What is Duchenne Muscular Dystrophy?
Duchenne muscular dystrophy (DMD) is a rapidly progressive form of muscular dystrophy that occurs primarily in boys. It is caused by a mutation in a gene, called the DMD gene, which encodes the muscle protein dystrophin. Boys with Duchenne muscular dystrophy do not make the dystrophin protein in their muscles. Duchenne muscular dystrophy is inherited in an X-linked recessive fashion; however, it may also occur in people from families without a known family history of the condition. Individuals who have DMD have progressive loss of muscle function and weakness, which begins in the lower limbs. In addition to the skeletal muscles used for movement, DMD may also affect the muscles of the heart. There is no known cure for Duchenne muscular dystrophy. Treatment is aimed at control of symptoms to maximize the quality of life.
Muscular dystrophies are a group of genetic conditions characterized by progressive muscle weakness and wasting (atrophy). The Duchenne and Becker types of muscular dystrophy primarily affect the skeletal muscles, which are used for movement, and the muscles of the heart. These conditions occur much more frequently in males than in females.
Duchenne and Becker muscular dystrophies have similar signs and symptoms and are caused by different mutations in the same gene. The two conditions differ in their severity, age of onset, and rate of progression. In people with Duchenne muscular dystrophy, muscle weakness tends to appear in early childhood and progress rapidly. Affected children may have delayed motor skills, such as sitting, standing, and walking. They are usually wheelchair-dependent by adolescence. The signs and symptoms of Becker muscular dystrophy are usually milder and exhibit a large range of variation. In most cases, muscle weakness becomes apparent later in childhood or adolescence and progresses at a much slower rate.
Both the Duchenne and Becker forms of muscular dystrophy are associated with a heart condition called dilated cardiomyopathy. This form of heart disease enlarges and weakens the heart (cardiac) muscle, preventing it from pumping blood efficiently. Dilated cardiomyopathy progresses rapidly and is life-threatening in many cases. In people with Duchenne muscular dystrophy, the signs and symptoms of cardiomyopathy typically appear in adolescence. The onset of cardiomyopathy in people with Becker muscular dystrophy is later, usually in early to mid-adulthood.
How common is Duchenne and Becker muscular dystrophy?
Duchenne and Becker muscular dystrophies together affect 1 in 3,500 to 5,000 newborn males. Between 400 and 600 boys in the United States are born with these conditions each year. Females are rarely affected by these forms of muscular dystrophy.
What genes are related to Duchenne and Becker muscular dystrophy?
Mutations in the DMD gene cause Duchenne and Becker muscular dystrophy.
The DMD gene provides instructions for making a protein called dystrophin. This protein helps stabilize and protect muscle fibers and may play a role in chemical signaling within cells. Mutations in the DMD gene alter the structure or function of dystrophin, or prevent any functional dystrophin from being produced. Muscle cells without this protein become damaged as muscles repeatedly contract and relax with use. The damaged fibers weaken and die over time, leading to the muscle weakness and heart problems characteristic of Duchenne and Becker muscular dystrophies.
How do people inherit Duchenne and Becker muscular dystrophy?
This condition is inherited in an X-linked recessive pattern. A condition is considered X-linked if the mutated gene that causes the disorder is located on the X chromosome, one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation must be present in both copies of the gene to cause the disorder. Males are affected by X-linked recessive disorders much more frequently than females. A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
In about two thirds of cases, an affected male inherits the mutation from a mother who carries an altered copy of the DMD gene. The other one third of cases probably result from new mutations in the gene.
In X-linked recessive inheritance, a female with one mutated copy of the gene in each cell is called a carrier. She can pass on the altered gene, but usually does not experience signs and symptoms of the disorder. Occasionally, however, females who carry a DMD gene mutation may have muscle weakness and cramping. These symptoms are typically milder than the severe muscle weakness and atrophy seen in affected males. Females who carry a DMD gene mutation also have an increased risk of developing heart abnormalities including dilated cardiomyopathy.
If Duchenne and Becker muscular dystrophy run in my husband's family, what are the chances that our children will have the condition?
When a genetic disorder is diagnosed in a family, family members often want to know the likelihood that they or their children will develop the condition. This can be difficult to predict in some cases because many factors influence a person’s chances of developing a genetic condition. One important factor is how the condition is inherited.
Duchenne and Becker muscular dystrophy are inherited in an X-linked recessive manner. Because of the difference in sex chromosomes, the probability of passing on an X-linked recessive disorder also differs between men and women. The sons of a man with an X-linked recessive disorder will not be affected, and his daughters will carry one copy of the mutated gene. With each pregnancy, a woman who carries an X-linked recessive disorder has a 50 percent chance of having sons who are affected and a 50 percent chance of having daughters who carry one copy of the mutated gene.
Although the chances of inheriting a genetic condition appear straightforward, factors such as a person’s family history and the results of genetic testing can sometimes modify those chances. In addition, some people with a disease-causing mutation never develop any health problems or may experience only mild symptoms of the disorder. Estimating the chance of developing or passing on a genetic disorder can be complex. Genetics professionals can help people understand these chances and help them make informed decisions about their health.
Causes of Duchenne Muscular Dystrophy
Duchenne muscular dystrophy is an inherited disorder that involves rapidly worsening muscle weakness. Duchenne muscular dystrophy is a rapidly-worsening form of muscular dystrophy muscular dystrophy. Other muscular dystrophies (including Becker's muscular dystrophy) get worse much more slowly.
Duchenne muscular dystrophy 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.
Because of the way the disease is inherited, males are more likely to develop symptoms than are women. The sons of females who are carriers of the disease (women with a defective gene but no symptoms themselves) each have a 50% chance of having the disease. The daughters each have a 50% chance of being carriers.
Duchenne muscular dystrophy occurs in approximately 1 out of every 3,600 male infants. Because this is an inherited disorder, risks include a family history of Duchenne muscular dystrophy.
Symptoms of Duchenne Muscular Dystrophy
Symptoms usually appear before age 6 and may appear as early as infancy. They may include:
- Fatigue Fatigue
- Mental retardation (possible, but does not worsen over time)
- Muscle weakness
- Begins in the legs and pelvis, but also occurs less severely in the arms, neck, and other areas of the body
- Difficulty with motor skills (running, hopping, jumping)
- Frequent falls
- Rapidly worsening weakness
- Progressive difficulty walking difficulty walking
- Ability to walk may be lost by age 12
By age 10, the person may need braces for walking. By age 12, most patients are confined to a wheelchair.
Exams and Tests for Duchenne Muscular Dystrophy
A complete nervous system (neurological), heart, lung, and muscle exam may show:
- Abnormal heart muscle (cardiomyopathy)
- Congestive heart failure or irregular heart rhythm (arrhythmias) -- rare
- Deformities of the chest and back (scoliosis)
- Enlarged calf muscles, which are eventually replaced by fat and connective tissue (pseudohypertrophy)
- Loss of muscle mass (wasting)
- Muscle contractures in the heels, legs
- Muscle deformities
- Respiratory disorders, including pneumonia and aspiration of food or fluid into the lungs (in late stages of the disease)
Tests may include:
- Electromyography (EMG)
- Genetic tests
- Muscle biopsy
- Serum CPK
Treatment for Duchenne Muscular Dystrophy
There is no known cure for Duchenne muscular dystrophy. Treatment aims to control symptoms to maximize quality of life. Gene therapy may become available in the future.
Activity is encouraged. Inactivity (such as bed rest) 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 to care for yourself.
Support Groups for Duchenne Muscular Dystrophy
You can ease the stress of illness by joining a support group where members share common experiences and problems. The Muscular Dystrophy Association is an excellent source of information on this disease.
Outlook / Prognosis for Duchenne Muscular Dystrophy
Duchenne muscular dystrophy leads to quickly worsening disability. Death usually occurs by age 25, typically from lung disorders.
Possible Complications of Duchenne Muscular Dystrophy
- Congestive heart failure (rare)
- Heart arrhythmias (rare)
- Mental impairment (varies, usually minimal)
- Permanent, progressive disability
- Decreased mobility
- Decreased ability to care for self
- Pneumonia or other respiratory infections
- Respiratory failure
When to Contact a Medical Professional
Call your health care provider if:
- Your child has symptoms of Duchenne muscular dystrophy
- Symptoms worsen, or new symptoms develop, particularly fever with cough or breathing difficulties
Prevention of Duchenne Muscular Dystrophy
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.