Myotonic dystrophy

What causes myotonic dystrophy?

Myotonic dystrophy is caused by mutations (changes) in either the DMPK gene (in type 1) or the CNBP (ZNF9) gene (in type 2). The specific kinds of mutations found in both types of myotonic dystrophy are trinucleotide repeat expansions. These types of mutations occur when a piece of DNA is abnormally repeated a number of times, which makes the gene unstable. The mutated gene makes an altered version of messenger RNA (mRNA), which is a copy of the gene that is normally used for protein production. The abnormal mRNA forms clumps inside the cell that interfere with the production of many proteins. These changes prevent cells in muscles and other tissues from functioning normally, leading to the signs and symptoms of myotonic dystrophy.

The exact functions of the DMPK and CNBP genes are not well understood. DMPK may play a role in communication within cells, specifically in cells of the heart, brain, and skeletal muscles. The CNBP gene gives the body directions to make a protein found mainly in the cells of the heart and skeletal muscles, where it is thought to regulate the activities of other genes.

Last updated on 05-01-20

How is myotonic dystrophy diagnosed?

Myotonic dystrophy is diagnosed by doing a physical exam. A physical exam can identify the typical pattern of muscle wasting and weakness of the jaw and neck muscles and the presence of myotonia. Men may have frontal balding.

There are several laboratory tests that can be used to clarify the clinical diagnosis of myotonic dystrophy. One test, called electromyography (EMG), involves inserting a small needle into the muscle. The electrical activity of the muscle is studied and usually shows characteristic patterns of myotonic dystrophy. Other laboratory tests may include a muscle biopsy, which can be used to determine if the muscle fibers are weaker than they should be (atrophied), or a blood test to determine if there are elevated levels of certain muscle enzymes.

The definitive test for myotonic dystrophy is a genetic test. For this test, a blood or saliva sample is analyzed to determine if there is a mutation in the DMPK __ or CNBP (ZNF9) genes.

The University of Washington provides more information on genetic testing for myotonic dystrophy in their publication titled, "Myotonic Dystrophy: Making an Informed Choice About Genetic Testing."

Last updated on 05-01-20

How is myotonic dystrophy inherited?

Myotonic dystrophy (DM) is inherited in an autosomal dominant pattern. This means that one copy of the altered gene in each cell of the body is enough to cause symptoms of the disease. We inherit one copy of each gene from our mother and the other from our father. Therefore, when a person with myotonic dystrophy goes on to have children, there is a 50% chance that each child will have myotonic dystrophy.

Any person who has myotonic dystrophy can pass the mutation on to his or her children, whether the children are male or female. Myotonic dystrophy type 1 exhibits an unusual genetic pattern called anticipation. Anticipation means the signs and symptoms of a genetic disease begin earlier in life and become more severe as the disease is passed on through generations. Congenital myotonic dystrophy type 1 occurs only when the disease is inherited from the mother.

Last updated on 05-01-20

What is the long-term outlook for people with myotonic dystrophy?

The long-term outlook (prognosis) for each person with myotonic dystrophy (including life expectancy) may depend on the type of myotonic dystrophy and the specific medical problems present. Myotonic dystrophy is a progressive disease, meaning that symptoms worsen as a person gets older.

Although evidence is limited, life expectancy appears to be reduced for people with myotonic dystrophy type 1 (DM1). The most common causes of death in people with DM1 are respiratory and cardiac (heart) symptoms. An increased risk of death may be associated with younger age of onset, more severe muscle weakness, and cardiac conduction defects. People with more mild symptoms of DM1 may have a normal lifespan.

Definitive information about prognosis in people with myotonic dystrophy type 2 is limited, but the condition generally runs a milder course. People with myotonic dystrophy type 2 may have a normal lifespan. While mobility may be impaired at an early age, the ability to walk is often retained until around 60-years-old.

Last updated on 05-01-20

What treatment is available for myotonic dystrophy?

There is currently no cure or specific treatment for myotonic dystrophy. Treatment is aimed at managing symptoms of the disease. Routine physical activity appears to help maintain muscle strength and endurance and to control musculoskeletal pain. Canes, braces, walkers, and scooters can help as muscle weakness progresses.

There are also medications that can lessen pain associated with myotonic dystrophy. Pain management can be achieved through the use of medications prescribed by a doctor. Heart problems associated with myotonic dystrophy can be treated through the insertion of a pacemaker, medications, and regular monitoring of cardiac function. Cataracts can be surgically removed. Testosterone replacement therapy may be used to treat infertility in males.

Current research is focusing on how we might be able to one day use gene- editing technology or other treatments to remove the clumps of RNA that cause the symptoms of myotonic dystrophy. However, this therapy is not yet possible in humans.

GeneReviews has more detailed information about the management of myotonic dystrophy type 1 and type 2.

Last updated on 05-01-20

In-Depth Information

Resources for myotonic dystrophy

GeneReviews provides current, expert-authored, peer-reviewed, full-text articles on myotonic dystrophy type 1 and myotonic dystrophy type 2. These articles describe the application of genetic testing to the diagnosis, management, and genetic counseling of patients with specific inherited conditions.

Last updated on 04-27-20

Name: Myotonic Dystrophy Foundation 1004-A O'Reilly Avenue
San Francisco, CA, 94129, United States
Phone: +1-415-800-7777 Toll Free: 1-86-MYOTONIC (1-866-968-6642) Email: Url:
Name: Muscular Dystrophy Association MDA 222 S Riverside Plaza Suite 1500
Chicago, IL, 60606, United States
Toll Free: 1-833-275-6321 (Helpline) Email: Url:
Name: Muscular Dystrophy UK 61A Great Suffolk Street
London, SE1 0BU, United Kingdom
Phone: (+44) 0 020 7803 4800 Toll Free: 0800 652 6352 (Helpline) Email: Url:
Name: Myotonic Dystrophy Support Group 19-21 Main Road Gedling
Nottingham NG4 3HQ
United Kingdom
Phone: 0808 169 1960 Email: Url:
Name: Myotonic Dystrophy Support and Information Facebook Group Url:
Myotonic Dystrophy 1; DM1 Online Mendelian Inheritance in Man. July 6, 2017; Reference Link Dalton JC, Ranum LPW, and Day JW. Myotonic Dystrophy Type 2 GeneReviews. July 3, 2013; Reference Link Learning About Myotonic Dystrophy National Human Genome Research Institute (NHGRI). June 5, 2017; Reference Link Bird TD. Myotonic Dystrophy National Organization for Rare Disorders. 2017; Reference Link O’Sullivan Smith C, Bennett RL, and Bird TD. Myotonic Dystrophy: Making an Informed Choice About Genetic Testing Department of Neurology, University of Washington School of Medicine. Reference Link Adult-Onset MMD1/MMD2 and Juvenile-Onset MMD1 Muscular Dystrophy Association (MDA). 2017; Reference Link Batra R, Nelles DA, Pirie E, Blue SM, Marina RJ, Wang H, Chaim IA, Thomas JD, Zhang N, Nguyen V, Aigner S, Markmiller S, Xia G, Corbett KD, Swanson MS, and Yeo GW. Elimination of Toxic Microsatellite Repeat Expansion RNA by RNA-Targeting Cas9 Cell. August 8, 2017; Reference Link Konieczny P, Selma-Soriano E, Rapisarda AS, Fernandez-Costa JM, Perez-Alonso M, and Artero R. Myotonic dystrophy: candidate small molecule therapeutics Drug Discovery Today. August 2, 2017; Reference Link Managing DM: FAQs Myotonic Dystrophy Foundation. Reference Link Darras BT and Chad DA. Myotonic dystrophy: Etiology, clinical features, and diagnosis UpToDate. June 6, 2017; Reference Link

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