What is it?
Muscular dystrophy (MD) is a group of inherited muscle diseases in which muscle fibers are unusually susceptible to damage. Muscles, primarily voluntary muscles, become progressively weaker. In the late stages of muscular dystrophy, fat and connective tissue often replace muscle fibers. Some types of muscular dystrophy affect heart muscles, other involuntary muscles and other organs.
The most common types of muscular dystrophy appear to be due to a genetic deficiency of the muscle protein dystrophin.
There's no cure for muscular dystrophy, but medications and therapy can slow the course of the disease.
Signs and symptoms vary according to the type of muscular dystrophy. In general, muscular dystrophy symptoms may include:
- Muscle weakness
- Apparent lack of coordination
- Progressive crippling, resulting in fixations (contractures) of the muscles around your joints and loss of mobility
Specific signs and symptoms vary among the different forms of MD. Each type is different in the age of onset, which parts of the body the symptoms primarily affect and how rapidly the disease progresses.
These types of muscular dystrophies are due to a genetic defect of the protein dystrophin.
Duchenne's muscular dystrophy is the most severe form of dystrophinopathy. It occurs mostly in young boys and is the most common form of MD that affects children. Signs and symptoms of Duchenne's MD may include:
- Frequent falls
- Large calf muscles
- Difficulty getting up from a lying or sitting position
- Weakness in lower leg muscles, resulting in difficulty running and jumping
- Waddling gait
- Mild mental retardation, in some cases
Signs and symptoms of Duchenne's usually appear between the ages of 2 and 3. It first affects the muscles of the pelvis, upper arms and upper legs. By late childhood, most children with this form of muscular dystrophy are unable to walk. Most die by their 20s or early 30s, often from pneumonia, respiratory muscle weakness or cardiac complications. Some people with Duchenne's MD may exhibit curvature of their spine (scoliosis).
Becker's muscular dystrophy is a milder form of dystrophinopathy. It generally affects older boys and young men, and progresses more slowly, usually over several decades. Signs and symptoms of Becker's MD are similar to those of Duchenne's. The onset of the signs and symptoms is generally around age 11, but may not occur until the mid-20s or even later. Those affected by Becker's MD usually are able to walk through their teens, and often well into adulthood.
Also known as Steinert's disease, this form of muscular dystrophy produces stiffness of muscles and an inability to relax muscles at will (myotonia), as well as the muscle weakness of the other forms of muscular dystrophy.
Although this form of MD can affect children, it often doesn't affect people until adulthood. It can vary greatly in its severity. Muscles may feel stiff after using them. Progression of this form of MD is slow. Besides myotonia, signs and symptoms of adult-onset myotonic dystrophy may include:
- Weakening of voluntary muscles that control your arms and legs, usually beginning with the limb muscles farthest from the torso — the muscles of the feet, hands, lower legs and forearms.
- Weakening of head, neck and face muscles, which may result in the face having a hollow, drooped appearance.
- Weakening of muscles involved in breathing and swallowing. Weaker breathing muscles may result in less oxygen intake and fatigue. Weaker swallowing muscles increase the risk of choking.
- Fainting or dizziness, which may indicate that the disease is interfering with the conduction of electrical signals that keep the heart rate normal.
- Weakening of muscles of hollow internal organs such as those in the digestive tract and the uterus. Depending on which part of the digestive tract is affected, you may experience problems with swallowing as well as constipation and diarrhea. Weakness of the uterine walls may cause problems during childbirth.
- Difficulty sleeping well at night and daytime sleepiness, and inability to concentrate because of the effect of the disease on the brain.
- Frontal balding in men.
- Clouding of the lenses of the eyes (cataracts).
- Mild diabetes.
Rarely, infants have this form of muscular dystrophy, in which case it's called congenital myotonic dystrophy. The infant form is more severe, although infants with myotonic dystrophy don't experience myotonia. Signs in infants may include:
- Severe muscle weakness
- Difficulty sucking and swallowing
- Difficulty breathing
- Cognitive impairment
Facioscapulohumeral muscular dystrophy
Also known as Landouzy-Dejerine dystrophy, this form involves progressive muscle weakness involving:
- Upper arms
- Pelvic area
- Lower arms
When someone with facioscapulohumeral MD raises his or her arms, the shoulder blades may stick out like wings. Progression of this form is slow, with some spurts of rapidly increasing weakness. Onset usually occurs during the teen to early adult years.
Other major types of muscular dystrophy
The other major types of muscular dystrophy include:
- Limb-girdle muscular dystrophy
- Congenital muscular dystrophy
- Oculopharyngeal muscular dystrophy
- Distal muscular dystrophy
- Emery-Dreifuss muscular dystrophy
- Myofibrillar myopathies
Limb-girdle muscular dystrophy
Muscles usually affected first by this form of muscular dystrophy include:
This form then progresses to the arms and legs, though progression is slow. Limb-girdle MD may begin from early childhood to adulthood.
Congenital muscular dystrophy
The term "congenital muscular dystrophy" refers to a group of inherited muscular dystrophies. Signs of these disorders may include:
- General muscle weakness
- Joint deformities
Congenital MD is apparent at birth or becomes evident before age 2. The course of this disorder varies significantly depending on the type. Some forms of congenital MD progress slowly and cause only mild disability, while others progress rapidly and cause severe impairment.
Oculopharyngeal muscular dystrophy
The first sign of this type of muscular dystrophy is usually drooping of the eyelids, followed by weakness of the muscles of the eye, face and throat, resulting in difficulty swallowing. Progression is slow. Signs and symptoms first appear in adulthood, usually in a person's 40s or 50s.
Distal muscular dystrophy
This group involves the muscles farthest away from the center of the body (distal muscles) — those of the hands, forearms, feet and lower legs. The severity is generally less than for other forms of MD, and this form tends to progress slowly. Distal MD generally begins in adulthood between the ages of 40 and 60.
Emery-Dreifuss muscular dystrophy
This form of muscular dystrophy usually begins in the muscles of the:
- Upper arms
Cardiac arrhythmias, stiffness of the spine and muscle contractures are other features of Emery-Dreifuss MD. Emery-Dreifuss MD usually begins in the childhood to early teen years and progresses slowly.
Myofibrillar myopathies (MFMs)
Though in some cases the MFMs affect only the muscles closest to the center of the body (proximal muscles) — such as the shoulder and hip muscles — the distal muscles also are usually involved. This group of muscle disorders also is commonly associated with:
- Stiffness of the spine
- Muscle contractures
- Nerve damage (peripheral neuropathy)
- Thickening and stiffening of the heart muscle (cardiomyopathy)
Muscular dystrophy is a general term for a group of inherited diseases involving a defective gene. Each form of muscular dystrophy is caused by a genetic mutation that's particular to that type of the disease. The most common types of muscular dystrophy appear to be due to a genetic deficiency of the muscle protein dystrophin.
Inheriting Duchenne's or Becker's MD
Duchenne's and Becker's muscular dystrophies are passed from mother to son through one of the mother's genes in a pattern called X-linked recessive inheritance. Boys inherit an X chromosome from their mothers and a Y chromosome from their fathers. The X-Y combination makes them male. Girls inherit two X chromosomes, one from their mothers and one from their fathers. The X-X combination determines that they are female.
The defective gene that causes Duchenne's and Becker's muscular dystrophies is located on the X-chromosome. Women who have only one X-chromosome with the defective gene that causes these muscular dystrophies are carriers and sometimes develop heart muscle problems (cardiomyopathy) and mild muscle weakness. The disease can skip a generation until another son inherits the defective gene on the X-chromosome. In some cases of Duchenne's and Becker's muscular dystrophies, the disease arises from a new mutation in a gene rather than from an inherited defective gene.
Patterns differ for other types of MD
Myotonic dystrophy and most MFMs are passed along in a pattern called autosomal dominant inheritance. If either parent carries the defective gene for myotonic dystrophy, there's a 50 percent chance the disorder will be passed along to a child.
Some of the less common types of muscular dystrophy are passed along in the same inheritance pattern that marks Duchenne's and Becker's muscular dystrophies. Other types of muscular dystrophy can be passed on from generation to generation and affect males and females equally. Still others require a defective gene from both parents.
A careful review of your family's history of muscle disease can help your doctor reach a diagnosis. In addition to a medical history review and physical examination, your doctor may rely on the following in diagnosing muscular dystrophy:
- Blood tests. Damaged muscles release enzymes, such as creatine kinase (CK), into your blood. High blood levels of CK suggest a muscle disease, such as muscular dystrophy.
- Electromyography. A thin-needle electrode is inserted through your skin into the muscle to be tested. Electrical activity is measured as you relax and as you gently tighten the muscle. Changes in the pattern of electrical activity can confirm a muscle disease. The distribution of the disease can be determined by testing different muscles.
- Ultrasonography. High-frequency sound waves are used to produce precise images of tissues and structures within your body. An ultrasound is a noninvasive way of detecting certain muscle abnormalities, even in the early stages of the disease.
- Muscle biopsy. A small piece of muscle is taken for laboratory analysis. The analysis distinguishes muscular dystrophies from other muscle diseases. Special tests can identify dystrophin and other markers associated with specific forms of muscular dystrophy.
- Genetic testing. Blood samples are examined for mutations in some of the genes that cause different types of muscular dystrophy. For Duchenne's and Becker's muscular dystrophies, standard tests examine just the portions of the dystrophin gene responsible for most cases of these types of MD. These tests identify deletions or duplications on the dystrophin gene in more than two-thirds of people with Duchenne's and Becker's MDs. The genetic defects responsible for Duchenne's and Becker's muscular dystrophies are harder to identify in other cases of those affected, but new tests that examine the entire dystrophin gene are making it possible to pinpoint tiny, less common mutations.