TYPES OF MUSCULAR DYSTROPHY
Muscular dystrophy (MD) is a group of inherited diseases in which the muscles that control movement (called voluntary muscles) progressively weaken. In some forms of this disease, the heart and other organs are also affected.
There are nine major forms of muscular dystrophy:
• Myotonic
• Duchenne
• Becker
• Limb-girdle
• Facioscapulohumeral
• Congenital
• Oculopharyngeal
• Distal
• Emery-Dreifuss
Muscular dystrophy can appear in infancy up to middle age or later, and its form and severity are determined in part by the age at which it occurs. Some types of muscular dystrophy affect only males; some people with MD enjoy a normal life span with mild symptoms that progress very slowly; others experience swift and severe muscle weakness and wasting, dying in their late teens to early 20s.
The various types of MD affect more than 50,000 Americans. Through advances in medical care, children with muscular dystrophy are living longer than ever before.
The Major Forms of Muscular Dystrophy
• Myotonic (also called MMD or Steinert's disease). The most common form of muscular dystrophy in adults, myotonic muscular dystrophy affects both men and women, and it usually appears any time from early childhood to adulthood. In rare cases, it appears in newborns (congenital MMD). The name refers to a symptom, myotonia -- prolonged spasm or stiffening of muscles after use. This symptom is usually worse in cold temperatures. The disease causes muscle weakness and also affects the central nervous system, heart, gastrointestinal tract, eyes, and hormone-producing glands. In most cases, daily living isn't restricted for many years. Those with myotonic MD have a decreased life expectancy.
• Duchenne. The most common form of muscular dystrophy in children, Duchenne muscular dystrophy affects only males. It appears between the ages of 2 and 6. The muscles decrease in size and grow weaker over time yet may appear larger. Disease progression varies, but many people with Duchenne (1 in 3,500 boys) need a wheelchair by the age of 12. In most cases, the arms, legs, and spine become progressively deformed, and there may be some cognitive impairment. Severe breathing and heart problems mark the later stages of the disease. Those with Duchenne MD usually die in their late teens or early 20s.
• Becker. This form is similar to Duchenne muscular dystrophy, but the disease is much milder: symptoms appear later and progress more slowly. It usually appears between the ages of 2 and 16 but can appear as late as age 25. Like Duchenne muscular dystrophy, Becker muscular dystrophy affects only males (1 in 30,000) and causes heart problems. Disease severity varies. Those with Duchenne can usually walk into their 30s and live further into adulthood.
• Limb-girdle. This appears in the teens to early adulthood and affects males and females. In its most common form, Limb-girdle muscular dystrophy causes progressive weakness that begins in the hips and moves to the shoulders, arms, and legs. Within 20 years, walking becomes difficult or impossible. Sufferers typically live to middle age to late adulthood.
• Facioscapulohumeral. Facioscapulohumeral refers to the muscles that move the face, shoulder blade, and upper arm bone. This form of muscular dystrophy appears in the teens to early adulthood and affects males and females. It progresses slowly, with short periods of rapid muscle deterioration and weakness. Severity ranges from very mild to completely disabling. Walking, chewing, swallowing, and speaking problems can occur. About 50% of of those with facioscapulohumeral MD can walk throughout their lives, and most live a normal life span.
• Congenital. Congenital means present at birth. Congenital muscular dystrophies progress slowly and affect males and females. The two forms that have been identified -- Fukuyama and congenital muscular dystrophy with myosin deficiency -- cause muscle weakness at birth or in the first few months of life, along with severe and early contractures (shortening or shrinking of muscles that causes joint problems). Fukuyama congenital muscular dystrophy causes abnormalities in the brain and often seizures.
• Oculopharyngeal. Oculopharyngeal means eye and throat. This form of muscular dystrophy appears in men and women in their 40s, 50s, and 60s. It progresses slowly, causing weakness in the eye and face muscles, which may lead to difficulty swallowing. Weakness in pelvic and shoulder muscles may occur later. Choking and recurrent pneumonia may occur.
• Distal. This group of rare diseases affects adult men and women. It causes weakness and wasting of the distal muscles (those farthest from the center) of the forearms, hands, lower legs, and feet. It is generally less severe, progresses more slowly, and affects fewer muscles than other forms of muscular dystrophy.
• Emery-Dreifuss. This rare form of muscular dystrophy appears from childhood to the early teens and affects only males. It causes muscle weakness and wasting in the shoulders, upper arms, and lower legs. Life-threatening heart problems are common and can also affect carriers -- those who have the genetic information for the disease but do not develop the full-blown version (including mothers and sisters of those with Emery-Dreifuss MD). Muscle shortening (contractures) occurs early in the disease. Weakness can spread to chest and pelvic muscles. The disease progresses slowly and causes less severe muscle weakness than some other forms of muscular dystrophy.
What Are the Causes of Muscular Dystrophy?
Muscular dystrophy is caused by defects in certain genes, with type determined by the abnormal gene. In 1986, researchers discovered the gene that, when defective or flawed, causes Duchenne muscular dystrophy. In 1987, the muscle protein associated with this gene was named dystrophin. Duchenne muscular dystrophy occurs when that gene fails to make dystrophin. Becker muscular dystrophy occurs when a different mutation in the same gene results in some dystrophin, but it's either not enough or it's poor in quality. Scientists have discovered and continue to search for the genetic defects that cause other forms of muscular dystrophy.
Most of the muscular dystrophies are a form of inherited disease called X-linked disorders or genetic diseases that mothers can transmit to their sons even though the mothers themselves are unaffected by the disease.
Men carry one X chromosome and one Y chromosome. Females carry two X chromosomes. Thus, in order for a girl to become affected by muscular dystrophy, both of her X chromosomes would have to carry the defective gene -- an extremely rare occurrence, since her mother would have to be a carrier (one defective X chromosome) and her father would have to have muscular dystrophy (since men carry just one X chromosome, the other is a Y chromosome).
A female who carries the defective X chromosome can pass the disease to her son (whose other chromosome is a Y, from the father).
A few muscular dystrophies aren't inherited at all and occur because of a new gene abnormality or mutation.
What Are the Symptoms of Muscular Dystrophy?
The symptoms of muscular dystrophy include:
• All forms: muscle weakness
• Myotonic: muscle spasms or stiffening after use, hand weakness, foot drop (inability to raise the front of one or both feet)
• Duchenne and Becker: clumsiness, frequent falling, difficulty getting up, waddling gait, curvature of the spine
How is Muscular Dystrophy Diagnosed?
Muscular dystrophy (MD) is diagnosed through a physical exam, a family medical history, and tests. These might include:
• A muscle biopsy (the removal and exam of a small sample of muscle tissue)
• DNA (genetic) testing
• Electromyography or nerve conduction tests (which use electrodes to test muscle and/or nerve function)
• Blood enzyme tests (to look for the presence of creatine kinase, which reveals deterioration of muscle fibers)
For Duchenne and Becker muscular dystrophies, the most common types of MD, a muscle biopsy may show whether dystrophin, a muscle protein, is missing or abnormal, and DNA testing is used to analyze the condition of the related gene. Genetic testing also is available for some forms of muscular dystrophy.
What Are the Treatments for Muscular Dystrophy?
There is no cure for any form of muscular dystrophy, but medications and therapy can slow the course of the disease. Human trials of gene therapy with the dystrophin gene are on the near horizon. For instance, scientists are researching ways to insert a working dystrophin gene into the muscles of boys with Duchenne and Becker muscular dystrophies.
Researchers are investigating the potential of certain muscle-building drugs to slow down or reverse the progression of muscular dystrophy. Other trials are looking into the effects of the dietary supplements creatine and glutamine on muscle energy production and storage.
Conventional Medicine for Muscular Dystrophy
Symptoms of muscular dystrophy often can be relieved through exercise, physical therapy, rehabilitative devices, respiratory care, and surgery:
• Exercise and physical therapy can minimize abnormal or painful positioning of the joints and may prevent or delay curvature of the spine. Respiratory care, deep breathing, and coughing exercises are often recommended.
• Canes, powered wheelchairs, and other rehabilitative devices can help those with MD maintain mobility and independence.
• Surgery can sometimes relieve muscle shortening. In Emery-Dreifuss and myotonic muscular dystrophy, it's often necessary to surgically implant a cardiac pacemaker.
In some cases of MD, disease progression can be slowed or symptoms relieved with medication:
• In Duchenne muscular dystrophy, corticosteriods may slow muscle destruction.
• In myotonic muscular dystrophy, phenytoin, procainamide, or quinine can treat delayed muscle relaxation.
Medications also can be prescribed for some muscular dystrophy-related heart problems.
Can You Prevent Muscular Dystrophy?
If you have a family history of muscular dystrophy, you may want to consult a genetic counselor before having children. The odds of passing the disease on to your children range from 25% to 50%. Carriers -- typically sisters and mothers of those with MD -- usually don't have the disease, but they may exhibit mild symptoms that give hints of it. They can pass the disease on to their children; their sons will get the disease and half the time, their daughters become carriers. For Duchenne and Becker muscular dystrophies, protein and DNA tests can identify carriers, and DNA probes can provide prenatal diagnosis. Tests for carriers of other forms of muscular dystrophy are under development.
