Synonyms
None

Disorder Subdivisions
Thomsen Disease (THD); Myotonia Congenita, Autosomal Dominant
Becker Disease; Myotonia Congenita, Autosomal Recessive (MCR); Generalized

General Discussion
Myotonia congenita is a rare genetic disorder in which an abnormality of voluntary (skeletal) muscle fiber membranes causes an unusually exaggerated response to stimulation (hyperexcitability). As a result, affected individuals have difficulty relaxing certain muscles after contracting them (myotonia), muscle stiffness (rigidity), and associated symptoms. Such symptoms tend to occur when attempting to move certain muscles after rest. In many cases, individuals with myotonia congenita also have abnormal enlargement of the muscles (hypertrophy), resulting in a herculean or body-builder like appearance.

Two main forms of myotonia congenita have been described: Thomsen disease and Becker disease. In individuals with Thomsen disease, symptoms and findings such as myotonia, associated muscle rigidity, and abnormal muscle enlargement may become apparent from infancy to approximately two to three years of age. In many cases, muscles of the eyelids, hands, and legs may be most affected. Thomsen disease is transmitted as an autosomal dominant trait.

In those with Becker disease, symptoms most commonly become apparent between the ages of four to 12 years. As in Thomsen type myotonia congenita, affected individuals develop myotonia, associated muscle rigidity, and abnormal muscle enlargement (hypertrophy). However, in contrast to Thomsen type, such symptoms are progressive and tend to be more severe. In addition, muscle hypertrophy may be particularly striking, and muscle weakness may be present. Becker disease is inherited as an autosomal recessive trait.