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At a glance

It is a rare genetic disorder characterized by myotonia, muscle stiffness, and abnormal muscle hypertrophy that gives the impression of Herculean or “bodybuilder-like” appearance. There are two main forms of myotonia congenita that 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.

Synonyms

Becker Disease; Autosomal Recessive Myotonia Congenita Syndrome; MCR; Thomsen Disease (THD); Autosomal Dominant Myotonia Congenita Syndrome.

History

First described in 1876 by Asmus Julius Thomas Thomsen (1815-1896) in Schleswig, Denmark. He was a Danish/German physician and was himself affected with the condition. There were more than 65 member of this large family over seven generations that were diagnosed over with this disease. In 1971, P. F. Becker, a German physician, described the more severe variant of myotonia congenita.

Classification

There are two forms for myotonia congenital based on the inheritance pattern. They are:

  • 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 be apparent in infancy, but most often by age 2 to 3 years of age. Symptoms stabilize after onset and do not progress.

Incidence

Epidemiological studies have estimated the incidence internationally at 0.3 to 0.6:100,000 in the general population. There are 500 persons affected in Sweden alone. In northern Scandinavia, the prevalence is estimated at 1:10,000. Becker’s myotonia congenita is about twice as common as Thomsen’s disease. It affects males and females in relatively equal numbers.

Genetic inheritance

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.

Pathophysiology

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.

Diagnosis

Action (eg, inability to release a handshake) and percussion (eg, 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 ...

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