Association française contre les myopathies (AFM)
What is myotonic dystrophy?
Myotonic dystrophy is an autosomal dominant inherited disease characterized by wasting of the muscles (muscular dystrophy), myotonia and multisystemic conditions. Myotonic dystrophy is one of the most common neuromuscular diseases in adults. Its prevalence rate is five cases out of 100,000 inhabitants.
What are the clinical signs?
Two major clinical forms can be distinguished, although both due to defects of the same gene.
The common form in adults, which combines:
- Progressive muscular dystrophy (weakness and atrophy);
- Myotonia: delayed relaxation of a muscle after an initial contraction (for example difficulty to release grip after shaking hands);
- Defects in other organs: eye (cataracts in nearly all patients aged 40 and more), nervous system (sleep, cognitive function, or mood disorders), heart (heart rhythm disorders and/or conduction disturbances sometimes causing sudden death), respiratory system (pneumopathies), digestive system, endocrinal glands.
The severity of the disease depends upon the age of onset, clinical signs and progression.
The congenital form combines a clinical picture of neonatal hypotonia and severe acute respiratory distress. If the child survives, the disease progresses to become a disabling condition, especially at the intellectual level. Its exclusive maternal transmission pattern is still not clearly understood.
What is the cause?
Myotonic dystrophy is an autosomal dominant inherited disease. There is a 50-50 chance that affected parents will transmit the disease to any of their children. The myotonic dystrophy gene has now been located and identified precisely (chromosome 19). It codes for a protein kinase called myotonine, the function of which remains unclear. The peculiar type of the genetic defect may partially explain the anticipation phenomenon; the severity of symptoms increases as the disease is passed on to the next generation.
How is it progressing?
Average age of onset is around age 20-25, but every age of onset can also be observed. However, the insidious nature of disorders makes early diagnosis difficult.
The progression of the disease is varying; it is sometimes well tolerated and consistent with social and vocational activities, and other times cause severe functional limitation (loss of the ability to walk after 15 to 20 years of progression, with slightly impaired intellectual functions). However, the earlier the age of onset, the more severe the resulting invalidity will be.
The disease progresses slowly and at a constant speed in a given subject. Within a single family, a minor form of the disease does not exclude the possibility of a congenital onset (earlier and more severe). However, the disease tends to increase in severity or decrease in age at onset as it is transmitted down to the next generations; this is called the anticipation phenomenon.
What are the suggested treatments and management strategies?
There is currently no curative treatment available for this disease. It can be managed through various means including medication and/or methods aiming to improve the individual's quality of life. Cardiac monitoring is especially required to prevent these complications and is performed through a pacemaker. Myotonia, pain, mood disorders may all be treated through efficient medications. Kinesitherapy provides some relief and comfort to these patients. Some safety measures should be followed regarding medication and anesthesia during surgical interventions, in particular because the intervention is minor and the disease often remains unknown; the most common interventions in these patients are cataract interventions or gall bladder removal surgery.
Living with myotonic dystrophy
Although this disease is well known in some families, its presence is stressful, particularly for women who are aware of the risk of transmitting a neonatal form of the disease. The most recent advancements in research and molecular genetics techniques have made genetic counselling available for all these families. Affected or not, every family member concerned with this disease may refer to a clinical geneticist to clarify the diagnosis for himself or herself and his or her progeny. In this case, a family genetic study is undertaken, which in case of pregnancy makes the prenatal diagnosis possible, at the request of parents. However, when the foetus is affected, it is difficult, with the current state of knowledge, to assess the potential severity of the condition to determine a prognosis. This is why the entire genetic counselling approach should be combined with caring support from the parents.
For more information
AFM: Association Française contres les Myopathies, 1 rue de l'Internationale, B.P. 59, 91002 Évry cedex.
Suggested readings (French only)
La dystrophie myotonique de Steinert. Monographie Myoline, AFM, 1993.
Dystrophie musculaire de Steinert. Fiche technique Myoline, AFM, 1993.