Familial 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 four forms.
During an attack of familial 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. There are four forms—hypokalemic, hyperkalemic, thyrotoxic, and Andersen-Tawil syndrome. In the hypokalemic form, the paralysis is caused by low levels of potassium in the blood. In the hyperkalemic form, the paralysis is caused by high levels of potassium in the blood. In the thyrotoxic form, the paralysis is caused by low levels of potassium in the blood, and people also have signs of an overactive thyroid gland (hyperthyroidism—see see Hyperthyroidism). In Andersen-Tawil syndrome, potassium levels can be high, low, or normal.
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 hypokalemic 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. Often, the person awakens the day after vigorous exercise with an attack of weakness. The weakness may be mild and limited to certain muscle groups or may affect all four limbs. However, eating meals rich in carbohydrates (sometimes hours or even the day before), emotional or physical stress, alcohol ingestion, and exposure to cold 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 and urine.
In the hyperkalemic form, 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, rest shortly after exercise, or exercise after meals may precipitate attacks. Myotonia (very stiff muscles caused by a delayed ability to relax the muscles after contracting them) is common. Myotonia of the eyelids may be the only symptom.
In the thyrotoxic form, attacks of weakness last hours to days and are usually precipitated by exercise, stress, or eating foods rich in carbohydrates, similar to the hypokalemic form. People have symptoms of hyperthyroidism, such as anxiety, tremors, palpitations, and heat intolerance.
In Andersen-Tawil syndrome, attacks of weakness usually begin before age 20 and are precipitated by rest after exercise. Episodes may last for days and occur monthly. People with this form may also have other disorders such as an abnormally curved spine (scoliosis), webbed fingers and toes (syndactyly), abnormally positioned fingers and toes (clinodactyly), a small chin (micrognathia), or low-set ears. Heart abnormalities may cause abnormal heart rhythms and increase the risk of sudden death.
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 level of potassium is abnormal, doctors usually do additional tests to be sure the abnormal level is not the result of other causes. Rarely, a doctor may give the person drugs by vein (intravenously) that increase or decrease the level of potassium in the blood to see whether an attack results.
Prevention and Treatment
People with the hypokalemic form can take potassium chloride in an unsweetened solution or have it injected into a vein 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, avoid alcohol after periods of rest, and avoid strenuous exercise. Acetazolamide, a drug that alters the blood's acidity, may help prevent attacks.
People with the hyperkalemic form can stop a mild attack by doing light exercise and eating a carbohydrate-rich meal. If an attack is persistent, drugs (such as a thiazide diuretic or inhaled albuterol) can help lower the potassium level. If an attack is severe, doctors give calcium, insulin, and glucose by vein. People can prevent attacks by eating frequent meals rich in carbohydrates and low in potassium and by avoiding fasting, strenuous activity after meals, and exposure to cold.
People with the thyrotoxic form are given potassium chloride (as in the hypokalemic form), and doctors closely monitor levels of potassium in the blood during severe attacks. To prevent attacks, doctors give people drugs to keep their thyroid gland functioning properly and beta-blockers (such as propranolol).
People with Andersen-Tawil syndrome may prevent attacks by making lifestyle changes, including tightly controlling levels of exercise or activity, and by taking acetazolamide. People may need a heart pacemaker or implantable cardioverter-defibrillator (see Pacemakers) to control heart symptoms.
Last full review/revision July 2013 by Michael Rubin, MDCM