Periodic paralysis is an autosomal dominant inherited disorder (only one affected parent is needed to pass the trait on to offspring) that causes sudden attacks of weakness and paralysis. There are several forms.

• Muscles do not respond normally to stimulation, usually when the blood potassium level is too low or high.
• Weakness is intermittent, affecting mainly the limbs, and is often brought on by exercising or eating too much or too little carbohydrates.
• The diagnosis is based on the symptoms and a check of the potassium level in the blood.
• Avoiding triggers that cause attacks and taking drugs can prevent attacks effectively.

During an attack of periodic paralysis, muscles do not respond to normal nerve impulses or even to artificial stimulation with an electronic instrument. The precise form that the disorder takes varies among different families. In some families, the paralysis is related to low levels of potassium in the blood (hypokalemia). In others, the paralysis is related to high levels of potassium in the blood (hyperkalemia). In a rare form, potassium levels are normal.

Symptoms and Diagnosis

During an attack of weakness, the person remains completely awake and alert. Muscles in the eye and face are not affected. Weakness may affect only certain muscles or all four limbs.

In the hyperkalemic form, attacks generally first appear before age 16 but may appear during the 20s and always by age 30. The attacks last up to 24 hours, occasionally even longer. Often, the person awakens the day after vigorous exercise with an attack of weakness. However, eating meals rich in carbohydrates (sometimes hours or even the day after) can also cause attacks. Eating carbohydrates and exercising vigorously drive sugar into cells. Potassium moves with the sugar, and the result is lowered potassium levels in the blood.

In the hyperkalemic form of the disorder, attacks often begin by age 10. The attacks last 15 minutes to 1 hour. Weakness tends to be less severe than in the hypokalemic form. Fasting, exercise, strenuous work, and exposure to cold may precipitate attacks.

A doctor's best clue to the diagnosis is a person's description of a typical attack. If possible, the doctor draws blood while an attack is in progress to check the level of potassium. If the levels of potassium are abnormal, doctors usually perform additional tests to be sure the abnormal levels are not from other causes. Occasionally, a doctor may give the person intravenous drugs that increase or decrease the levels of potassium in the blood to see whether an attack results.

Prevention and Treatment

Acetazolamide, a drug that alters the blood's acidity, may prevent attacks in all types of periodic paralysis. People with the hypokalemic form can take potassium chloride in an unsweetened solution while an attack is in progress. Usually symptoms improve considerably within an hour. People with the hypokalemic form should also avoid meals rich in carbohydrates and salt and avoid alcohol or strenuous exercise.

People with the hyperkalemic form can prevent attacks by eating frequent meals rich in carbohydrates and low in potassium and by avoiding fasting, strenuous activity, and exposure to cold. If an attack is severe or persistent, drugs (such as a thiazide diuretic or inhaled albuterol) can help lower the potassium level.

Last full review/revision January 2008 by Michael Rubin, MD