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 many or too few carbohydrates.
The diagnosis is based on the symptoms, a check of the potassium level in the blood, and the results of an exercise test.
Avoiding triggers that cause attacks and taking certain drugs can prevent attacks effectively.
Attacks of familial periodic paralysis occur in response to the amount of potassium in a person's blood. Potassium is an electrolyte Overview of Electrolytes Well over half of the body's weight is made up of water. Doctors think about the body's water as being restricted to various spaces, called fluid compartments. The three main compartments are... read more and is necessary for the normal functioning of cells, nerves, and muscles (see Overview of Potassium's Role in the Body Overview of Potassium's Role in the Body Potassium is one of the body's electrolytes, which are minerals that carry an electric charge when dissolved in body fluids such as blood. (See also Overview of Electrolytes.) Most of the body’s... read more ). During an attack of familial periodic paralysis, muscles do not respond to normal nerve impulses or even to artificial stimulation with an electronic instrument.
Familial periodic paralysis is autosomal dominant Dominant disorders Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are made of a very long strand... read more , which means only one affected parent is needed to pass the trait on to offspring.
The precise form that the disorder takes varies among different families. There are four forms:
Hypokalemic
Hyperkalemic
Thyrotoxic
Andersen-Tawil syndrome
In the hypokalemic form, the paralysis is caused by low levels of potassium in the blood (hypokalemia Hypokalemia (Low Level of Potassium in the Blood) In hypokalemia, the level of potassium in blood is too low. A low potassium level has many causes but usually results from vomiting, diarrhea, adrenal gland disorders, or use of diuretics. A... read more ).
In the hyperkalemic form, the paralysis is caused by high levels of potassium in the blood (hyperkalemia Hyperkalemia (High Level of Potassium in the Blood) In hyperkalemia, the level of potassium in blood is too high. A high potassium level has many causes, including kidney disorders, drugs that affect kidney function, and consumption of too much... read more ).
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 Hyperthyroidism Hyperthyroidism is overactivity of the thyroid gland that leads to high levels of thyroid hormones and speeding up of vital body functions. Graves disease is the most common cause of hyperthyroidism... read more 
).
In Andersen-Tawil syndrome, potassium levels can be high, low, or normal.
Symptoms of Familial Periodic Paralysis
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 trigger 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 triggered 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 triggered 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 Scoliosis Scoliosis is abnormal curvature of the spine. Scoliosis can be present at birth or can develop during adolescence. Mild forms may cause only mild discomfort, but more severe forms can cause... read more 
), webbed fingers and toes (syndactyly Finger and Toe Defects The fingers and toes may be abnormally formed, incompletely formed, or missing at birth. Birth defects, also called congenital anomalies, are physical abnormalities that occur before a baby... read more 
), abnormally positioned fingers and toes (clinodactyly), a small chin (micrognathia Jaw Defects The jaw can be missing, deformed, or incompletely developed at birth. Birth defects, also called congenital anomalies, are physical abnormalities that occur before a baby is born. "Congenital"... read more 
), or low-set ears Ear Defects Ears can be missing, deformed, or incompletely developed at birth. Birth defects, also called congenital anomalies, are physical abnormalities that occur before a baby is born. "Congenital"... read more 
. Heart abnormalities may cause abnormal heart rhythms and increase the risk of sudden death.
Diagnosis of Familial Periodic Paralysis
Description of an attack
Level of potassium in the blood during an attack
Exercise test
Genetic testing
A doctor’s best clue to the diagnosis of familial periodic paralysis 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.
In the past, doctors based the diagnosis on provocative testing. In provocative testing, doctors give a person drugs by vein (intravenously) that increase or decrease the level of potassium in the blood to see whether an attack results. However, provocative testing may cause serious side effects such as paralysis of the breathing muscles or an abnormal heart rhythm. Thus, provocative testing has been replaced by a safer exercise test. In the exercise test, doctors have the person vigorously exercise a single muscle for 2 to 5 minutes to see if that makes the muscle weak. Doctors record electrical activity from the muscle before and after exercise to see if it was affected.
Prevention and Treatment
Depends on the form
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 or 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 Artificial Pacemakers There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more 
or implantable cardioverter-defibrillator Implantable Cardioverter-Defibrillator (ICD) There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more to control heart symptoms.
More Information
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
Muscular Dystrophy Association: Information on research, treatment, technology, and support for people living with a periodic paralysis
