Myotonia refers to a neurological symptom that describes the difficulty for muscle relaxation after contraction. An involvement pattern may exist, although it can also affect all muscle groups.
Among the inherited causes, two fundamental groups exist:
- Myotonic dystrophy (or myotonic muscular dystrophy, whose main form is also known as DM1, or Steinert disease): its prevalence is estimated to be 1 in 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 a trinucleotide expansion in the DMOK gene, which is why its diagnosis requires a specific test. Also, a second form of myotonic dystrophy with later onset and a typically milder course exists, called proximal myotonic myopathy (DM2, or PROMM), associated with a CCTG repeat expansion in the CNBP gene.
- Non-dystrophic myotonias belong to the channelopathy group, and their genetic defects lead to the appearance of symptoms that include myotonia, feebleness, myalgia, paralysis episodes, etc.