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MYOTONIC DYSTROPHY BOOKLET

MYOTONIC DYSTROPHY An informational booklet for individuals and families

Written by: Wendy C. McKinnon, M.S., Genetic Counselor Vermont Regional Genetics Center

Introduction

This booklet is written for individuals and families with Myotonic Dystrophy, also known as Steinert's disease or Dystrophia Myotonica (DM). The booklet describes what DM is, how it is inherited, and recent discoveries about DM.

DM is an inherited disorder that affects about one in every 8000 people. In general, DM consists of muscle weakness and myotonia (inability of muscles to relax after use) which gets more severe over time. Specific problems in other systems of the body can also occur. Since DM can affect many tissues and organs it is called a "multisystem" disorder.

Myotonic Dystrophy is an extremely variable condition. It can vary in severity, the systems of the body it affects, and the age of onset, even in the same family. People with milder symptoms may never require special medical attention and thus never be diagnosed as having DM. On the other hand, newborn babies who are more severely affected may die during infancy without the diagnosis of DM ever having been made. If a newborn is diagnosed with DM, this may be the first time the family learns about the condition. Recent discovery of the gene alteration which causes DM helps explain this condition's incredible variation.

What are the clinical features of Myotonic Dystrophy?

It is important to realize that any one individual with DM is unlikely to have all of the features described here, but will probably have some of them and these may vary greatly from one individual to another.

What other muscles are affected in DM?

Speech requires the coordination of the muscles of the voice box (larynx), the throat , the tongue, the lips, and the roof of the mouth (palate). If any of these muscles are affected by DM, speech may sound slurred or indistinct. It is also common for individuals with DM to have somewhat "nasal" speech.

How are the heart and blood vessels affected?

There is nothing unusual about the structure of the heart muscles in individuals with DM. However, there can be abnormalities in how the heart beats. An irregular beat is referred to as a cardiac arrhythmia. Some people have no symptoms as a result of a cardiac arrhythmia. Other people have palpitations ("fluttering"), chest pain or tightness.

What other systems are affected by DM?

Individuals with DM can have abnormalities of the endocrine system. The endocrine system consists of a number of glands that produce substances called hormones which are carried by the bloodstream to various parts of the body.

What is the age of onset of Myotonic Dystrophy?

Symptoms of DM can first occur at various ages. In fact, there appear to be four stages of onset: congenital (at birth), juvenile (childhood), classic (20-40 years) and late (after 40 years of age).

The genetic nature of Myotonic Dystrophy

DM is caused by a genetic alteration in the myotonin protein kinase gene located on chromosome 19. Since our chromosomes come in pairs, we have two copies of the myotonin protein kinase gene

What is the genetic alteration in the myotonin protein kinase gene?

The genetic alteration found in the myotonin protein kinase gene is a repeating sequence of three specific nucleotides (the building blocks which make up DNA, which in turn make up genes).

Can the clinical features vary from person to person in the same family?

Problems can range from very mild to very severe even within the same family, and it is not always obvious who in the family has the DM gene alteration.

What if a doctor suspects an individual has Myotonic Dystrophy

If a doctor suspects that an individual has DM because features of the condition are present, genetic testing may be performed to confirm the diagnosis. Genetic testing involves obtaining about two teaspoons of blood. DNA is isolated from blood cells and then the number of CTG repeats in the DM gene can be determined. Testing takes an average of 2-3 weeks. Its cost varies somewhat but in 1994 is approximately $300.00

What if an individual has no clinical features of DM but has a family history of DM?

If an individual diagnosed with DM is identified as having the CTG repeat expansion, a family member who has no features of the condition may also consider having genetic testing.

What about having children?

When one member of a couple has DM, there are a number of options available when thinking about having children.

Prenatal Diagnosis

Prenatal diagnosis is available to couples who would like to know during a pregnancy whether their fetus has inherited the DM gene from an affected parent. Prenatal diagnosis can be discussed with a genetic counselor.

Genetic Counseling

Genetic counseling is often useful for individuals and families with questions and concerns about specific genetic conditions

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There are no specific treatments currently available for individuals with DM. It is hoped that with advances in research, a better understanding of the underlying cause of DM will lead to specific treatments for its symptoms. Medications are available that your doctor can prescribe to reduce myotonia, especially if it interferes with daily activities. Physical therapy and occupational therapy, as well as regular exercise may help minimize the progression of muscle weakness. Ankle and leg braces may help support these muscles.

Additional Resources
Muscular Dystrophy Association (MDA) National Headquarters 3300 East Sunrise Drive Tucson, AZ 85718-3208 Telephone (602) 529-2000 / Fax (602) 529-5300	The National Society of Genetic Counselors (NSGC) Executive Office 233 Canterbury Drive Wallingford, PA 19086 Telephone (610) 872-7608

Selected Bibliography

Brook, JD, et al, Molecular Basis of Myotonic Dystrophy: Expansion of a Trinucleotide (CTG) Repeat at the 3' End of a Transcript Encoding a Protein Kinase Family Member. Cell 68:799-808, 1992.

Acknowledgements

I thank Marisa Ladoulis for her efforts in writing this booklet. I thank all those who took the time to review the booklet (Alan Guttmacher, Kelly Ormond, Barb Biesecker, E. Stanley Emery, and Rup Tandan) Your critiques were invaluable. I thank Gary Nelson of the University of Vermont for the illustrations and the Department of Pediatrics of the University of Vermont College of Medicine for financial support. In addition, I thank Pauline Rainville who gave me the inspiration to write this booklet.

This booklet was published by the Vermont Regional Genetics Center. The Vermont Regional Genetics Center is a combined effort of the Department of Pediatrics of the University of Vermont College of Medicine, the Vermont Department of Health, and the New York State Department of Health.

Wendy C. McKinnon, M.S. October, 1994