Duchenne muscular dystrophy

Definition

Duchenne muscular dystrophy is an inherited disorder, characterized by rapidly-worsening muscle weakness that starts in the legs and pelvis, and later affects the whole body.

Alternative Names

Causes

Duchenne muscular dystrophy is a rapidly-worsening form of muscular dystrophy. It is caused by a defective gene for dystrophin (a protein in the muscles). However, it often occurs in people without a known family history of the condition. This disorder is marked by worsening loss of muscle function, which begins in the lower limbs.

Duchenne muscular dystrophy is inherited in what is known as an X-linked recessive pattern. The defective gene is found on the X chromosome. Because women have two X chromosomes, if one contains a normal copy of the gene, that gene will make enough of the protein to prevent symptoms. But boys have an X chromosome from their mother and a Y from father, so if the X chromosome is defective, there is no second X to make up for it and they will develop the disease.

The sons of carrier females (women with one defective chromosome but no symptoms themselves) each have a 50% chance of having the disease, and the daughters each have a 50% chance of being carriers.

Symptoms usually appear before age 6 and may appear as early as infancy. There is progressive muscle weakness of the legs and pelvis, which is associated with a loss of muscle mass (wasting). Muscle weakness also occurs in the arms, neck, and other areas, but not as severely or as early as in the lower half of the body.

Calf muscles initially grow larger -- the enlarged muscle tissue is eventually replaced by fat and connective tissue (a condition called pseudohypertrophy). Muscle contractures occur in the legs. Thus, the muscles are unusable because the muscle fibers shorten and fibrosis (scarring) occurs in connective tissue.

By age 10, braces may be required for walking, and by age 12, most patients are confined to a wheelchair. Bones develop abnormally, causing skeletal deformities of the spine and other areas.

Muscular weakness and skeletal deformities contribute to frequent breathing disorders. Cardiomyopathy occurs in almost all cases. Intellectual impairment may occur, but it is not inevitable and does not worsen as the disorder progresses.

Duchenne muscular dystrophy occurs in approximately 2 out of 10,000 people. Because this is an inherited disorder, risks include a family history of Duchenne muscular dystrophy. In contrast, Becker muscular dystrophy is a form that progresses (gets worse) much more slowly.

Symptoms

• Muscle weakness
  • Rapidly progressive
  • Frequent falls
  • Difficulty with motor skills (running, hopping, jumping)
• Progressive difficulty walking
  • Ability to walk may be lost by age 12
• Fatigue
• Intellectual retardation (possible)
• Skeletal deformities
  • Chest and back (scoliosis)
• Muscle deformities
  • Contractures of heels, legs
  • Pseudohypertrophy of calf muscles

Exams and Tests

Muscle wasting (atrophy) begins in the legs and pelvis, then progresses to the muscles of the shoulders and neck, followed by loss of arm muscles and respiratory muscles. Calf muscle enlargement (pseudohypertrophy) is quite obvious.

Cardiomyopathy is commonly present, but signs of congestive heart failure or arrhythmias (irregular heartbeats) are rare. Respiratory disorders are common during the later stages, including pneumonia

• A serum CPK is highly elevated.
• A neurological exam shows weakness and lack of coordination or balance.
• An EMG (electromyography) shows that weakness is caused by destruction of muscle tissue, rather than nerve damage.
• A muscle biopsy confirms the diagnosis.

Treatment

There is no known cure for Duchenne muscular dystrophy. Treatment is aimed at control of symptoms to maximize the quality of life. Gene therapy may become available in the future.

Activity is encouraged. Inactivity (such as bedrest) can worsen the muscle disease. Physical therapy may be helpful to maintain muscle strength and function. Orthopedic appliances (such as braces and wheelchairs) may improve mobility and the ability for self-care.

Support Groups

The stress of illness can often be helped by joining a support group where members share common experiences and problems. See muscular dystrophy - support group. The Muscular Dystrophy Association is an excellent source of information on this disease.

Outlook (Prognosis)

Duchenne muscular dystrophy results in rapidly progressive disability. Death usually occurs by age 25, typically from respiratory (lung) disorders.

Possible Complications

• Deformities
• Permanent, progressive disability
  • Decreased mobility
  • Decreased ability for self-care
• Mental impairment (varies, usually minimal)
• Pneumonia or other respiratory infections
• Respiratory failure
• Cardiomyopathy
• Congestive heart failure (rare)
• Heart arrhythmias (rare)

When to Contact a Medical Professional

Call your health care provider if your child has symptoms that indicate Duchenne muscular dystrophy.

Call your health care provider if symptoms worsen, or new symptoms develop, particularly fever with cough or breathing difficulties.

Prevention

Genetic counseling is advised if there is a family history of the disorder. Duchenne muscular dystrophy can be detected with about 95% accuracy by genetic studies performed during pregnancy.