Myotonia refers to a neurological symptom that describes difficulty in muscle relaxation after a contraction. There may be a pattern of involvement, although all muscle groups can also be affected.
Among inherited causes, there are basically two groups:
- Dystrophic myotonia (or myotonic muscular dystrophy, the main form of which is also known as DM1 or Steinert’s disease): its prevalence is estimated at 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 problems. The molecular mechanism consists of an expansion of trinucleotides in the DMOK gene, so its diagnosis requires a specific test. There is also a second form of dystrophic myotonia with a later onset and typically milder course, called proximal myotonic myopathy (DM2 or PROMM), associated with the expansion of a CCTG repeat within the CNBP gene.
- Non-dystrophic myotonias belong to the group of channelopathies and their genetic defect conditions the appearance of symptoms that include myotonia, as well as weakness, myalgia, episodes of paralysis, etc.
