's muscular dystrophy, except that it progresses at a much slower rate. The disorder is inherited with an X-linked recessive inheritance pattern -- the gene is located on the X chromosome. Since women have two X chromosomes, if one X chromosome has the defective gene, the second X chromosome will have a working copy of the gene to compensate. In these cases, some women have much milder symptoms because of this ability to compensate. Men have an X and a Y and because they don't have another X to compensate for the defective gene, they will develop symptoms if they inherit the defective gene.
People with this disorder experience 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 as in the lower half of the body. Calf muscles initially enlarge (an attempt by the body to compensate for loss of muscle strength), but 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. Bones develop abnormally, causing skeletal deformities of the chest and other areas. Cardiomyopathy (damage to the heart) does not occur as commonly with this disorder as it does with Duchenne's muscular dystrophy. Cognitive problems may accompany the disorder, but they are not inevitable and do not worsen as the disorder progresses.
Becker's muscular dystrophy occurs in approximately 3-6 in 100,000 male births. Symptoms usually appear in men at about age 12, but may sometimes begin later. The average age of becoming unable to walk is 25-30. Women rarely develop symptoms. Because this is an inherited disorder, risks include having a family history of Becker's muscular dystrophy.
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