Myotonic dystrophy is a progressive disease in which the muscles are weak
and slow to relax after contraction.
Myotonic dystrophy (DM), also called dystrophia myotonica, myotonia
atrophica, or Steinert disease, is a common form of
. 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
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.
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
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
. 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
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
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
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.
Parents should let the doctor know as soon as possible if there is a
history of DM. Otherwise, they should contact their pediatrician if the
child shows any signs of delayed or abnormal growth, or unexplained muscle
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
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.
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.
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.
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.
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.
Electrocardiagram (ECG, EKG)
—A record of the electrical activity of the heart, with each wave
being labeled as P, Q, R, S, and T waves. It is often used in the
diagnosis of cases of abnormal cardiac rhythm and myocardial damage.
—A diagnostic test that records the electrical activity of
muscles. In the test, small electrodes are placed on or in the skin; the
patterns of electrical activity are projected on a screen or over a
loudspeaker. This procedure is used to test for muscle disorders,
including muscular dystrophy.
—A group of inherited diseases characterized by progressive
wasting of the muscles.
—A sleep disorder characterized by periods of breathing cessation
lasting for 10 seconds or more.
Trinucleotide repeat expansion
—A sequence of three nucleotides that is repeated too many times
in a section of a gene.
The International Myotonic Dystrophy Consortium (IDMC). "New
nomenclature and DNA testing guidelines for myotonic dystrophy type 1
54 (2000): 1218–21.
Meola, Giovanni. "Myotonic Dystrophies."
Current Opinion in Neurology
13 (2000): 519–25.
International Myotonic Dystrophy Organization.
P.O. Box 1121, Sunland, CA 91041-1121. (866) 679-7954 or (818)951-2311.
Web site: http://www.myotonicdystrophy.org.
Muscular Dystrophy Association.
3300 East Sunrise Dr., Tucson, AZ 85718. (520) 529-2000 or (800)
572-1717. Web site: http://www.mdausa.org.
Bird, Thomas D. "Myotonic Dystrophy Type 1."
[cited August 9, 2004]. Available online at:
Smith, Corrine O'Sullivan.
Myotonic Dystrophy: Making an
Informed Choice About Genetic Testing.
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