Definition

Myotonia congenita is an inheritable congenital (present from birth) disorder characterized by slow relaxation of voluntary muscles, such as those in the legs.

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Alternative Names

Thomsen's disease

Causes, incidence, and risk factors

Myotonia congenita can be either an autosomal dominant or autosomal recessive disease. With the autosomal dominant form, a person only needs to inherit the mutation from one parent to be affected. Inheriting the recessive form means a person must inherit the mutation from both parents to be affected. The gene that causes this condition resides on chromosome 7.

The cause of myotonia congenita is believed to be an abnormality in the chloride channels of muscle cells (chloride ions are required for a muscle to relax). The abnormal chloride channels also cause an accumulation of potassium outside the cells and an activation of sodium channels in the muscle cells (sodium ions trigger muscle contraction).

When the cells have more than enough sodium but not enough chloride, abnormal repetitive electrical discharges cause a stiffness called myotonia.

Signs and tests

  • There is often a family history of myotonia congenita.
  • An EMG demonstrates myotonic potentials.
  • A muscle biopsy may show an absence of type 2B fibers.

Treatment

Treatment for symptoms of myotonia congenita includes mexiletine, phenytoin, and procainamide.

Expectations (prognosis)

Affected people do well. Symptoms only occur when a movement is first started. After a few repetitions, the muscle relaxes and the movement becomes normal. Symptoms may improve later in life.

Complications

  • Frequent choking, gagging, or difficulty swallowing in an infant
  • Aspiration pneumonia, related to swallowing difficulties

Calling your health care provider

Call your health care provider if symptoms of myotonia congenita develop.

Prevention

Genetic counseling may be of interest to prospective parents with a family history of myotonia congenita.

Lots More Information:
•Autosomal dominant
•Autosomal recessive
•Aspiration pneumonia
Review Date:5/16/2006
Reviewed By: Daniel Kantor, M.D., Director of the Comprehensive MS Center, Neuroscience Institute, University of Florida Health Science Center, Jacksonville, FL. Review provided by VeriMed Healthcare Network.