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:
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.
|Comparison of MMD1 and MMD2|
|age of onset||birth to adulthood||8 to 60 years|
|earlier onset with each generation||often occurs||doesn't appear to occur|
|neck muscle weakness||common, early||common, early|
|hip and thigh weakness||late||early|
|distal muscle weakness||prominent||mostly hands|
|weakness anywhere||can occur||can occur|
|muscle pain||can occur||often occurs|
|enlargement of calf muscles||does not occur||occurs|
|early cataracts (of the eyes)||occurs||occurs|
|early balding in males||common||common|
|cardiac rhythm abnormalities||common||variable|
|cardiac muscle degeneration||can occur, especially late in disease course||not common|
|excessive daytime sleepiness||common||variable|
|cognitive impairment||occurs often; can be mild to severe||can occur; generally mild|
|respiratory abnormalities||common, particularly sleep-disordered breathing and inadequate breathing||not common|
|gastrointestinal disturbances||difficulty swallowing, constipation, diarrhea, gallstones can occur||not common|
|uterine dysfunction during labor and delivery||can occur||not common|
|high blood sugar because of insulin resistance||can occur||can occur|