Myotonia congenita

Definition

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

Alternative Names

Causes

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.

Symptoms

The hallmark of this condition is the myotonia -- the inability of the muscle to quickly relax after voluntary contraction. For example, after a handshake, the affected individual is only very slowly able to open and disengage his hand.

Early symptoms may include gagging and difficulty in swallowing, because oropharyngeal muscles are slow to relax. Initial movements may be stiff but improve with immediate repetition.

Children with myotonia congenita often appear to be muscular and well-developed. There may not be symptoms of myotonia congenita until the child is 2 or 3 years old.

Exams 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.

Outlook (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.

Possible Complications

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

When to Contact a Medical Professional

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.