Myotonic dystrophy (DM) is a type of muscular dystrophy, a group of genetic disorders that cause progressive muscle loss and weakness. In DM, muscles are often unable to relax after contraction. Other manifestations may include cataracts, intellectual disability and heart conduction problems. In men, there may be early balding and an inability to father children. While myotonic dystrophy can occur at any age, onset is typically in the 20s and 30s.

Myotonic dystrophy is caused by a genetic mutation in one of two genes. Mutation of the DMPK gene causes myotonic dystrophy type 1 (DM1). Mutation of CNBP gene causes type 2 (DM2). DM is typically inherited, following an autosomal dominant inheritance pattern, and it generally worsens with each generation. A type of DM1 may be apparent at birth. DM2 is generally milder. 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 can help relax muscles. Pain, if it occurs, may be treated with tricyclic antidepressants and nonsteroidal anti-inflammatory drugs (NSAIDs).

Myotonic dystrophy affects about 1 in 2,100 people, a number that was long estimated to be much lower (often cited as 1 in 8,000), reflecting that not all patients have immediate symptoms and, once they do have symptoms, the long time it typically takes to get to the right diagnosis. 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.