Myotonia permanens is a very rare, persistent and more severe form of potassium-aggravated myotonia (PAM, see this term).Prevalence is unknown.Continuous and severe 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. Occasionally, patients have muscle hypertrophy, especially of the neck and shoulders. Paralysis is never observed. Severe stiffness of the pharyngeal and respiratory muscles may provoke episodes of hypoxia and acidosis that can compromise the respiratory function. Close monitoring is necessary during surgery as rigidity and rhabdomyolysis may occur. Depolarizing agents can cause severe ventilation problems due to a paradoxical increase of stiffness in respiratory muscles and must be avoided.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.