Myotonia refers to a neurological symptom that describes difficulty in muscle relaxation after contraction. There may be an involvement pattern, although any muscle group can be affected.
Among hereditary myotonias, there are mainly two groups:
- Dystrophic myotonia (or myotonic dystrophy, whose main form is also known as DM1 or Steinert disease): Its prevalence is estimated to be about 1/8 000 individuals. Clinically, it is a multisystem disease where myotonia is accompanied by muscle weakness, cardiac conduction problems, cataracts, and endocrine and gastrointestinal alterations. Its molecular mechanism consists of a trinucleotide expansion in the DMPK gene; therefore, its diagnosis requires a specific test.
- Non-dystrophic myotonias belong to the group of channelopathies, and their genetic defect determines the occurrence of symptoms including myotonia as well as weakness, myalgias, episodes of paralysis, etc.