Merck Manual

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Myotonic Dystrophy

(Steinert Disease)


Michael Rubin

, MDCM, Weill Cornell Medical College

Last full review/revision Jul 2020| Content last modified Jul 2020
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Myotonic dystrophy is a rare muscular dystrophy. This disorder affects the ability to relax the muscles at will.

Myotonia refers to delayed relaxation after muscle contraction, which can cause muscle stiffness. Muscular dystrophies are a group of inherited muscle disorders in which one or more genes needed for normal muscle structure and function are defective, leading to muscle weakness and muscle wasting (dystrophy) of varying severity.

Myotonic dystrophy is an autosomal dominant disorder, which means only one affected parent is needed to pass the trait on to offspring. This dystrophy affects males and females and about 1 in 8,000 people.


Symptoms of myotonic dystrophy begin during adolescence or young adulthood.

The disorder causes myotonia. Myotonia is the delayed ability to relax the muscles after contracting them. Other main symptoms are weakness and wasting of arm and leg muscles (especially in the hands) and muscles in the face. Drooping eyelids are also common. The heart muscle also becomes weak (cardiomyopathy) and the heart rhythm may become abnormal.

Symptoms of myotonic dystrophy begin during adolescence or young adulthood and can range from mild to severe. People with the most severe form of the disorder have extreme muscle weakness and many other symptoms, including cataracts, small testes (in men), premature balding in the front (in men), irregular heartbeats, diabetes, and intellectual disability. They usually die by about age 54.

Mothers with myotonic dystrophy may have children with a severe form of myotonia that appears during infancy. Infants have severely reduced muscle tone (hypotonia, or "floppiness"), feeding and breathing problems, bone deformities, facial weakness, and delays in development of thought processes and physical movement. Up to 40% of infants do not survive, usually because of respiratory failure and perhaps cardiomyopathy. Up to 60% of survivors have intellectual disability.


  • Genetic testing

The diagnosis of myotonic dystrophy is based on characteristic symptoms, the person's age when symptoms began, and family history.

Genetic testing is done to confirm the diagnosis.


  • Drugs to relieve muscle stiffness

Treatment of myotonic dystrophy with mexiletine or other drugs (for example, quinidine, phenytoin, carbamazepine, or procainamide) may relieve the stiffness, but these drugs do not relieve the weakness, which is the most bothersome symptom to the person. Also, each of these drugs has undesirable side effects. The only treatment for muscle weakness is supportive measures, such as ankle braces (for footdrop) and other devices.

More Information

The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

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