Definition:

Non-dystrophic myotonias are rare diseases caused by mutations in key skeletal muscle ion channels. The major clinical manifestation is muscle stiffness as a consequence of the myotonia. Additional common symptoms include pain, weakness and fatigue. They are considered to be distinct from myotonic dystrophy because of the absence of progressive weakness and systemic features. Non-dystrophic myotonias include myotonia congenita, paramyotonia congenita, and sodium channel myotonias.