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Rare genetic disorder characterized by myotonia, muscle stiffness, and abnormal muscle hypertrophy that gives the impression of Herculean or “bodybuilder-like” appearance. Two main forms of myotonia congenita have been described: Thomsen disease and Becker disease. In Thomsen disease, symptoms and findings are usually apparent from infancy to approximately 2 to 3 years of age. In many cases, muscles of the eyelids, hands, and legs are most affected. In Becker disease, symptoms most commonly become apparent between the ages of 4 and 12 years. Affected individuals develop progressive myotonia; however, muscle rigidity and hypertrophy tend to be more severe.

  • Becker type: Typically manifests between the age of 4 and 12 years. Rarely, it has been reported that the onset may occur as late as approximately 18 years of age. Symptoms are more severe in this type.
  • Thomsen type: The onset of the disease may because apparent in infancy but most often by age 2 to 3 years of age. Symptoms stabilize after onset and do not progress.

First described in 1876 by Thomas Thomsen in Schleswig, Denmark. Thomsen was himself affected with the condition. In 1971, P.F. Bécker, a German physician, described the more severe variant of myotonia congenita.

Epidemiological studies have estimated the incidence internationally at 6:100,000 population. Five hundred persons are affected in Sweden alone. Becker's myotonia congenita is about twice as common as Thomson's disease.

The mutation in both Becker disease and Thomsen disease resides in the muscle chloride channel gene CLCN1, whose locus is on chromosome 7q35. Transmission is autosomal dominant for Thomsen disease and recessive for Becker disease.

Decreased conductance of the muscle chloride channel. There is a reduction in Cl- ions entering the cell; it remains relatively depolarized, resulting in spontaneous oscillations in membrane potential and clinical myotonia. The symptoms may respond to class I antiarrhythmic drugs such as mexiletine.

Action (e.g., inability to release a handshake) and percussion (e.g., sustained contraction after stimulation of a tendon reflex) myotonia. Attacks of muscle stiffness are usually painless and relieved by exercise (warm-up effect). Enhancement of myotonia by cold. Typical electromyographic tracing; myotonic discharge when a needle is inserted in the muscle. Muscle biopsy shows no signs of dystrophy. Creatine kinase level mildly elevated.

The onset of disease is in early infancy in Thomsen disease and later in Becker disease. The child shows decreased ability to relax muscles after contraction; blepharospasm (myotonia of the eyelids); and diffuse hypertrophy of muscles (buttocks, neck, back, shoulders). The disease affects mainly the lower limbs in Becker disease. No cardiac involvement, unlike in myotonic dystrophy. Heiman-Patterson et al. (1988) described two sisters with myotonia congenita who, on halothane contracture testing of skeletal muscle in vitro, had findings consistent with susceptibility to malignant hyperthermia. This could be a nonspecific abnormal response to the test caused by myotonia itself. Other case reports of diffuse muscle ...

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