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Duchenne and Becker Muscular Dystrophies
(Duchenne's Muscular Dystrophy; Becker's Muscular Dystrophy)
Duchenne muscular dystrophy and Becker muscular dystrophy cause weakness in the muscles closest to the torso.
These dystrophies are caused by defects in genes responsible for muscle function, which lead to muscle weakness that develops during childhood or adolescence, nearly always occur in boys.
Both dystrophies are characterized by physical weakness.
The diagnosis is based on the results of tests done on samples of blood and a sample of muscle tissue.
Physical therapy and sometimes prednisone or surgery provides some help.
Duchenne muscular dystrophy is the most common and severe form of muscular dystrophy and begins during early childhood. Becker muscular dystrophy, although closely related to Duchenne muscular dystrophy, begins later during adolescence and causes milder symptoms. These dystrophies nearly always occur in boys. On average, 1 of 4,700 boys born has Duchenne muscular dystrophy, whereas on average 1 of 30,000 boys born has Becker muscular dystrophy.
The gene defect that causes Duchenne muscular dystrophy is different from the gene defect that causes Becker muscular dystrophy, but both defects involve the same gene. The gene for either of these traits is recessive and is carried on the X chromosome. Therefore, although a female can carry the defective gene, she will not develop the disease because the normal gene on one X chromosome compensates for the gene defect on the other X chromosome. However, any male who receives the defective gene will have the disease because he has only one X chromosome (see page Inheritance Patterns : X-Linked Inheritance).
Boys with Duchenne muscular dystrophy lack almost all of the muscle protein dystrophin, which is important for maintaining the structure of muscle cells. Boys with Becker muscular dystrophy produce dystrophin, but because the protein structure is altered, the dystrophin does not function properly or the amount of dystrophin is insufficient.
The main symptom caused by Duchenne muscular dystrophy and Becker muscular dystrophy is weakness of the muscles, including the heart muscle and the muscles for breathing.
Duchenne muscular dystrophy begins between the ages of 2 years and 3 years. The first symptoms are developmental delay (particularly a delay in starting to walk) and difficulty walking, running, jumping, or climbing stairs. Children fall frequently, which often causes arm or leg fractures. Children walk with a waddle, frequently walk on their toes, and have difficulty rising from the floor.
Weakness in the shoulder muscles usually follows and gets steadily worse. As the muscles weaken they also enlarge, but the abnormal muscle tissue is not strong. In boys with Duchenne muscular dystrophy, the heart muscle also gradually enlarges and weakens, causing problems with the heartbeat. Heart complications occur in about one third of children by age 14 and in all affected people over age 18. However, because children are not able to exercise, the weakened heart muscle does not cause symptoms until the disease has progressed. About one third of children have mild, nonprogressive (that is, will not become worse) intellectual impairment that affects mostly verbal ability.
In boys with Duchenne muscular dystrophy, the arm and leg muscles usually contract around the joints, so that the elbows and knees cannot fully extend. Eventually, an abnormally curved spine (scoliosis) develops. Fat and other tissues replace certain enlarged muscle groups, particularly the calf muscles. By age 12, most children with the disease are confined to a wheelchair. Increasing weakness of the respiratory muscles also makes children susceptible to pneumonia and other illnesses, and most die by the age of 20.
In boys with Becker muscular dystrophy, weakness is less severe and first appears a little later, at about age 12. Children are usually able to walk until at least age 15, and many children remain able to walk into adulthood. The pattern of weakness resembles that of Duchenne muscular dystrophy. However, very few adolescents become confined to a wheelchair. Most people survive into their 30s or 40s.
Doctors suspect muscular dystrophy based on characteristic symptoms, for example, a young boy becomes weak and grows weaker, particularly when there is a family history of muscular dystrophy or unexplained weakness in boys. Doctors do blood tests to measure the levels of the enzyme creatine kinase. Creatine kinase leaks out of muscle cells, causing levels in the blood to be abnormally high. However, high blood levels of creatine kinase do not necessarily mean that a person has muscular dystrophy because other muscle diseases may also cause elevated levels of this enzyme.
Duchenne muscular dystrophy is diagnosed when blood tests show the gene for the protein dystrophin to be absent or abnormal and when a muscle biopsy (removal of a piece of muscle tissue for examination under a microscope) shows extremely low levels of dystrophin in the muscle. When looking under the microscope, doctors see dead tissue and abnormally large muscle fibers.
Similarly, Becker muscular dystrophy is diagnosed when blood tests show the gene for the protein dystrophin is abnormal or occurs in only very small amounts and a muscle biopsy shows low levels of dystrophin in the muscle, but not as low as in Duchenne muscular dystrophy.
Children who have Duchenne muscular dystrophy undergo electrocardiography (see page Electrocardiography) and echocardiography (see page Echocardiography and Other Ultrasound Procedures) to detect heart problems. These tests are done at the time children are diagnosed or by 6 years of age.
Close family members of children who have either Duchenne or Becker muscular dystrophy can have tests to detect the gene. Prenatal tests on a fetus to can help determine whether the child is likely to be affected.
Neither Duchenne nor Becker muscular dystrophy can be cured. Physical therapy, gentle exercise, and sometimes wearing ankle braces at night may help prevent the muscles from contracting permanently around joints. Leg braces may temporarily help maintain the ability to walk or stand. Sometimes surgery is needed to release tight, painful muscles or to correct scoliosis. Boys need fewer calories because they are less active. They should avoid overeating.
Children with breathing problems may wear special masks that help breathing (see page Alternatives). If the mask does not help the breathing enough, doctors may insert a plastic tube directly into the windpipe (trachea) through a small incision in the front of the neck (a procedure called tracheostomy). The tube is attached to a mechanical breathing device (ventilator). A tracheostomy may allow children with Duchenne dystrophy to live into their 20s. Children with heart problems may be given drugs such as angiotensin-converting enzyme inhibitors and beta-blockers.
Children who have Duchenne muscular dystrophy, are over age 5, and who have significant muscle weakness may be given prednisone. Prednisone, a corticosteroid, is taken by mouth daily and may temporarily improve strength. However, long-term use causes many side effects (see Corticosteroids: Uses and Side Effects). Prednisone has not been adequately studied for treatment of children with Becker muscular dystrophy.
Gene therapy that would enable muscles to produce dystrophin and thereby relieve the weakness is not yet available. Families should consult a genetic counselor for help in evaluating the risk of passing the muscular dystrophy trait to their children.
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