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. They are the most common muscular dystrophies.
These dystrophies are caused by defects in genes responsible for muscle function, which lead to muscle weakness that develops during childhood or adolescence, and 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.
Duchenne muscular dystrophy and Becker muscular dystrophy are the most common muscular dystrophies.
Duchenne muscular dystrophy is 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 about 1 of 5,000 to 1 of 6,000 live male births. Most affected 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, called the dystrophin 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 disorder 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 from the mother will have the disorder because males have only one X chromosome (see 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 disorder. However, because these boys are not able to exercise, the weakened heart muscle does not cause symptoms until the disorder has progressed. About one third of boys with the disorder have mild intellectual impairment that will not become worse and 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 disorder need to use a wheelchair. Increasing weakness of the respiratory muscles also makes them susceptible to pneumonia and other illnesses, and, if they are not using a mechanical ventilator (a machine that helps air get in and out of the lungs) to aid breathing, most die by the age of 20. However, children who do use a mechanical ventilator may live into their 30s and beyond.
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
Genetic tests
Sometimes 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. Doctors also often do a test called electromyography (EMG), which helps record a muscle's electrical activity.
Duchenne muscular dystrophy is diagnosed by doing genetic tests (DNA tests) on a blood sample to identify mutations in the dystrophin gene. If genetic tests cannot confirm the diagnosis, doctors do a muscle biopsy (removal of a piece of muscle tissue for examination under a microscope) to determine levels of the protein dystrophin in the muscle. When looking at the muscle tissue under the microscope, doctors see dead tissue and abnormally large muscle fibers. The protein dystrophin cannot be detected or is detected in extremely low levels in people who have Duchenne muscular dystrophy.
Similarly, Becker muscular dystrophy is diagnosed when genetic tests show defects in the dystrophin gene. A muscle biopsy shows a low level of the protein 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, sometimes medications that increase production of the protein dystrophin
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 (mechanical ventilator). A tracheostomy may allow children with Duchenne dystrophy to live into their 30s and beyond.
Children with heart problems may be given medications such as angiotensin-converting enzyme inhibitors and beta-blockers.
Prednisone or deflazacort is taken by mouth daily. When taken for a long time, these medications 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 medications causes many side effects, such as weight gain, puffiness in the face, and an increased risk of spine and bone problems (see sidebar Corticosteroids: Uses and Side Effects
Some people who have Duchenne muscular dystrophy and who have certain mutations of the dystrophin
Families should consider genetic counseling for help in evaluating the risk of passing the muscular dystrophy gene to their children.
More Information
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
Muscular Dystrophy Association: Information on research, treatment, technology, and support for people living with Duchenne muscular dystrophy and Becker muscular dystrophy
