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 their 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 signals occur in the muscles, causing a stiffness called myotonia.

The hallmark of this condition is myotonia -- the muscles are unable 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 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 look muscular and well-developed. They 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:

• Electromyography (EMG, a test of the electrical activity of the muscles)
• Genetic testing
• Muscle biopsy  

Mexiletine is a medication that treats symptoms of myotonia congenita. Other treatments include:

• Phenytoin
• Procainamide
• Quinine
• Tocainide

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 trouble swallowing in an infant
• Long-term (chronic) joint problems
• Weakness of the abdominal muscles

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

Couples who want to have children and who have a family history of myotonia congenita should consider genetic counseling.

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

Chinnery PF. Muscle diseases. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 429.

Kerchner GA, Lenz RA, Ptacek LJ. Channelopathies: Episodic and electrical disorders of the nervous system. In: Bradley WG, Daroff RB, Fenichel GM, Jankovic J, eds. Neurology in Clinical Practice. 5th ed. Philadelphia, Pa: Butterworth-Heinemann: 2008:chap 68.