Definition of Myotonia congenita:

Myotonia congenita is an inherited condition that affects muscle relaxation. It is congenital, meaning that it is present from birth.

Causes, incidence, and risk factors:

Myotonia congenita is caused by a genetic change (mutation). It is passed down from either one or both parents to the children (inherited).

Myotonia congenita is caused by a problem in the part of the muscle cells that are needed for muscles to relax. Abnormal repeated electrical discharges occur in the muscles, causing a stiffness called myotonia.

Symptoms:

The hallmark of this condition is the myotonia -- the inability of the muscle to quickly relax after contracting. For example, after a handshake, the person is only very slowly able to open and pull away his hand.

Early symptoms may include:

• Difficulty in swallowing
• Gagging
• Stiff movements that improve when they are repeated
• Shortness of breath or tightening of the chest at the beginning of exercise

Children with myotonia congenita often appear to be muscular and well-developed. The child may not have symptoms of myotonia congenita until age 2 or 3.

Signs and tests:

The doctor may ask if there is a family history of myotonia congenita.

Tests include:

• Genetic testing
• Muscle biopsy
• Test of the electrical activity in muscles (EMG)

Treatment:

Treatment for symptoms includes:

• Mexiletine
• Phenytoin
• Procainamide

Expectations (prognosis):

People with this condition can 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:

• Aspiration pneumonia caused by swallowing difficulties
• Frequent choking, gagging, or difficulty swallowing in an infant
• Abdominal muscle weakness
• Chronic joint problems

Calling your health care provider:

Call your health care provider if your child has symptoms of myotonia congenita.

Prevention:

Genetic counseling may be of interest to couples who want to have children and have a family history of myotonia congenita.