Myotonic dystrophy type 2: An inherited disorder of the muscles and other body systems characterized by progressive muscle weakness, prolonged muscle contractions (myotonia), clouding of the lens of the eye (cataracts), cardiac abnormalities, balding, and infertility. Type 2 myotonic dystrophy is caused by mutation of a different gene than type 1 myotonic dystrophy and tends to be milder than type 1.
Only about 2% of people with myotonic dystrophy reportedly have type 2. The gene for type 2 myotonic dystrophy was only discovered in 2001, which may help explain the low number of diagnoses to date. Type 2 myotonic dystrophy may be as common as type 1 myotonic dystrophy in families with German ancestry.
Mutations in the ZNF9 gene cause type 2 myotonic dystrophy. These mutations are abnormally long expansions of a tetranucleotide repeat sequence in the DNA. The disease more severe as the disorder is passed down from one generation to the next. This phenomenon is called anticipation. Myotonic dystrophy, type 2 is inherited in an autosomal dominant pattern, which means one copy of the altered gene is sufficient to cause the disorder. In most cases, an affected person has one affected parent.
Myotonic dystrophy type 2 is also called dystrophica myotonia 2; DM2; myotonic dystrophy 2; myotonic myopathy, proximal; proximal myotonic myopathy; PROMM; and Ricker syndrome.