Thomsen disease

Myotonia congenita
Classification and external resources
Specialty	neurology
ICD-10	G71.1
ICD-9-CM	359.2
OMIM	160800
DiseasesDB	8736
MedlinePlus	001424
MeSH	D009224
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Congenital myotonia, also called myotonia congenita, is a congenital neuromuscular channelopathy that affects skeletal muscles (muscles used for movement). It is a genetic disorder. The hallmark of the disease is the failure of initiated contraction to terminate, often referred to as delayed relaxation of the muscles (myotonia) and rigidity. Symptoms include delayed relaxation of the muscles after voluntary contraction (myotonia), and may also include stiffness, hypertrophy (enlargement), transient weakness in some forms of the disorder (from certain genetic mutations), and cramping.

Two types of myotonia congenita exist, an autosomal dominant form and an autosomal recessive form. Autosomal dominant myotonia congenita (OMIM #160800) is also called Thomsen disease, after Danish/German physician Asmus Julius Thomas Thomsen (1815–1896), who himself had the disease and who wrote the first description of it in the medical literature (in 1876). Autosomal recessive myotonia congenita (OMIM #255700) is also called generalized myotonia, recessive generalized myotonia (RGM), Becker disease, and Becker myotonia, after the German professor Peter Emil Becker, who discovered its recessive nature.

The term congenital in its sense of "clinically apparent from birth" applies only to Thomsen disease, as the clinical onset of Becker myotonia may be delayed up to the age of 4 to 6 years. But in either form of myotonia congenita, the term's strictest sense reflects that the disease is genetically present from birth, although the clinical onset may be delayed.

With the advent of genetic testing, it has recently been found that some typically recessive mutations may occur in a dominant fashion in some individuals. The reason for this is not known.

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