Myotonia Congenita

Myotonia congenita is a genetic muscle disorder of chromosome 7 that begins at birth. When voluntary muscles are stimulated, they stiffen and have difficulty relaxing. Individuals with myotonia congenita often have abnormally enlarged muscles that make them look like a body builder. Myotonia congenita affects both males and females but it is not known how often it occurs. It generally does not get worse over time.

The two main types of myotonia congenita are Thomsen disease, which begins in infancy, and Becker disease (sometimes called generalized myotonia), which usually begins between the ages four and 12. Thomsen disease, the less common form, is inherited in an autosomal dominant manner, meaning a child only needs to receive one copy of the defective gene in order to develop myotonia congenita. Becker disease, the more common form, is inherited in an autosomal recessive manner, meaning that the child must inherit two copies of the defective gene, one from each parent, in order to develop myotonia congenita.

Symptoms

The main symptoms of myotonia congenita are:

• muscle stiffness (rigidity) when stimulated – there is a feeling of muscle tightness but usually not pain
• difficulty relaxing contracted muscles
• abnormal enlargement of the muscles (hypertrophy)

Diagnosis

Diagnosis of myotonia congenita is based on the symptoms. A special muscle test called an electromyelogram (EMG) is done to check the health of the muscles. There is a more common muscle disorder which has similar symptoms called myotonic muscular dystrophy (caused by a genetic defect on chromosome 19), so a genetic test is usually done to exclude it. Testing for myotonia congenita, however, is difficult, because many different genetic defects on chromosome 7 may cause the disorder.

Treatment

An effective non-medical treatment to prevent muscle stiffness is warming up. Individuals with mytonia congenita find that using low-intensity movements before bigger ones helps prevent rigidity. During sustained movement, the stiffness fades away.

Some individuals are helped by the drug mexiletine (Mexitil). Antiseizure drugs such as phenytoin (Dilantin) may help. Other drugs such as quinine or diuretics may be effective.

_Sources:

"NINDS Myotonia Congenita Information Page." NINDS – Disorders Index. 14 Feb 2007. National Institute of Neurological Disorders . 30 Apr 2007 <http://www.ninds.nih.gov/disorders/myotoniacongenita/myotoniacongenita.htm>.

"Myotonia Congenita." NORD - Index of Rare Diseases. 2006. National Organization for Rare Disorders. 30 Apr 2007 <http://www.rarediseases.org/search/rdbdetail_abstract.html?disname=Myotonia%20Congenita>.

Stimson, Dan. "Myotonia Congenita." QUEST Vol 8 Dec 2001 30 Apr 2007 <http://www.mda.org/publications/Quest/q86myo.html>.