A very rare, persistent and more severe form of potassium-aggravated myotonia. Begins during childhood (usually before 10 years of age) and involves mainly the face, neck, limbs, and thoracic muscles. It can be aggravated by exercise or potassium ingestion and less often by cold. Myotonia permanens is a muscle sodium channelopathy due to missense mutations of the SCN4A gene encoding the alpha subunit of the skeletal muscle voltage-gated sodium channel Nav1.4. Transmission is autosomal dominant. [from
SNOMEDCT_US]