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

Thomsen's disease; Becker's disease

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.

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.

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 for symptoms includes:

• Mexiletine
• Phenytoin
• Procainamide

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.

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

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

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

Barohn RJ. Muscle diseases. Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2008:chap 447.

Bernard G, Shevell MI. Channelopathies: a review. Pediatr Neurol. 2008 Feb;38(2):73-85.