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Prevalence is estimated at 1 between and 10/100,000.

Onset occurs early in life and affected individuals are often identified by family members within the first few months after birth. The myotonia is unusual in that it is relieved by exercise (warm-up effect).

Both forms of the disease (Thomsen myotonia and Becker myotonia) are caused by loss of function mutations in the gene encoding the chloride channel (CLCN1) that plays a role in muscle cell repolarisation.

The clinical diagnosis can easily be confirmed by electromyography (EMG), which reveals myotonic discharges in association with hyperexcitation of the muscle fibre membrane. Testing under exercise and cold stress allows detailed characterisation of the myotonia and helps orientate the molecular diagnosis. Identification of mutations in the chloride channel gene can be considered as diagnostic following a positive EMG under exercise and cold stress.

The mode of transmission may be autosomal dominant (Thomsen myotonia) or autosomal recessive (Becker myotonia). Genetic counselling should be proposed and the results of the molecular analysis should be interpreted with care as the same mutations have been associated with both dominant and recessive modes of transmission depending on the family studied.

Treatment revolves around sodium channel blocking agents such as mexiletine, carbamazepine or diphenylhydantoin.