Fact Sheet # 20
Is Myotonic Dystrophy also known by other names?
Yes, Myotonic Dystrophy is also known as Steinert’s disorder or Dystrophia Myotonica.
What is Myotonic Dystrophy?
Myotonic dystrophy is the most common adult form of muscular dystrophy - the general term for a group of hereditary disorders where the most prominent feature is muscle weakness and progressive wasting of voluntary muscles. Myotonic Dystrophy is associated with myotonia (delayed relaxation of muscles after contraction). Myotonic Dystrophy can affect the tissues and organs of many body systems in addition to the voluntary muscle system. Clinical features include cataracts, disturbance of the heart rhythm, hormonal problems, frontal balding (in males), testicular atrophy, difficulty in swallowing (dysphagia) and learning difficulties in children. Consequently, myotonic dystrophy may present itself in a variety of ways. While affecting predominantly adults, it also occurs in infancy and childhood. Congenital Myotonic Dystrophy is the early childhood form of Myotonic Dystrophy.
What causes Myotonic Dystrophy?
A faulty gene that is located on chromosome 19 causes myotonic dystrophy. This defective gene has a change (mutation) in one end region of the gene, which is unstable. This change is the expansion of a series of three building blocks (nucleotides) of DNA. This series of three nucleotides is called a "triplet" repeat. The expansion of the triplet repeats is from just a few copies in healthy (unaffected) individuals, to several hundreds in individuals affected with myotonic dystrophy. Once the change is established it often (not always) increases in the following generations, leading to an increase in severity of the disorder as the repeat increases in size. The normal gene produces a protein that is likely to be important in membrane function of the muscle and other organs. In myotonic dystrophy the expansion in the gene is thought to result in an abnormal function of the protein.
What are the symptoms?
Symptoms may present at birth, in childhood or at any age. Myotonic dystrophy can affect the tissues and organs of many body systems in addition to muscle. The following should be noted:
- Many affected individuals show visible signs of the disorder before the age of 20 but a significant number of individuals do not develop clear-cut symptoms until after 50. However, when myotonic dystrophy is suspected (because it is present in other members of the family) careful examination may reveal typical abnormalities before the individual complains of either weakness or myotonia. The course of the disorder can vary widely between individuals within a single family. The range of symptoms is from very mild, so that the individual does not know they have the disorder, to severe, where death may occur within 20 years after appearance of the initial symptoms.
- Muscle weakness and muscle wasting. The first muscles to be affected are those of the face, neck, hands, forearms and feet. Weakness is slowly progressive but most patients remain mobile.
- Muscle stiffness or ‘myotonia’ (delayed relaxation of muscles after contraction) is characteristic, especially affecting the hands.
- Symptoms involving the other organ systems, include blindness due to cataracts, swallowing difficulties, frontal balding in men, diabetes, heart problems (due to an irregular heart beat) and testicular atrophy may occur.
- Symptoms vary between individuals. Individuals who present at a young age are more likely to develop muscular problems.
- Congenital myotonic dystrophy is the severest form and can be fatal. It presents at birth. Infants have profound difficulty with sucking and swallowing and may have severe respiratory difficulties. Motor development is delayed and they may show some learning problems and may be developmentally delayed.
Which muscles are affected?
Muscles in the face, jaw and neck area are mainly involved. The large, weight-bearing muscles of the legs and thighs are much less affected. Myotonia or muscle stiffness characteristically affects the hands. Muscle of the oesophagus can also be involved which leads to swallowing problems. Cardiac (or heart muscle) can also be affected.
How is Myotonic Dystrophy inherited?
Myotonic dystrophy can be transmitted from generation to generation by individuals who themselves have inherited the defective gene and have the disorder. Since the defective gene is transmitted in an autosomal dominant pattern, only one faulty myotonic dystrophy gene, derived from either the father or mother, is required to produce the disorder in a child. There is therefore a 50% chance that a child will be affected. Since a parent can be more mildly affected than a child, it is not unusual for a child to present with myotonic dystrophy before the diagnosis has been made in the affected parent. Further, if a father is the transmitting parent, a child is likely to be similarly affected to his father. However, if the mother is the transmitting parent, she may have a child who may present as early as birth with myotonic dystrophy. As this is a genetic condition, genetic counselling is strongly recommended. Genetic counselling provides information on the inheritance pattern, risks to other family members, prognosis, psycho-social support, as well as information about diagnostic testing, carrier testing, preclinical and prenatal testing (where available).
How is Myotonic Dystrophy diagnosed?
A physical examination will usually reveal the typical pattern of muscle weakness and wasting as well as the presence of myotonia. Special laboratory tests, which may include a muscle biopsy, can confirm the diagnosis. The myotonic dystrophy gene has been characterised and therefore a diagnosis can be made at the genetic level, by providing a blood sample. The test is positive in 100% of individuals with the condition. Thus, muscle biopsies should no longer be used to confirm a diagnosis of myotonic dystrophy. The genetic test can be carried out before any symptoms arise and can therefore also be useful for prenatal or preclinical diagnosis.
Is there a cure?
No, unfortunately no cure has yet been found for myotonic dystrophy.
Is there any treatment?
No specific treatment has yet been found for the muscle weakness and wasting in myotonic dystrophy although ankle and leg braces can help to support muscles as weakness progresses. Medications are advised for particular symptoms, such as muscle stiffness (myotonia) or for heart irregularity. Pacemakers may also be necessary for heart irregularities in some instances.
Is there a risk during anaeshesia?
Yes, anaesthesia is a risk for patients with myotonic dystrophy. It is therefore essential that your family doctor, surgeon and anaesthetist be told that you have this condition, even if you are only mildly affected. If surgery is necessary, it should be carefully planned and done in a hospital with proper back-up. The main problems are because of weak breathing muscles and sensitivity to anaesthetics and drugs used, as well as the disturbances in heart rhythm. It is always recommended that the presence of a muscular disorder be mentioned to your doctor.
Is there any research conducted on Myotonic Dystrophy in South Africa?
Extensive research has been conducted on myotonic dystrophy at the Department of Human Genetics, SAIMR and University of the Witwatersrand. International researchers are working on how the change in the gene and protein results in this muscle disorder.
Please contact your local MDF office for further information.
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