Hyperkalemic periodic paralysis HyperKPP is a genetic disease that causes episodes of extreme muscle weakness associated with an increase of the potassium levels in the blood. Muscle weakness during an attack usually affects the arms and legs and muscles of the eyes, throat, and trunk. Most often, these episodes involve a temporary inability to move muscles in the arms and legs. Episodes usually begin before age 20, most commonly between infancy and age 10. Normally an episode lasts for 15 minutes to an hour, but in some people the episodes may last a few days to a week. Episodes tend to increase in frequency until about age 50, after which they may occur less frequently. Factors that can trigger attacks include rest after strenuous exercise, potassium-rich foods, stress, fatigue, and exposure to cold. Depolarizing anesthetics must be avoided. Muscle strength usually returns to normal between episodes, although many people continue to experience mild stiffness, particularly in muscles of the face and hands. Studies suggest more than 80% of people with hyperkalemic periodic paralysis over age 40 have permanent muscle weakness, most often affecting the leg muscles. About one third may develop a chronic progressive myopathy.
Hyperkalemic periodic paralysis is caused by mutations in the SCN4A gene and is inherited in an autosomal dominant manner. Diagnosis is based on clinical symptoms including the increase of blood potassium level during an episode, but normal levels of blood potassium in between episodes. Genetic testing can confirm the diagnosis. Treatment is focused on avoiding triggers and decreasing the severity of an episode. At the first sign of muscle weakness, episodes in many people may be prevented or stopped by mild exercise and/or eating carbohydrates, inhalation of salbutamol, or intravenous calcium gluconate.