What Is 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:
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 Becker 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.