Muscular Dystrophy
(Duchenne Muscular Dystrophy · Becker's Muscular Dystrophy · Facioscapulohumeral Muscular Dystrophy · Limb-Girdle Muscular Dystrophies)
In this condition factsheet:
The Facts on Muscular Dystrophy
Muscular dystrophy is a term that refers to a group of muscle disorders in which the face, arm, leg, spine, or heart muscles gradually shrink and weaken over time. The brain, eyes, intestines, and reproductive and hormone-secreting organs can also be affected.
There are a variety of types of muscular dystrophy, including:
- Duchenne muscular dystrophy
- myotonic dystrophy
- Becker's muscular dystrophy
- facioscapulohumeral muscular dystrophy
- the limb-girdle muscular dystrophies
- Emery-Dreifuss muscular dystrophy
- oculopharyngeal dystrophy
- distal muscular dystrophies
- mitochondrial myopathies
The different types are distinguished by factors such as the:
- age at which symptoms usually start
- pattern of muscle weakness
- speed at which the disease progresses
- involvement of other tissues besides muscle
- pattern of inheritance
Muscular dystrophy is rare. For example, although Duchenne Muscular Dystrophy is one of the more common types, it occurs in about only 1 out of every 20,000 people. The other types of muscular dystrophy are even rarer.
Causes of Muscular Dystrophy
All muscular dystrophies are inherited. Each type of muscular dystrophy is associated with a distinct genetic mutation. The nature of the gene mutation and which chromosome it is located on determine the characteristics of the muscular dystrophy and the way the disease is passed from one generation to the next.
The gene responsible for Duchenne and Becker's muscular dystrophies normally produces a protein called dystrophin, which helps muscle cells keep their normal shape and function. The faulty gene that causes muscular dystrophies either does not produce enough dystrophin or produces dystrophin that doesn't work properly. This gene is located on the X chromosome.
Women have two X chromosomes, but men have only one X chromosome (men also have a Y chromosome, which does not have a copy of the dystrophin gene). Girls don't usually get the disease because the healthy gene on one X chromosome can produce enough normal dystrophin to compensate for the defective gene on the other X chromosome. They often show no symptoms of muscular dystrophy. Boys who inherit the defective gene are not so lucky since they do not have another copy of the good gene and are not able to make enough dystrophin to keep their muscles healthy.
A woman who has the abnormal gene is called a carrier. Each of her children (boy or girl) has a 50% chance of getting the gene from her. If the child is a boy and inherits the defective gene, he will develop the muscle disease. If it is a girl, she will be a carrier who can then pass the gene on to her own children.
Any woman in an affected family can be tested before deciding to get pregnant to find out if she is a carrier. If she is not a carrier, she cannot pass the disease on to any of her children. A fetus can also be tested for the disease during pregnancy.
Symptoms and Complications of Muscular Dystrophy
The symptoms and age of onset depend on the type of muscular dystrophy. Symptoms of muscular dystrophy often include:
- problems with coordination and mobility with frequent falls
- muscle weakness
- joint stiffness
Duchenne muscular dystrophy symptoms appear early, usually between the ages of 2 to 3. The condition progresses quickly, affecting the legs and hips first, and leads to the need for a wheelchair by the age of 12. Symptoms include those listed above as well as fatigue, problems with the bones in the back and chest, and intellectual disability. People with this condition often develop large calves due to large fatty deposits that accumulate alongside muscle. They may eventually also have trouble breathing and develop pneumonia, or develop heart problems. They may also have an increased risk of bone fractures in the arms and legs.
Becker's muscular dystrophy which usually starts around age 16, is much less severe and progresses more slowly than Duchenne muscular dystrophy. Symptoms include those listed above, plus heart disease, problems with the curvature of the spine, fatigue, problems with thinking, and breathing difficulties.
Symptoms of facioscapulohumeral muscular dystrophy (also called Landouzy-Dejerine muscular dystrophy) include weak facial and shoulder muscles and difficulty with lifting arms, whistling, and closing eyes. It affects mainly the upper body and can also cause hearing loss, speech problems, and changes in heart rhythm. Symptoms often start around ages 9 to 30, but may also start at a later stage.
Limb-girdle muscular dystrophies cause weakness and wasting in the arm and leg muscles. There are over 20 different sub-types of this muscular dystrophy. Symptoms, such as low back pain, usually start to appear in childhood or early adulthood. Muscle weakness starts in the pelvic area and moves to the shoulders and other body parts.
Emery-Dreifuss muscular dystrophy causes wasting of the muscles in the lower legs as well as the upper arms. It may also affect other areas such as the shoulders, chest, and face. Usually, the muscle contractures (shortening of muscle) appear before the muscle weakness appears. Heart disease is common and can lead to serious problems such as abnormal heartbeats.
Oculopharyngeal muscular dystrophy is an adult-onset dystrophy (i.e., appears after the age of 45) that primarily affects those of French Canadian and Jewish descent. In Quebec, 1 in 1000 people has this type of muscular dystrophy. Symptoms include difficulty swallowing (dysphagia), tongue weakness, weakness in the proximal muscles, difficulty looking up or double-vision (diplopia), and closing or drooping eyelids (ptosis). It also causes tongue and facial muscle weakness.
Distal muscular dystrophy usually starts showing symptoms between age 40 to 60 and causes weakness in the ankles, making it difficult to walk.
Mitochondrial myopathies are caused by defects in the genes of the mitochondria, which are specialized units found inside cells that create the energy needed for cells to work. Symptoms include those listed above, plus heart problems, seizures, gastrointestinal tract disorders, skeletal muscle issues, and problems with vision and hearing.
Myotonic muscular dystrophy usually does not start to produce symptoms until after the age of 20 (except for the congenital form). Some people don't have symptoms until as late as age 50.This type of muscular dystrophy can also affect the heart (arrhythmias), eyes (cataracts), lungs, digestive tract, and brain function and can be associated with diabetes. Myotonic dystrophy is an autosomal dominant disease, meaning that 1 out of 2 two children is likely to inherit it if their parent was diagnosed with the disorder.
