There are two major types of Myotonic Dystrophy (DM) and two types more have been discovered during last few years.

Myotonic Dystrophy Type 1 (DM1) & Myotonic Dystrophy Type 2 (DM2)

They are both caused by a genetic defect, on different genes, even though many symptoms are similar. Both Myotonic Dystrophy Type 1 (DM1) and Myotonic Dystrophy Type 2 (DM2) are characterized by skeletal and smooth muscle weakness and myotonia, heart abnormalities, cataracts and insulin resistance.

DM1 – Myotonic Dystrophy Type 1 is without any doubt the most common type and results from an abnormal DNA expansion in the DMPK gene on chromosome 19.

DM2 – That arises from an abnormal expansion of DNA in the ZNF9 gene on chromosome 3.

Myotonic Dystrophy Type 1 (DM1)

Within DM1 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. Below are listed and explained the three subtypes of Myotonic Dystrophy Type 1 (DM1).

Congenital-Onset DM1 —This type of Myotonic Dystrophy begins at or around the time of birth and it’s the most severe of the three types. Occurs when a mother, unaware of having DM1 passes the gene’s mutation to her child. Characterized by severe muscle weakness, cognitive impairment and other developmental abnormalities. (In DM2, there have been no reported cases of a congenital form).

Juvenile-onset DM1 —This other type of Myotonic Dystrophy begins between early childhood and early adolescence (after birth but before being adolescent) and is characterized by cognitive and behavioral symptoms, muscle weakness, myotonia (difficulty relaxing muscles after use) and other symptoms.

Adult-onset DM1 —The last type of Myotonic Dystrophy may begins after the end of adolescence through old age. Depending on severity of symptoms, patients with adult DM1 may be categorized as having either the mild or classical form of the disease. Individuals with the mild form of adult-onset DM1 are often not aware they have the disorder. Muscle symptoms (if any) may be attributed to general stiffness or arthritis.

This type is characterized by slowly progressive weakness, myotonia, cardiac abnormalities and, sometimes, mild to moderate cognitive difficulties.

Myotonic Dystrophy Type 2 (DM2)

The Myotonic Dystrophy Type 2 (DM2) known as well as 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.

DM2 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 DM1.

In general, the disorder is not as severe as DM1. However, it may affect walking ability earlier than DM1, because it causes early weakening of the hip muscles. This type of Myotonic Dystrophy is rare compared to DM1, except in people of German descent.

Other Types Of Myotonic Dystrophy

There are two more additional forms (respectively called DM3 and DM4) recently discovered, as a small number of individuals have been seen who have the characteristics symptoms of myotonic dystrophy, but they seem to do not have none of the typical genetic mutations which cause these disorders.

Considerable debate exists as to whether these individuals truly represent a new form of myotonic dystrophy or whether they simply present unique diagnostic challenges.