A form of potassium-aggravated myotonia which shows dramatic improvement with the use of acetazolamide. Symptoms generally manifest during childhood (before 10 years old), with myotonia of the facial, limbs and/or intercostal muscles that is triggered by potassium ingestion, fasting and mildly by cold exposure and exercise. Muscle stiffness is generally painful. Acetazolamide-responsive myotonia 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]