Definition:

Myotonia congenita is a specific inherited disorder of muscle membrane hyperexcitability caused by reduced sarcolemmal chloride conductance due to mutations in CLCN1, the gene coding for the main skeletal muscle chloride channel ClC-1. Impaired functioning of the ClC-1 leads to an increase in sarcolemmal excitability that clinically presents as delayed muscular relaxation (myotonia). Myotonia congenita may be inherited as either an autosomal dominant (Thomsen disease) or recessive trait (Becker disease). The predominant features of Thomsen disease are a painless, transient, muscle stiffness with a predilection for both the upper extremity and the facial muscles. Compared with Thomsen disease, Becker disease is more common, more insidious, and has initial symptoms that occur later in childhood. Two additional forms of myotonia congenita have been described: myotonia levior and fluctuating myotonia congenita. Like Becker and Thomsen disease, both of these conditions are associated with a defect in the ClC-1. Whether these two entities are truly distinct disorders is under debate, and some propose that they are variants of Thomsen disease.