Myotonic Dystrophy

Myotonic dystrophy type 1 (DM1)

Myotonic dystrophy type 1 (DM1) is an inherited disorders that affects skeletal and smooth muscles, as well as the eyes, heart , endocrine and central nervous system. The clinical presentation varies from mild to severe and symptoms may appear any time between birth and adulthood. Individuals with mild DM1 have only cataract and mild myotonia (sustained muscle contraction), and there is no major muscle weakness.

• Typical individuals with DM1 have muscle weakness and wasting, on top of myotonia and cataract. They often also have cardiac conduction abnormalities and hormonal problems.
• Babies with congenital DM1 present have significant floppiness and severe generalized weakness at birth, often with respiratory insufficiency. Intellectual disability or learning disability is common.

Cause

This disorder result from a specific genetic change (mutation) on chromosome number 19. The length of a particular ‘triple repeat sequence’ (CTG) is expanded that vary from a slight expansion in mildly affected individuals to a very large expansion in severely affected children. This is a 'dominant' inheritance pattern. Men and women are equally likely to be affected and to pass on the disorder, but affected women are more likely to have a severely affected child. The disorder tends to be more severe in successive generations.

Myotonic dystrophy type 2 (DM2)

DM2 , previously named ‘proximal myotonic myopathy ‘(PROMM)), is an important condition to consider in individuals presenting with proximal muscle weakness around the shoulders and pelvis or a “limb-girdle weakness”. Myotonia is often mild or absent. Cataract and heart involvement is less common. Children affected at birth or a “congenital form” has not been reported in DM2.

Cause

DM2 is an autosomal dominant genetic disorder. Unlike DM1, the size of the repeated DNA expansion (CCTG repeats) does not relate to the age of onset or disease severity in DM2. The disease does not tend to be worse in children of affected patients unlike in DM1.

Diagnosis

DM2 is confirmed by blood test that analyzed the number of CCTG repeats.