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Duchenne Muscular Dystrophy and Becker Muscular Dystrophy

(Duchenne's Muscular Dystrophy; Becker's Muscular Dystrophy)

By Michael Rubin, MDCM, Professor of Clinical Neurology;Attending Neurologist and Director, Neuromuscular Service and EMG Laboratory, Weill Cornell Medical College;New York Presbyterian Hospital-Cornell Medical Center

Muscular dystrophies are a group of inherited muscle disorders in which one or more genes needed for normal muscle structure and function are defective, leading to muscle weakness of varying severity. 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.

  • Treatment includes physical therapy and sometimes surgery for both dystrophies and prednisone or deflazacort for Duchenne dystrophy.

Duchenne muscular dystrophy is the second most common (facioscapulohumeral dystrophy is the most common) and the most severe form of muscular dystrophy. It 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. Together, Duchenne muscular dystrophy and Becker muscular dystrophy affect 5 of 1,000 people. Most people have Duchenne 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 males have only one X chromosome (see 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.

Symptoms

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. Only boys develop symptoms.

Duchenne muscular dystrophy

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. Boys with Duchenne muscular dystrophy fall frequently, which often causes arm or leg fractures. They 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 boys with Duchenne muscular dystrophy by age 14 and in all those over age 18 affected by the disease. However, because these boys are not able to exercise, the weakened heart muscle does not cause symptoms until the disease has progressed. About one third of boys with the disease 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 boys with the disease need to use a wheelchair. Increasing weakness of the respiratory muscles also makes them susceptible to pneumonia and other illnesses, and most die by the age of 20.

Becker muscular dystrophy

In boys with Becker muscular dystrophy, weakness is less severe and first appears a little later, at about age 12. They are usually able to walk until at least age 15, and many remain able to walk into adulthood. The pattern of weakness resembles that of Duchenne muscular dystrophy. However, very few adolescents need to use a wheelchair. Most people survive into their 30s or 40s.

Diagnosis

  • Blood tests

  • Muscle biopsy

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. In muscular dystrophy, 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 boy 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 or 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. 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 and echocardiography to detect heart problems. These tests are done at the time the boy is diagnosed or by 6 years of age.

Close family members of children who have either Duchenne or Becker muscular dystrophy can have DNA tests of blood to detect the gene. Prenatal tests on a fetus to can help determine whether the child is likely to be affected.

Treatment

  • Physical therapy and ankle or leg braces

  • Sometimes angiotensin-converting enzyme inhibitors and beta-blockers

  • Sometimes surgery

  • For Duchenne dystrophy, prednisone or deflazacort

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. 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 machine that helps air get in and out of the lungs (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.

People who have Duchenne muscular dystrophy who are over age 5 and have significant muscle weakness may be given prednisone or deflazacort, which are corticosteroids. Prednisone or deflazacort is taken by mouth daily. When taken for a long time, these drugs have many benefits, such as improving strength, allowing children to walk for a few more years, maintaining heart and lung functions, and increasing survival by 5 to 15 years. However, long-term use of these drugs causes many side effects, such as weight gain, puffiness in the face, and an increased risk of spine and bone problems (see Corticosteroids: Uses and Side Effects). Prednisone and deflazacort have not been adequately studied for treatment of Becker muscular dystrophy.

Gene therapy that would enable muscles to produce dystrophin and thereby relieve the weakness is not yet available. Families should consider genetic counseling for help in evaluating the risk of passing the muscular dystrophy gene to their children.

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