DM1 is the most common form of myotonic muscular dystrophy diagnosed in children, with a prevalence ranging from 1 per 100,000 in Japan to 3-15 per 100,000 in Europe. The prevalence may be as high as 1 in 500 in regions such as Quebec, possibly due to the founder effect. In most populations, DM1 appears to be more common than DM2. However, recent studies suggest that type 2 may be as common as type 1 among people in Germany and Finland.

The incidence of congenital myotonic dystrophy is thought to be about 1:20,000. DM occurs in about 1 per 7,000–8,000 people and has been described in people from all over the world. It affects males and females approximately equally. About 30,000 people in the United States are affected.

Myotonic dystrophy is a long term genetic disorder that affects muscle function. Symptoms include gradually worsening muscle loss and weakness. Muscles often contract and are unable to relax. Other symptoms may include cataracts, intellectual disability, and heart conduction problems. In men there may be early balding and an inability to have children.

Myotonic dystrophy is an autosomal-dominant disorder which is typically inherited from a person's parents. There are two main types: type 1 (DM1) due to mutations in the DMPK gene and type 2 (DM2) due to mutations in the CNBP gene. The disorder generally worsens in each generation. A type of DM1 may be apparent at birth. DM2 is generally milder. They are types of muscular dystrophy. Diagnosis is confirmed by genetic testing.

There is no cure. Treatments may include braces or wheelchairs, pacemakers, and non invasive positive pressure ventilation. The medications mexiletine or carbamazepine are occasionally helpful. Pain if it occurs may be treated with tricyclic antidepressants and nonsteroidal anti inflammatory drugs (NSAIDs).

Myotonic dystrophy affects more than 1 in 8,000 people worldwide. While myotonic dystrophy can occur at any age, onset is typically in the 20s and 30s. It is the most common form of muscular dystrophy that begins in adulthood. It was first described in 1909 with the underlying cause of type 1 determined in 1992.