Potassium-aggravated myotonia (PAM), which is also known as sodium channel myotonia (SCM), is a group of pure myotonic disorders caused by mutations in the SCN4A gene. PAM includes three diseases with very similar phenotypes: myotonia fluctuans, myotonia permanens, and acetazolamide-sensitive myotonia. These disorders differ from the other nondystrophic myotonias in that: (1) the myotonia is exacerbated by potassium ingestion; (2) the myotonia does not worsen with cold exposure; and (3) there is no major weakness.
