Exercise-induced delayed-onset myotonia, Fluctuating myotonia
Myotonia fluctuans (MF) is a form of potassium-aggravated myotonia (PAM, see this term) which is cold insensitive, dramatically fluctuating and profoundly worsened by potassium ingestion.Prevalence is unknown.Fluctuating myotonia develops during childhood or adolescence and involves the extraocular, bulbar and limb muscles. Eyelid myotonia is often the first sign of the disease. Episodes of stiffness vary in severity and frequency and may be separated by prolonged periods of normality. Myotonia is aggravated by potassium ingestion and appears with a delayed onset (10-30 min) after exercise (exercise-induced delayed-onset myotonia). Cold has no effect on myotonia. Paralysis is never observed. Episodic weakness is rare.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.