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Incidence

Hyperkalemic periodic paralysis is caused by mutations in the SCN4A gene and is inherited in an autosomal dominant manner. It affects an estimated 1 in 200,000 people. In 30 to 40 percent of cases, the cause of hyperkalemic periodic paralysis is unknown. Changes in other genes, which have not been identified, likely cause the disorder in these cases.

Clinical Characteristics

Hyperkalemic periodic paralysis is a condition that causes episodes of extreme muscle weakness or paralysis, usually beginning in infancy or early childhood. It is a genetic disease that causes episodes of extreme muscle weakness and 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, usually 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. Muscle strength usually returns to normal between attacks, although many affected people continue to experience mild stiffness (myotonia), particularly in muscles of the face and hands. The attacks never occur during exercise and may be aborted if the individual begins mild exercise. 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 should also 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.

Precipitants

Factors that can trigger attacks include rest after exercise, potassium-rich foods such as bananas and potatoes, stress, fatigue, alcohol, pregnancy, exposure to hot or cold temperatures, certain medications, and periods without food (fasting).

Provocation Tests

Provocation tests are usually not recommended. Ingesting potassium can trigger attacks in affected individuals, even if blood potassium levels do not go up. Hyperkalemic periodic paralysis is diagnosed with an exercise stress test. Individuals are exercised for 30 minutes to a heart rate greater than 120 beats per minute, followed by absolute rest. In normal individuals, serum potassium will rise during the exercise phase, then decline to baseline during the rest phase. In periodic paralysis patients, serum potassium will start to decline at rest, but then rise again in 10 to 20 minutes with accompanying weakness. The prolonged exercise test is positive in 80% of cases (decrease of over 30% of the compound muscle action potential after exercise).

Diagnostic Procedures

Diagnosis is based on clinical history, EMG and genetic tests. Hyperkalemia during attacks can be very mild and fugitive. Diagnosis is based on clinical symptoms including the increase of blood potassium level during an episode, but normal levels of blood potassium level in between episodes. Genetic testing can confirm the diagnosis. It is caused by mutations in the SCN4A gene.