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Myotonia congenita is a genetic disease characterized by the inability of the skeletal muscles to quickly relax after voluntary movements. Symptoms typically begin in childhood and vary from person to person. They may include muscle stiffness, muscle weakness, and attacks of weakness brought on by movement after rest. There are two forms of myotonia congenita: Becker disease, which is the most common and severe, generalized form; and Thomsen disease, which is a rare and milder form.[488] Both forms are caused by mutations in the CLCN1 gene. The Becker type is inherited in an autosomal recessive manner, and the Thomsen type is inherited in an autosomal dominant manner.[489] Treatment may include medication for muscle stiffness, such as mexiletine, carbamazepine, or phenytoin. Exercise may temporarily alleviate myotonia. People with myotonia congenita may be at increased risk for harmful side effects of anesthesia. Therefore, it is recommended that relatives of a person with the disease are tested during childhood.[13247]
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