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 dystrophy
- 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 only 1 out of every 50,000 people. The other types of muscular dystrophy are even rarer.
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
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 3 to 5. 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.
Becker's muscular dystrophy which usually starts around age 12, 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 10 to 26, 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 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.
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 occurs after age 35 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 children is likely to inherit it.
Making the Diagnosis
If a person has weak muscles that become weaker over time, they may have muscular dystrophy. High levels of a chemical called creatine kinase (CPK) found in the blood may also help diagnose muscular dystrophy. High levels of creatine kinase don't give a definite diagnosis, since many other muscle diseases can also cause this.
Testing for very low levels of dystrophin in the muscles can also be used to diagnose Duchenne muscular dystrophy. For other types of muscular dystrophy, specific blood tests can be done to help your doctor determine which type of muscular dystrophy you have.
A test called a nerve conduction study involves passing tiny electrical shocks through nerves. The nerve conduction study is performed along with a test called electromyography (EMG). EMG involves inserting a needle-like probe to detect abnormal spontaneous muscle activity. These studies can be performed to test whether the muscles and nerves are working properly.
A muscle biopsy may also be used to help with diagnosis. In this procedure, a doctor surgically removes a small piece or sample of the muscle and looks at it under the microscope. A muscle with muscular dystrophy has many dead and abnormally large muscle fibres. As the disease progresses, the dead muscle is replaced with fat and other kinds of tissue.
Genetic counselling can help with making the diagnosis. It can also open the way to clinical trials. Couples who have a history of muscular dystrophy in their families may want to go for genetic counselling to identify the carriers of the affected gene(s). It can also help with family planning. The counsellor will help determine the risk of having a child with the disease and will give guidance on how to proceed. In spite of the many advances made in genetics of muscular dystrophy, the responsible gene(s) cannot be identified in up to half of patients.
Treatment and Prevention
There is no cure for muscular dystrophy, but exercise and physical therapy can keep the muscles from tightening up around the joints. Assistive devices such as braces and wheelchairs can also help people with muscular dystrophy. People with Duchenne muscular dystrophy can take a corticosteroid medication (like prednisone) to improve muscle strength and function as well as reduce the progression of scoliosis (a curvature of the spine). Every effort should be made to prevent viral infections that could progress to pneumonia.
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