Section 5. Bone, Joint, and Muscle Disorders

Chapter 73. Muscular Dystrophy and Related Disorders

Topics: Introduction | Duchenne and Becker Muscular Dystrophies | Other Muscular Dystrophies | Myotonic Myopathies | Periodic Paralysis

Duchenne and Becker Muscular Dystrophies

Duchenne and Becker muscular dystrophies cause weakness in the muscles closest to the torso.

These dystrophies are the most common muscular dystrophies and nearly always occur in boys. On average, 1 of 3,300 boys born has Duchenne dystrophy, whereas on average 1 of 18,000 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.

Boys with Duchenne muscular dystrophy lack almost totally 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.

Symptoms

In boys with Duchenne muscular dystrophy, the first symptoms are developmental delay (particularly a delay in starting to walk), difficulty walking or climbing stairs, and falling. Starting between the ages of 3 and 7, the gait becomes waddling, and the child has difficulty rising from a sitting position.

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 over 80% of boys with Duchenne muscular dystrophy, the heart muscle also enlarges and weakens, causing problems with the heartbeat, which show up on an electrocardiogram.

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. By age 10 or 12, most children with the disease are confined to a wheelchair. The increasing weakness also makes them 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. The pattern of weakness resembles that of Duchenne muscular dystrophy. However, very few adolescents become confined to a wheelchair. The average age of death is 42 years.

Diagnosis

Doctors suspect muscular dystrophy when a young boy becomes weak and grows weaker. An enzyme (creatine kinase) leaks out of muscle cells, causing levels of creatine kinase in the blood to be abnormally high. However, high blood levels of creatine kinase do not necessarily mean that a person has muscular dystrophy; 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 a muscle biopsy (see Section 5, Chapter 59) shows extremely low levels of dystrophin in the muscle. Under the microscope, the muscle generally shows dead tissue and abnormally large muscle fibers. In the late stages of muscular dystrophy, fat and other tissues replace the dead muscle tissue. Similarly, Becker muscular dystrophy is diagnosed when blood tests show the gene for the protein dystrophin to be abnormal and a muscle biopsy shows low levels of dystrophin in the muscle, but not as low as in Duchenne muscular dystrophy.

Other tests to support the diagnosis include electrical studies of muscle function (electromyography) and nerve conduction studies (see Section 6, Chapter 77).

Families with members who have either Duchenne or Becker muscular dystrophy are advised to consult a genetic counselor for help in evaluating the risk of passing the muscular dystrophy trait on to their children. In families with a history of these disorders, doctors can perform prenatal tests on the fetus to determine if the child is likely to be affected.

Treatment

Neither Duchenne nor Becker muscular dystrophy can be cured. Physical therapy and exercise help prevent the muscles from contracting permanently around joints. Sometimes surgery is needed to release tight, painful muscles.

Prednisone, a corticosteroid, taken by mouth daily, improves the person's strength. However, long-term use causes many side effects (see Section 5, Chapter 67), so it is not given to every child with muscular dystrophy. Use of prednisone is generally reserved for people whose muscle weakness has severely interfered with the normal activities of daily living. Creatine, a supplement taken by mouth, has recently been shown to improve strength. Gene therapy that would enable muscles to produce dystrophin and thereby relieve the weakness is under investigation but has so far not proved successful.