Definition
Myotonic dystrophy is a progressive disease in which the muscles are weak and slow to relax after contraction.
Description
Myotonic dystrophy (DM), also called dystrophia myotonica, myotonia atrophica, or Steinert disease, is a common form of muscular dystrophy. DM is an inherited disease. It causes general weakness, usually beginning in the muscles of the hands, feet, neck, or face. It slowly progresses to involve other muscle groups, including the heart and a wide variety of other organ systems.
There are four types of DM as determined by when symptoms appear. These are:
- Congenital: Severe symptoms are apparent at birth.
- Juvenile: Symptoms appear between birth and adolescence.
- Adult: Symptoms appear in individuals ages 20–40.
- Late onset: Mild symptoms appear after age 40.
Transmission
DM is an inherited disease. It is passed from parent to child through an autosomal dominant pattern of inheritance. In the case of DM, one copy of each gene is inherited from each parent. In an autosomal dominant pattern of inheritance, only one of these two copies needs to have the mutation (change) or defect in order for the child to have DM. Therefore, there is a 50 percent chance that a parent who has DM will pass it onto each child. This percentage is not changed by results of other pregnancies. In each pregnancy, a parent with DM has a 50% chance of having a child with DM.
Demographics
Myotonic dystrophy is an uncommon disease occurring in about one out of every 8,000 individuals. It is found worldwide. The congenital form of DM is much rarer, occurring in only about one out of every 100,000 births. DM affects males and females approximately equally.
Causes and Symptoms
The most common type of DM is called DM1, which is caused by a mutation in a gene called myotonic dystrophy protein kinase (DMPK). The DMPK gene is located on chromosome 19. The specific mutation that causes DM1 is called a trinucleotide repeat expansion. In people who have DM1, a particular unit of the gene is repeated too many times—more than the normal range of five to 38 times—and thus this section of the gene is too big and is unstable. The enlarged section of the gene is called a trinucleotide repeat expansion.
People who have repeat numbers in the normal range will not develop DM1 and cannot pass it to their children. Having more than 50 repeats causes DM1. People who have 38–49 repeats have what is called a premutation. They do not develop DM1, but can pass DM1 on to their children.
Myotonic dystrophy has an effect called "anticipation." This means that when a person with repeat numbers in the affected or premutation range (above 38) has children, the expansion grows larger, and the child has more of the repeated genetic unit (a higher repeat number). As a result, symptoms of the disease tend to appear at an earlier age in children than in their affected parent. Anticipation happens more often when a mother, rather than the father, passes DM1 to children. Occasionally, repeat sizes stay the same or even get smaller when they are passed to a person's children.
In general, the more repeats above 38 an individual has, the earlier the age of onset of symptoms and the more severe the symptoms. Having repeat numbers greater than 1,000 causes congenital myotonic dystrophy. However, this is a general rule. It is not possible to look at a person's repeat number and predict at what age he or she will begin to have symptoms or how the condition will progress.
Some families with symptoms of DM do not have a mutation in the DMPK gene. Instead, they have a mutation in a gene on chromosome 3 that causes four units within the gene to be repeated. This genetic defect is called DM2 or proximal myotonic myopathia (PROMM). Symptoms of DM2 are almost never apparent at birth. This defect has only been decoded since 2001; therefore, less is known about how it functions.
Symptoms of DM vary in severity, and not everyone will have all of the symptoms. In general, myotonic dystrophy causes weakness and delayed muscle relaxation called myotonia. Exactly how the repeat of genetic information causes myotonia, the inability to relax muscles, is not yet understood. The disease somehow blocks the flow of electrical impulses across the muscle cell membrane. Without proper flow of charged particles, the muscle cannot return to its relaxed state after it has contracted.
The most severe form of DM, congenital myotonic dystrophy, may appear in newborns of mothers who have DM1. Congenital myotonic dystrophy is marked by severe weakness, poor sucking and swallowing responses, respiratory difficulty, delayed motor development, and mental retardation. Death in infancy is common in babies with congenital DM.
Symptoms of juvenile and adult onset DM include facial weakness and a slack jaw, drooping eyelids called ptosis, and muscle wasting in the forearms and calves. A person with DM has difficulty relaxing his or her grasp, especially in the cold. DM affects the heart muscle, causing irregularities in the heartbeat. It also affects the muscles of the digestive system, causing constipation and other digestive problems. DM may cause cataracts in the eye, retinal degeneration, low IQ, early frontal balding, skin disorders, atrophy of the testicles, and diabetes. It can also cause sleep apnea, a condition in which normal breathing is interrupted during sleep. DM increases the need for sleep and decreases motivation. Often, severe disabilities do not set in until about 20 years after symptoms begin. Most people with myotonic dystrophy maintain the ability to walk, even late in life.
Some people who have a trinucleotide repeat expansion in their DMPK gene do not have DM symptoms or have very mild symptoms that go unnoticed. It is not unusual for a woman to be diagnosed with DM after she has an infant with congenital myotonic dystrophy.
When to Call the Doctor
Parents should let the doctor know as soon as possible if there is a family history of DM. Otherwise, they should contact their pediatrician if the child shows any signs of delayed or abnormal growth, or unexplained muscle weakness.
Diagnosis
Diagnosis of DM is not difficult once the disease is considered. However, the diagnosis may be masked because symptoms can begin at any age, can be mild or severe, and can occur with a wide variety of associated complaints. Diagnosis of DM begins with a careful medical history and a thorough physical examination to determine the distribution of symptoms and to rule out other causes. A family history of DM or unexplained weakness helps to establish the diagnosis.
Genetic testing, usually using a blood sample, establishes a definitive diagnosis of DM. The DNA in the blood cells is examined and the number of repeats in the affected gene is determined. Other tests may be done to help establish the diagnosis, but only rarely would other testing be needed. An electromyogram (EMG) is a test used to examine how muscles respond to stimulation. Characteristic changes revealed by this test, and seen in DM, help distinguish it from other muscle diseases. Removing a small piece of muscle tissue for microscopic examination is called a muscle biopsy. DM is marked by characteristic changes in the structure of muscle cells that can be seen on a muscle biopsy. An electrocardiogram could be performed to detect abnormalities in heart rhythm associated with DM. These symptoms often appear later in the course of the disease.
If genetic testing in a family has identified a DMPK mutation, it is possible to test a fetus during pregnancy. Testing can be done at 10–12 weeks gestation by a procedure called chorionic villus sampling (CVS) that involves removing a tiny piece of the placenta and analyzing DNA from its cells. It can also be done by amniocentesis after 14 weeks gestation by removing a small amount of the amniotic fluid surrounding the fetus and analyzing the cells in the fluid. Each of these procedures carries a small risk of miscarriage. Those who are interested in learning more should check with their doctor or genetic counselor.
Treatment
Myotonic dystrophy cannot be cured, and no treatment can delay its progression. However, many of its symptoms can be treated. Physical therapy can help preserve or increase strength and flexibility in muscles. Ankle and wrist braces can support weakened limbs. Occupational therapy is used to develop tools and techniques to compensate for loss of strength and dexterity. A speech-language pathologist can provide retraining for weakness in the muscles controlling speech and swallowing.
Irregularities in heartbeat may be treated with medication or a pacemaker. A yearly electrocardiogram is usually recommended. Diabetes mellitus in DM is treated in the same way that it is in the general population. A high-fiber diet can help prevent constipation. Sleep apnea may be treated with surgical procedures to open the airways or with nighttime ventilation. Treatment of sleep apnea may reduce drowsiness. Lens replacement surgery is available when cataracts develop.
Prognosis
The course of myotonic dystrophy varies. When symptoms appear earlier in life, disability tends to become more severe. Occasionally people with DM may require a wheelchair later in life. Children with congenital DM often die in infancy. If they survive, they usually require special educational programs and physical and occupational therapies. Respiratory infections pose a danger if weakness becomes severe.
Prevention
There is no way to prevent the genetic mutations that cause DM. However, it is possible to test someone who is at risk for developing DM1 before symptoms arise, to see whether he or she inherited an expanded trinucleotide repeat. This is called predictive testing. Predictive testing cannot determine the age at which someone will begin to have symptoms or the course of the disease.
Another procedure, called preimplantation diagnosis, allows a couple to have a child that does not have the genetic condition. This procedure is still experimental. Those interested in learning more about the procedure should check with their doctor or genetic counselor.
Parental Concerns
Pregnant woman should be cared for by an obstetrician familiar with the particular problems of DM because complications can occur during pregnancy, labor, and delivery.
It is advisable for children or adults with DM to wear a medical alert bracelet. Some emergency medications may have dangerous effects on the heart rhythm in a person with DM. Adverse reactions to general anesthesia may also occur.
See also Muscular dystrophy.
Resources
Periodicals
The International Myotonic Dystrophy Consortium (IDMC). "New nomenclature and DNA testing guidelines for myotonic dystrophy type 1 (DM1)." Neurology 54 (2000): 1218–21.
Meola, Giovanni. "Myotonic Dystrophies." Current Opinion in Neurology 13 (2000): 519–25.
Organizations
International Myotonic Dystrophy Organization. P.O. Box 1121, Sunland, CA 91041-1121. (866) 679-7954 or (818)951-2311. Web site: www.myotonicdystrophy.org.
Muscular Dystrophy Association. 3300 East Sunrise Dr., Tucson, AZ 85718. (520) 529-2000 or (800) 572-1717. Web site: www.mdausa.org.
Web Sites
Bird, Thomas D. "Myotonic Dystrophy Type 1." GeneReviews [cited August 9, 2004]. Available online at: www.genetests.org/profiles/myotonic_d/details.html.
Smith, Corrine O'Sullivan. Myotonic Dystrophy: Making anInformed Choice About Genetic Testing. University of Washington. Available online at: www.depts.washington.edu/neurogen/Myotonic.pdf.
"What Is Myotonic Muscular Dystrophy?" MuscularDystrophy Association [cited October 9, 2004]. Available online at: www.mda.org/publications/fa-mmd-qa.html.
[Article by: Tish Davidson, A.M. Karen M. Krajewski, M.S., C.G.C.]
