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|Duchenne Muscular Dystrophy|
Duchenne muscular dystrophy (Duchenne MD) is the most common childhood form of MD, as well as the most common of the muscular dystrophies overall, accounting for approximately 50 percent of all cases. It affects approximately one in 3,500 male births.
Duchenne MD usually becomes apparent when an affected child begins to walk. Progressive weakness and muscle wasting (a decrease in muscle strength and size) caused by degenerating muscle fibers begins in the upper legs and pelvis before spreading into the upper arms. Other symptoms include loss of some reflexes, a waddling gait, frequent falls and clumsiness (especially when running), difficulty when rising from a sitting or lying position or when climbing stairs, changes to overall posture, impaired breathing, lung weakness, and cardiomyopathy (heart muscle weakness that interferes with pumping ability). Many children are unable to run or jump. The wasting muscles, in particular the calf muscle (and, less commonly, muscles in the buttocks, shoulders, and arms), may be enlarged by an accumulation of fat and connective tissue, causing them to look larger and healthier than they actually are (called pseudohypertrophy). As the disease progresses, the muscles in the diaphragm that assist in breathing and coughing may weaken. Patients may experience breathing difficulties, respiratory infections, and swallowing problems. Bone thinning and scoliosis (curving of the spine) are common. Some children are mildly mentally impaired. Between ages 3 and 6, children may show brief periods of physical improvement followed by progressive muscle degeneration. Children with Duchenne MD are typically wheelchair-bound by age 12 and usually die in their late teens or early twenties from progressive weakness of the heart muscle, respiratory complications, or infection.
Duchenne MD is caused by mutations in the DMD gene, which makes a protein called dystrophin. This protein helps stabilize and protect muscle fibers and may play a role in chemical signaling within cells. Mutations in the DMD gene alter the structure or function of dystrophin, or prevent any functional dystrophin from being produced. As a result, muscle fibers become damaged with repeated use. The damaged fibers weaken and die over time, leading to the muscle weakness and heart problems characteristic of Duchenne MD.
Duchenne MD is inherited in an X-linked recessive pattern. Males are affected by X-linked recessive disorders much more frequently than females. In about two thirds of cases, an affected male inherits the mutation from a mother who carries one altered copy of the DMD gene. The other one third of cases probably result from new mutations in the gene. Females who carry one copy of a DMD mutation may have some signs and symptoms related to the condition, but these are typically milder.