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Genetic and Rare Diseases Information Center (GARD)

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Myotonic dystrophy

Other Names for this Disease
  • Dystrophia myotonica
  • Myotonia atrophica
  • Myotonia dystrophica
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Your Question

A doctor recommended that I undergo some genetic testing following my biopsy that revealed myotonic dystrophy. What is this condition, and what may be the underlying cause? Also, I cannot find a more affordable genetic lab than the one I was referred to? Can you help locate one?

Our Answer

We have identified the following information that we hope you find helpful. If you still have questions, please contact us.

What is myotonic dystrophy?

Myotonic dystrophy is an inherited condition that affects the muscles and other body systems. It is the most common form of muscular dystrophy that begins in adulthood, usually in a person's 20s or 30s. This condition is characterized by progressive muscle loss and weakness, particularly in the lower legs, hands, neck, and face. People with myotonic dystrophy often have prolonged muscle tensing (myotonia) and are not able to relax certain muscles after use. The severity of the condition varies widely among affected people, even among members of the same family.[1]

There are two types of myotonic dystrophy: myotonic dystrophy type 1 and myotonic dystrophy type 2. The symptoms in people with myotonic dystrophy type 2 tend to be milder than in those with type 1. Although both types are inherited in an autosomal dominant pattern, they are caused by mutations in different genes. Myotonic dystrophy type 1 is caused by mutations in the DMPK gene, while type 2 is caused by mutations in the CNBP gene.[1]
Last updated: 3/12/2014

What causes myotonic dystrophy?

Myotonic dystrophy is caused by mutations called nucleotide repeat expansions in either the DMPK gene (in type 1) or the CNBP gene (in type 2). Nucleotide repeat expansions occur when a piece of DNA is abnormally repeated a number of times, which makes the gene unstable. In myotonic dystrophy, the gene instability leads to a series of events that ultimately prevent cells in muscles and other tissues from acting normally, leading to the features of the condition.

The exact functions of these genes in not well understood. The DMPK gene may play a role in communication within cells, specifically in cells of the heart, brain, and skeletal muscles. The CNBP gene gives directions to make a protein found mainly in cells of the heart and skeletal muscles, where it is thought to regulate the activities of other genes.[1]
Last updated: 3/12/2014

What are nucleotide repeats or expansions? How does this specific type of mutation cause myotonic dystrophy? 

As a quick review, the information in the genes of our DNA is in the form of a genetic code made up of four "letters" (also called nucleotides) : A, T, C, and G. The information in each gene is arranged as a string of "code words"; each "word" is made up of three of the four letters (e.g. ATC, CGG). Each code word is referred to as a triplet. In some genes, the same triplet is repeated with the DNA sequence. This is a sequence of "triplet repeats". The number of times that a triplet code is repeated in a gene can be critical.[2]

Myotonic dystrophy type 1 is caused by the expansion of the triplet repeat CTG in the DMPK gene. The disease occurs when the CTG segment is abnormally repeated from 50 to 5,000 times.[3]  Myotonic dystrophy type 2 is caused by expansion of a string of four nucleotides: C, C, T, and G. When the CCTG string is abnormally repeated 75 to more than 11,000 times the disease occurs.[4] In both types of myotonic dystrophy, the mutations that cause the conditions result in the production of abnormal products that are normally used in the making of proteins. The abnormal product interacts with certain proteins to form clumps within the cell. The changes that occur prevent cells in muscles and other tissues from working properly, leading to the signs and symptoms seen in myotonic dystrophy types 1 and 2.[4][3]
Last updated: 6/17/2009

Where I can find information about which laboratories are offering genetic testing for myotonic dystrophy? How can I find out how much these laboratories charge for genetic testing? lists the names of clinical and research laboratories offering genetic testing for myotonic dystrophy. (Click here to learn more about the differences between genetic testing performed by clinical versus research laboratories.) Visit the links below to obtain the contact information for these laboratories. Please note: Most laboratories do not accept contact from patients and their families; therefore, you may have to work with your health care provider to learn more, including to find out about costs.

Myotonic dystrophy type 1
Clinical Testing
Research Testing

Myotonic dystrophy type 2
Clinical Testing
Research Testing
Last updated: 6/17/2009

How can I find a genetics professional in my area?

Genetics clinics are a source of information for individuals and families regarding genetic conditions, treatment, inheritance, and genetic risks to other family members. More information about genetic consultations is available from Genetics Home Reference. To find a genetics clinic, we recommend that you contact your primary healthcare provider for a referral.

The following online resources can help you find a genetics professional in your community:

Last updated: 1/21/2014

How can I find additional resources for myotonic dystrophy?

To locate additional information and supportive resources for myotonic dystrophy, click here to view the GARD resource Web page for this condition. If you are interested in learning more about the specific types of myotonic dystrophy, click here for the GARD resource Web page on type 1 and click here for the GARD resource Web page on type 2.
Last updated: 6/17/2009