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Myotonic dystrophy

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A heritable genetic disorder resulting in the slowly progressing muscle weakness and loss


Myotonic dystrophy is a genetic disorder that may soon lead to apparent muscle weakness and wasting (muscular dystrophy). The first symptoms often manifest by the age of 30s. Other symptoms include intellectual disability, heart conduction problems, endocrine changes, failing vision, slurred speech, ptosis (i.e. upper eyelid drooping), and opacity of the eye lenses. The affected individual suffers from myotonia, which is the inability to relax voluntary muscle. In males, the condition may lead to early balding and inability to have children. There are two types of myotonic dystrophy: (1) myotonic dystrophy type 1 (also called Steinert's disease) and (2) myotonic dystrophy type 2 (also called proximal myotonic myopathy). The myotonic dystrophy type 2 is rarer and involves milder symptoms than type 1.

Myotonic dystrophy is inherited in an autosomal-dominant pattern. It is an example of trinucleotide repeat disorders. In myotonic dystrophy type 1, the affected gene is DMPK gene on chromosome 19 whereas in myotonic dystrophy type 2, the affected gene is ZNF9 gene on chromosome 3. Both of these genes have an expansion of triplet repeat. In myotonic dystrophy type 1, there is an expansion of CTG (cytosine-thymine-guanine) triplet repeat. In myotonic dystrophy type 2, there is an expansion of CCTG (cytosine-cytosine-thymine-guanosine) tetranucleotide repeat.


  • myotonia atrophica
  • dystrophia myotonica

See also:

  • trinucleotide repeat disorder