The two major types of myotonic muscular dystrophy (MMD) — MMD1 and MMD2 — are both caused by genetic defects.

MMD1, the most common type, results from an abnormal DNA expansion in the DMPK gene on chromosome 19.

MMD2 arises from an abnormal expansion of DNA in the ZNF9 gene on chromosome 3.

Within MMD1 there are additional subtypes, depending on a person’s age at onset of symptoms. The age of onset is roughly correlated with the size of the DNA expansion, with larger expansions associated with earlier disease onset.

The subtypes of MMD1 are:

• congenital-onset MMD1 — begins at or around the time of birth and is characterized by severe muscle weakness, cognitive impairment and other developmental abnormalities
• juvenile-onset MMD1 — begins during childhood (after birth but before adolescence) and is characterized by cognitive and behavioral symptoms, muscle weakness, myotonia (difficulty relaxing muscles after use) and other symptoms
• adult-onset MMD1 — begins in adolescence or early adulthood and is characterized by slowly progressive weakness, myotonia, cardiac abnormalities and, sometimes, mild to moderate cognitive difficulties

MMD2 — sometimes called PROMM (proximal myotonic myopathy) — has not been seen in a congenital-onset form and rarely begins in childhood. Therefore, it is not described in subtypes.

MMD2 tends to involve the proximal muscles (close to the center of the body) rather than the distal muscles (far from the center of the body) that are the first to be affected in MMD1. In general, the disorder is not as severe as MMD1. However, it may affect walking ability earlier than MMD1, because it causes early weakening of the hip muscles. MMD2 is rare compared to MMD1, except in people of German descent.

For more, see Causes/Inheritance.

Below is a general comparison of the major features of MMD1 and MMD2.