Myotonia Congenita

Myotonia congenita (MC) is a genetic neuromuscular channelopathy that affects skeletal muscle fibers (striated muscle controlled by the somatic nervous system).[1] Myotonia, defined as a delay or failure of relaxation in contracted skeletal muscle, is considered to be the hallmark of the disease and results in prolonged rigidity resulting in stiffness, cramping, and muscle hypertrophy. MC is caused by mutations in the CLCN1 gene, which codes for voltage-gated chloride (CIC-1) channels within the sarcolemmal membrane.[2] Defective CIC-1 channels cause inappropriate hyperexcitability of skeletal muscle cells resulting in repetitive depolarization and myotonia.[3] Historically MC has been classified into two distinct conditions: Becker disease and Thomsen disease. Becker disease (BD) is inherited in an autosomal recessive pattern and classically results in a more severe myotonic picture, which can progress to permanent weakness. Thomsen disease (TD) is an autosomal dominant condition presenting earlier in childhood than BD and may be associated with milder features.[4] Advanced sequencing techniques have facilitated the identification of over 200 pathogenic mutations throughout the CLCN1 gene.[5] As knowledge of the variety of these mutations and their phenotypes increases, the classification of MC has become less distinct and more diverse.
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