Myotonic dystrophy
What is muscular dystrophy?
There are a number of different types of muscular dystrophy. They are muscle diseases which have 3 features in common; they are hereditary, they are progressive, and each causes a characteristic, selective pattern of muscle wasting and weakness.
What is myotonic dystrophy?
Myotonic dystrophy is the most common adult form of muscular dystrophy. Myotonic dystrophy is caused by a defective gene. Unlike any of the other muscular dystrophies, the muscle weakness is accompanied by myotonia (delayed relaxation of muscles after contraction) and by a variety of abnormalities in addition to those of muscle. The disorder is also known as Steinert's disease and dystrophia myotonica.
Is the myotonia a serious problem?
Delayed relaxation of voluntary muscles after contractions is often a problem even before muscle weakness is apparent. However, this is generally less noticeable after the early stages. The myotonic symptom that is probably the most troublesome is the inability to release the hand from a grip.
Which muscles are affected in myotonic dystrophy?
Myotonic dystrophy shows an early pattern of muscle wasting that is unique among the major muscular dystrophies. The first muscles to be affected are those of the face, neck, hands, forearms, and feet (as opposed to the hip and shoulder muscles in the other dystrophies).
Why is myotonic dystrophy called 'multisystemic'?
Myotonic dystrophy can affect the tissues and organs of many body systems in addition to the voluntary muscle system. The list of abnormalities that can be produced by the defective gene in addition to muscle wasting and myotonia is extremely diverse. Consequently myotonic dystrophy may present itself in what one expert has called a ‘bewildering variety of ways’. The list includes cardiac disease, cataracts, testicular atrophy, respiratory impairment and adverse reactions to anaesthesia, difficulty in swallowing (dysphagia) and other gastrointestinal tract involvement, mental disorders (including mental retardation), excessive output of insulin and abnormal carbohydrate metabolism, and excessive sleeping.
What is the age of onset?
50 per cent of those with the disorder show visible signs by about 20 years of age, but a significant number do not develop clear-cut symptoms until after age 50. However, when myotonic dystrophy is suspected (because it is present in other members of the family) careful examination may reveal typical abnormalities before obvious symptoms appear. There is also a less common form of myotonic dystrophy with onset in infancy.
How is myotonic dystrophy diagnosed?
A physical examination will usually reveal the typical pattern of muscle weakness and wasting and the presence of myotonia. Special laboratory tests, which may include a muscle biopsy, confirm the diagnosis. In addition, early in the disease process, weakness and wasting of facial, jaw, and neck muscles, and frontal balding of men often give the face a characteristic look. Complicating the immediate recognition of myotonic dystrophy is the fact that patients frequently first seek medical attention for one of the conditions accompanying muscle involvement. Eye problems, especially, are what may bring a patient to the doctor's office.
What causes myotonic dystrophy?
As in all hereditary disorders, the disease process originates in genetic material that is defective. However, the specific biochemical basis for this process in myotonic dystrophy is unknown. A growing body of evidence strongly suggests that myotonic dystrophy is a disease of cell membranes affecting a variety of cell types.
How does a person get myotonic dystrophy?
Myotonic dystrophy is transmitted from generation to generation by individuals — men or women — who themselves have inherited the defective gene and have the disease. Because the defective gene is dominant, only one myotonic dystrophy gene derived from either the father or mother is required to produce the disease in an offspring. There is a 50-50 chance that, if one parent has the disease, such transmission will occur. The technical term for this mode of inheritance is 'autosomal dominant'.
Do people with myotonic dystrophy become disabled?
The course of myotonic dystrophy varies too widely, even in the same family, to permit a general statement about its effects. On the one hand, there are people with the disorder whose symptoms are so mild they hardly know anything is wrong. Whatever muscle weakness they experience is something they take for granted and adapt to. In some cases, the only symptom may be a cataract. Nevertheless these people do indeed have myotonic dystrophy and can transmit a serious case of the disease to their children.
For the most part, weakness and wasting slowly progress to the point of some disability, moving beyond the muscles originally involved to those of the shoulders, hips, and thighs. As a rule, disability rarely becomes severe until 15 to 20 years after the onset of symptoms. The older a person is when muscle weakness is first noticed, the slower is the progression and the less serious the consequences.
What is congenital myotonic dystrophy?
As the name indicates, congenital myotonic dystrophy is present at birth, almost invariably in infants whose mothers have cases of myotonic dystrophy so mild they are unaware of them. Congenital myotonic dystrophy is a rare variant of the disease with striking differences from the form it takes among adults. In the newborn period, an affected infant is profoundly weak, has difficulty with sucking and swallowing, and may have severe respiratory difficulties. Myotonia is not a feature of the condition at this stage. Motor development is usually delayed in these children, and they may show some signs of mental retardation. Generally, the condition improves through the early years but deteriorates during late childhood and adolescence, when the 'adult' features of the disease gradually emerge. Whether or not the progression is then more rapid than in cases with onset in adult life has not been established.
Is there any specific treatment?
No specific treatment has yet been found for the muscle weakness and wasting in myotonic dystrophy, although ankle and leg braces can help to support muscles as weakness progresses. However, there are medications that can relieve the myotonia. Heart problems, cataracts, and other abnormalities associated with the disease can also be treated.
What research is being done?
Research scientists are concentrating on cell membrane defects and on isolating and mapping the defective gene. It is known that the defect for myotonic dystrophy is on the long arm of chromosome 19. Ultimately, knowledge of the defective gene will allow accurate tests for those members of a myotonic dystrophy family who show no signs of the disease. It will allow early antenatal diagnosis as well as the possibility for an effective treatment.
Last Reviewed: 13 June 2007
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