This disorder affects muscle relaxation and is present from birth. Those with myotonia congenita have a genetic mutation that affects the component of muscle cells necessary for muscle relaxation. The myotonia, or stiffness, occurs due to repeated abnormal electrical discharges in the muscles. This disorder can affect any skeletal muscle, but the legs are most often affected, sometimes making it difficult for patients to walk when the stiffness is severe. There is two different types of this condition. Becker disease tends to appear in the later years of childhood and the muscle stiffness seems to be more severe, especially in boys. Over time, those with Becker disease can develop permanent muscle weakening. Those with Thomsen disease do not experience the possible progressive and permanent muscle weakness. Thomsen disease tends to be apparent earlier than Becker disease.
How Common is this Genetic Disorder?
This genetic disorder affects approximately 1 in 100,000 people throughout the world. It affects approximately 1 in 10,000 people in northern Scandinavia and seems to be most common there.
Genetics of Myotonia Congenita
CLCN1 gene mutations are responsible for causing myotonia congenita. This gene is responsible for giving the commands to make the protein that is absolutely necessary for normal skeletal muscle function. When this gene is mutated, the function or structure of chloride channels is altered, resulting in ion flow not being properly regulated which decreases chloride ion movement into skeletal muscle cells. When the chloride ion flow is disrupted, it triggers prolonged muscle contractions. These contractions are the hallmark of myotonia. Both forms of this condition have different inheritance patterns. An autosomal dominant pattern is responsible for Thomsen disease. An autosomal recessive pattern is responsible for Becker disease.
Symptoms of Myotonia Congenita
Myotonia is best defined as a muscle inability to quickly relax after it is contracted and this is the hallmark of myotonia congenita. The early symptoms include:
- Difficulty swallowing
- Stiff movements that get better when repeated
- Chest tightening or shortness of breath when starting exercise
Children may not have noticeable symptoms until they are two to three years of age. Children with this condition, often appear to be well-developed and muscular.
Diagnosing this Genetic Disorder
Doctors will begin diagnosis with a physical exam and asking whether or not there is a family history of this disorder. Other diagnostic testing may include:
- Muscle biopsy
- Genetic testing
- Electromyography to test the muscle’s electrical activity
How is Myotonia Congenita Treated?
Medications are used to treat this condition. The medications used include:
- Mexiletine: An anti-arrhythmic drug.
- Phenytoin: An anti-convulsant drug.
- Procainamide: An anti-arrhythmic drug.
Medline Plus. (2010). Myotonia Congenita. Retrieved on June 24, 2010 from Medline Plus: https://www.nlm.nih.gov/medlineplus/ency/article/001424.htm
Genetics Home Reference. (2007). Myotonia Congenita. Retrieved on June 24, 2010 from Genetics Home Reference: https://ghr.nlm.nih.gov/condition/myotonia-congenita
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