Myotonic Dystrophy (DM)
Myotonic Dystrophy (DM)
What is myotonic dystrophy (DM)?
Myotonic dystrophy (DM) is a form of muscular dystrophy that affects muscles and many other organs in the body.
The word myotonic is the adjective for the word myotonia, an inability to relax muscles at will. The term muscular dystrophy means progressive muscle degeneration, with weakness and shrinkage of the muscle tissue.
Myotonic dystrophy often is abbreviated as “DM” in reference to its Greek name, dystrophia myotonica. Another name used occasionally for this disorder is Steinert disease, after the German doctor who originally described the disorder in 1909.
What causes DM?
DM is divided into two types.
Type 1 DM (DM1) occurs when a gene on chromosome 19 called DMPK contains an abnormally expanded section.
Type 2 DM (DM2) is caused by an abnormally expanded section in a gene on chromosome 3 called ZNF9. DM2 was originally called PROMM, for proximal myotonic myopathy, a term that has remained in use but is somewhat less common than the term DM2.
The expanded sections of DNA in these two genes appear to have many complex effects on various cellular processes.
What are the symptoms of DM?
DM causes weakness of the voluntary muscles, although the degree of weakness and the muscles most affected vary greatly according to the type of DM and the age of the person with the disorder.
Myotonia, the inability to relax muscles at will, is another feature of DM. For example, it may be difficult for someone with DM 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 DM (see Types of DM), 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 DM, in both type 1 and type 2.
Overall intelligence is usually normal in people with DM, but learning disabilities and an apathetic demeanor are common in the type 1 form. In congenital DM1, 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 DM1 begins, the more profound the symptoms tend to be. For more, see Signs and Symptoms.
In general, DM2 has a better overall prognosis than DM1. The symptoms are often relatively mild and progress slowly. DM2 rarely occurs during childhood, and there is no known congenital-onset form.
What is the progression of DM?
The progression of DM varies greatly among individuals, but in general, symptoms progress slowly.
The most common type of DM1 — 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 DM1 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 DM1, 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 DM1 and follow its usual progression.
The childhood-onset form of DM1 — 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.
DM2 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 DM, the muscles affected first in DM2 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.
DM2 is quite rare, except in Germany and in people of German descent. Not as much is known about DM2 as about DM1.
What is the status of research on DM?
Identification of the genetic mutations underlying DM1 and DM2, and understanding at least in part how the mutations cause disease, has opened up avenues for therapy development in DM.
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). For more, see Research and In Focus: Myotonic Dystrophy, particularly DM Research: Seeking to Free Proteins from a ‘Toxic Web’.