Myotonia congenita is an inheritable congenital (present from birth) disorder characterized by slow relaxation of voluntary muscles, such as those in the legs.
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 defective genes from one parent, while inheriting the recessive form requires defective genes from both parents. The gene that causes this condition resides on chromosome 7. The cause of myotonia congenita is believed to be an abnormality in the muscle cells' chloride channels. (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, procainamide, and quinine.
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.