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
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
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 ...