Myotonic Muscular Dystrophy (MMD)
What is myotonic muscular dystrophy (MMD)?
Myotonic muscular dystrophy (MMD) is a form of muscular dystrophy that affects muscles and many other organs in the body.
The word myotonic is the adjective for the wordmyotonia, an inability to relax muscles at will. The term muscular dystrophy means progressive muscle degeneration, with weakness and shrinkage of the muscle tissue.
Myotonic muscular dystrophy often is abbreviated as “DM” in reference to its Greek name,dystrophia myotonica. Other names for this disorder include simply myotonic dystrophy and, occasionally, Steinert disease, after the German doctor who originally described the disorder in 1909.
(MDA, which covers more than 40 forms of neuromuscular disease, uses the abbreviation “MMD” because “DM” refers to another disease in its program, dermatomyositis.)
What causes MMD?
MMD is divided into two types.
Type 1 MMD (MMD1) occurs when a gene on chromosome 19 called DMPK contains an abnormally expanded section.
Type 2 MMD (MMD2) is caused by an abnormally expanded section in a gene on chromosome 3 called ZNF9. MMD2 was originally called PROMM, for proximal myotonic myopathy, a term that has remained in use but is somewhat less common than the term MMD2.
The expanded sections of DNA in these two genes appear to have many complex effects on various cellular processes.
What are the symptoms of MMD?
MMD causes weakness of the voluntary muscles, although the degree of weakness and the muscles most affected vary greatly according to the type of MMD and the age of the person with the disorder.
Myotonia, the inability to relax muscles at will, is another feature of MMD. For example, it may be difficult for someone with MMD to let go of someone's hand after shaking it.
As the disease progresses, the heart can develop an abnormal rhythm and the heart muscle can weaken. The muscles used for breathing can weaken, causing inadequate breathing, particularly during sleep.
In addition, in type 1 MMD (see Types of MMD), the involuntary muscles, such as those of the gastrointestinal tract, can be affected. Difficulty swallowing, constipation and gallstones can occur. In females, the muscles of the uterus can behave abnormally, leading to complications in pregnancy and labor.
The development of cataracts (opaque spots in the lenses of the eyes) relatively early in life is another characteristic of MMD, in both type 1 and type 2.
Overall intelligence is usually normal in people with MMD, but learning disabilities and an apathetic demeanor are common in the type 1 form. In congenital MMD1, which affects children from the time of birth, there can be serious impairment of cognitive functioning. These children also may have problems with speech, hearing and vision.
Generally, the earlier MMD1 begins, the more profound the symptoms tend to be. For more, see Signs and Symptoms.
In general, MMD2 has a better overall prognosis than MMD1. The symptoms are often relatively mild and progress slowly. MMD2 rarely occurs during childhood, and there is no known congenital-onset form.
What is the progression of MMD?
The progression of MMD varies greatly among individuals, but in general, symptoms progress slowly.
The most common type of MMD1 — the "adult-onset" form — begins in adolescence or young adulthood, often with weakness in the muscles of the face, neck, fingers and ankles. The weakness is slowly progressive for these and eventually other muscles.
When MMD1 begins earlier in life than adolescence — the congenital-onset and juvenile-onset forms of the disease — it may be quite different in progression from the adult-onset type. Children with congenital-onset MMD1, once they survive the crucial neonatal period of respiratory muscle weakness with the help of assisted ventilation, usually show improvements in motor and breathing functions over the first year or so. They may have cognitive impairment, delayed speech, difficulty eating and drinking and various other developmental delays. Most will learn to walk. As adolescence approaches, children begin to show symptoms of the adult-onset form of MMD1 and follow its usual progression.
The childhood-onset form of MMD1 — beginning after infancy but before adolescence — is more often characterized by cognitive and behavioral abnormalities than by physical disabilities. Eventually, muscle symptoms develop, to varying degrees.
MMD2 is, in general, a milder disease than type 1. It does not appear to have a congenital-onset form and rarely begins in childhood.
In contrast to type 1 MMD, the muscles affected first in MMD2 are the proximal muscles — those close to the center of the body — particularly those around the hips. However, some finger weakness may be seen early as well. The disorder progresses slowly, but mobility may be impaired early because of weakness of the large, weight-bearing muscles.
MMD2 is quite rare, except in Germany and in people of German descent. Not as much is known about MMD2 as about MMD1.
What is the status of research on MMD?
Identification of the genetic mutations underlying MMD 1 and MMD2, and understanding at least in part how the mutations cause disease, has opened up avenues for therapy development in MMD.
Most of the strategies currently in development aim to block the harmful effects of the expanded DNA in the DMPK gene (type 1) or the ZNF9 gene (type 2). As of 2012, studies in mice are encouraging, and human trials are on the near horizon. For more, see Research and In Focus: Myotonic Muscular Dystrophy, particularly MMD Research: Seeking to Free Proteins from a ‘Toxic Web’.