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Myotonic Dystrophy Information

Author: Thomas C. Weiss

Published: 2009-03-10 : (Rev. 2010-07-26)

Synopsis and Key Points:

Myotonic Dystrophy is a progressive disease involving muscle weakness in which the muscles are slow to relax after contracting.

Main Digest

Myotonic Dystrophy (DM) is a progressive disease involving muscle weakness in which the muscles are slow to relax after contracting.

Myotonic Dystrophy

DM is also referred to by additional names; 'Myotonia Atrophica,' Dystrophia Myotonica,' or, 'Steinert's Disease,' and is a common form of Muscular Dystrophy. DM affects both females and males nearly equally, and is an inherited disease. Approximately thirty-thousand people in America are affected by DM, the symptoms of which appear at any time from infancy to adulthood. DM causes a general weakness in the person affected, commonly starting in the muscles of their face, neck, feet, or hands. The muscle weakness progresses slowly through additional muscle groups, including the person's heart, and affects a number of organ systems.

There is a severe form of DM known as, 'Congenital Myotonic Dystrophy,' which can appear in newborns of mothers who have DM. The term, 'Congenital,' means that the condition is present in the person when they are born. Approximately one in twenty-thousand persons have Myotonic Dystrophy (DM). Approximately one in seven to eight-thousand people have DM, which has been found in people from around the world.

Causes and Symptoms of DM

The most common for of Myotonic Dystrophy is called DM1; it is caused by a mutation in a gene called, 'Myotonic Dystrophy Protein Kinase (DMPK). The DMPK gene is found on chromosome 19q, and when there is a mutation in this gene a person develops DM1. The particular mutation associated with DM1 is referred to as a, 'Trinucleotide Repeat Expansion.'

There are some families who present symptoms of DM, yet do not have a mutation in their DMPK gene. Medical science has discovered that the DM in a number of these families is caused through a mutation in a gene present on chromosome three. Families with this genetic mutation are said to have DM2.

Scientists have linked Congenital Myotonic Dystrophy to a region on chromosome seven which contains a Muscle Chloride Channel gene.

Myotonic Dystrophy and Trinucleotide Repeats

In the DMPK gene is a section of genetic code referred to as a, 'CTG Repeat.' The letters, 'CTG,' stand for three particular nucleotides which are complex organic molecules. These nucleotides are known as, 'Cytosine,' 'Thymine,' and, 'Guanine,' and are repeated a number of times. In persons with DM1 the sequence of these nucleotides is repeated more than the usual thirty-seven times in an average person, making the section of the gene to large. The resulting enlargement of the gene section is referred to as a, 'Trinucleotide Repeat Expansion.'

Persons who have repeat numbers in the common range do not develop DM1, nor will they pass DM1 to their children. Persons who have more than fifty repeats experience DM1. Persons who have thirty-eight to forty-nine repeats have a, 'pre-mutation,' yet will not develop DM1; however, they may pass the disease along to their children. Persons with repeats greater than one-thousand experience Congenital Myotonic Dystrophy.

Generally, the greater the number of repeats the person has in the affected range, the earlier the age of onset of symptoms and severity of those symptoms they will experience. However, it is impossible to observe a person's repeat number and predict the age at which they will present symptoms, or the progression of their condition.

The exact way that the trinucleotide repeat expansion causes Myotonia, as well as the person's inability to relax muscles, is something that is not yet understood. Somehow this disease blocks the flow of electrical impulses across the person's muscle cell membranes. Lacking an appropriate flow of charged particles, the person's muscle cannot return to a relaxed state after contracting.

Science has found that DM2 is caused by a Cytosine-Cytosine-Thymine-Guanine (CCTG) expansion on chromosome three at locus 3q21. It is still unknown as to how this particular repeat affects a person's muscle cell function.

Myotonic Dystrophy and Anticipation

Occasionally, when someone who has repeat numbers in either the pre-mutation or affected range has children, their expansion grows larger; this is referred to as, 'Anticipation.' Anticipation may result in an earlier age of onset in children than that experienced by their affected parent. Anticipation occurs more often when a mother passes DM1 to her child then when it is passed from the father. On occasion, repeat sizes remain the same, or may get smaller, when they are passed to the person's child.

Myotonic Dystrophy and Inheritance

Both females and males, in approximately equal numbers, are affected by DM which is inherited through autosomal dominant inheritance. Autosomal dominant inheritance also means that only one gene in the pair needs to have the mutation in order for the person to be affected by DM. Due to the fact that a person only passes on copy of each gene to their children, there is a fifty-percent chance the person with DM will pass it to each of their children. Further expansion of the person's Trinucleotide repeat also has bearing on whether their children will develop DM1. People who have repeat numbers which are in the average range cannot pass DM1 to their children.

Symptoms of Myotonic Dystrophy

DM involves symptoms with a range of severity. Persons with DM will not all experience every symptom presented below.

Myotonic Dystrophy causes delayed muscle relaxation and weakness referred to as, 'Myotonia.' The symptoms of DM may include a slack jaw, facial weakness, muscle wasting in the person's calves and forearms, and drooping eyelids. Persons with DM may have difficulty with relaxing their grasp, particularly in the cold. DM affects the person's heart muscle, presenting irregularities in their heartbeat. DM may also affect the person's eyes, causing retinal degeneration and cataracts. DM may also cause frontal balding, a lower IQ, skin disorders, diabetes, and atrophy of the testicles. The disease may cause sleep apnea, an increased need for sleep, and decreased motivation. The experience of more severe disabilities usually do not begin until around twenty years after the symptoms of DM have started. Most persons with Myotonic Dystrophy remain able to walk, even late in life.

Congenital Myotonic Dystrophy, a severe form of DM, can appear in newborns of mothers who have DM1. The symptoms of Congenital Myotonic Dystrophy include severe weakness, breathing difficulties, poor swallowing and sucking responses, mental retardation, and delayed motor development. Death during infancy is common in this type of DM.

There are some persons who have a Trinucleotide repeat expansion present in their DMPK gene, yet do not experience symptoms, or experience very mild symptoms which go unnoticed. The potential exists for a woman to be diagnosed with DM after she has an infant with Congenital Myotonic Dystrophy.

Testing for Myotonic Dystrophy

There is a test for persons who are at risk for developing DM1 prior to presenting symptoms in order to see whether they inherited an expanded Trinucleotide repeat. The test is referred to as, 'Predictive Testing.' Predictive testing is unable to determine the age of onset at which a person will experience symptoms, or the course the disease will take within that person.

Diagnosing DM

Diagnosis of DM is not difficult, once the disease is considered. One of the issues is that symptoms may begin at any age, be either severe or mild, and may happen along with a large variety of related complaints. A diagnosis of DM starts with a review of the person's medical history, as well as a complete physical examination in order to determine the distribution of symptoms the person is experiencing, as well as to rule out other potential causes of those symptoms. The person's family history of DM, or unexplained weakness, also helps to establish a diagnosis.

Genetic testing for DM1 provides a definitive diagnosis; this testing is usually starts with a blood sample. DNA in the person's blood cells is examined for the number of repeats in the DMPK gene. Other forms of tests may be performed to assist in the establishment of a diagnosis, but are rarely needed. An Electromyogram (EMG) is a test which is used in order to examine the response of the person's muscles to stimulation. There are characteristic changes which are seen in persons with DM that help to distinguish the disease from other muscles diseases. A Muscle Biopsy may be done, allowing medical professionals to examine the person's muscle tissue under a microscope for characteristic changes in the structure of muscle cells associated with DM. An Electrocardiogram is another test that may be performed in order to detect abnormalities which are characteristic of DM.

Prenatal Testing for DM

Testing in order to determine whether an unborn child is affected is possible if genetic testing in family has identified a DMPK mutation. The testing can be performed at ten to twelve weeks gestation through a procedure referred to as, 'Chorionic Villus Sampling (CVS),' which involves the removal of a tiny piece of placenta, followed by DNA analysis from its cells. The test may also be done by Amniocentesis after fourteen weeks gestation through removal of a small amount of the amniotic fluid surrounding the baby, followed by analysis of the fluid. Each of the tests presents a small risk of miscarriage; persons interested in these tests should learn more about them and check with either their doctor or genetic counselor.

A procedure referred to as, 'Pre-implantation Diagnosis,' allows a couple to have a child that is not affected with the genetic condition their family experiences. The procedure is experimental and is not yet widely available. Persons who are interested in learning more about the procedure should check with either their doctor or genetic counselor.

In Houston, Texas, a group of researchers has reported that they have successfully developed a technique for the detection of the CCTG expansion which causes DM2, as well as estimating the size of the repeat expansion.

Treatment of Myotonic Dystrophy

At this time there is no cure for DM; there is also no treatment available that can delay its progression. There is no standardized treatment for DM because the precise reasons for muscle weakness are not yet fully understood. Many of the symptoms persons with DM experience can be treated. Physical therapies can be used in order to either preserve or increase the person's muscle flexibility and strength. Wrist and ankle braces may be used to support the person's limbs. Occupational therapies may be used in order to develop techniques and tools to compensate for loss of dexterity and strength. Speech-language pathologists may provide retraining for weakness in the muscles controlling speech and swallowing experienced by persons with DM.

Medication or a pacemaker can be used to treat irregularities in a person's heartbeat related to DM. Yearly Electrocardiograms are usually recommended in order to monitor the person's heartbeat. Diabetes experienced by persons with DM is treated in the same manner it is in persons who experience it in the general population. A diet that is high in fiber can help to prevent constipation. There is a surgical procedure which opens a person's airways, or night time ventilation that can help person's with DM who have Sleep Apnea. Treatment for Sleep Apnea can help to reduce drowsiness. Lens replacement surgery is available for persons with DM who have developed cataracts. Women who are pregnant should be followed by an obstetrician who is familiar with problems that are particular to DM because complications may occur during pregnancy, labor and delivery.

Wearing a Medical ID bracelet is something that is advised for persons with DM. Some emergency medications can have dangerous effects on a person's heart rhythm if they have DM. There are also some adverse reactions that may happen while the person is under a general anesthesia.

Prognosis of DM

The course of Myotonic Dystrophy varies from person to person. Should symptoms appear earlier in a person's life, the disability tends to become more severe. On occasion, persons with DM may require the use of a wheelchair later in their lives. Children with Congenital Myotonic Dystrophy often require special education programs, as well as occupational and physical therapy. Persons with both types of DM are at risk of respiratory infections when weakness becomes severe.

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