A form of potassium-aggravated myotonia which is cold insensitive, dramatically fluctuating and profoundly worsened by potassium ingestion. Fluctuating myotonia develops during childhood or adolescence and involves the extraocular, bulbar and limb muscles. Myotonia fluctuans is a sodium muscle 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]