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Myotonia congenita is a genetic disorder that causes a person's muscles to tense and relax abnormally. An individual with the disease cannot voluntarily relax certain muscles, causing them to stick in a rigid position. Possible symptoms include pain, frequent cramps, and mobility issues. Myotonia congenita can be present at birth or develop in early childhood and usually persists throughout a patient's life. Depending on the severity of symptoms, treatment may consist of regular doses of muscle-relaxing medications, anticonvulsants, and ongoing physical therapy.
Muscle movement problems associated with myotonia congenita are caused by abnormalities in the way electrical impulses are transmitted in the body. A gene called CLCN1 normally produces proteins that stabilize electrical activity inside muscles, allowing them to contract and relax on command. In the case of myotonia congenita, a defect in the CLCN1 gene results in missing or non-functioning proteins.
There are two forms of myotonia congenita that differ in the way they are inherited. The more common form, called Thomsen disease, is an autosomal dominant disorder that usually causes symptoms from birth. It can be inherited if just one parent carries a mutated copy of the CLCN1 gene. Becker disease is an autosomal recessive disorder that requires both parents to possess defective copies of the gene. It typically appears between the ages of four and 12 and tends to cause more debilitating symptoms than Thomsen disease.
Young children with either form of the disease can experience temporary, sporadic muscle spasms that cause their legs or arms to stiffen. Episodes generally only last a few seconds at a time, and movements tend to become easier as they are repeated. If the muscles of the face, throat, and chest are affected, a patient may occasionally have breathing and eating difficulties. Chronic joint and muscle pain are common with myotonia congenita. Overactive muscles tend to become large and well defined as a child ages, which can give him or her a very muscular appearance.
In most cases, the severity and frequency of muscle contraction problems can be eased with medications. Patients may be prescribed daily doses of such drugs as phenytoin or mexiletine that reduce electrical activity in the nerves that supply skeletal muscles. Physical therapy can help many children, adolescents, and adults with myotonia congenita learn how to remain fully mobile despite their conditions. Since symptoms tend to improve with repetitive motions, patients can learn exercises to perform before engaging in certain activities to reduce the chances of having symptomatic episodes.