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.