Familial periodic paralysis is a rare inherited disorder that causes sudden attacks of weakness and paralysis. There are 4 different forms, which involve abnormalities in how electrolytes, such as sodium and potassium, are moved in and out of cells.
Muscles do not respond normally to stimulation if 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 medications 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 and is necessary for the normal functioning of cells, nerves, and muscles (see Overview of Potassium's Role in the Body). 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, which means only one affected parent is needed to pass the disorder on to offspring.
The precise form that the disorder takes varies among different families. There are 4 forms:
Hypokalemic
Hyperkalemic
Thyrotoxic
Andersen-Tawil syndrome
In hypokalemic periodic paralysis, the paralysis is caused by low levels of potassium in the blood (hypokalemia).
In hyperkalemic periodic paralysis, the paralysis is caused by high levels of potassium in the blood (hyperkalemia).
In thyrotoxic periodic paralysis, the paralysis is caused by low levels of potassium in the blood, and people also have signs of an overactive thyroid gland (hyperthyroidism).
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 4 limbs.
Hypokalemic periodic paralysis
In this 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 4 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.
Hyperkalemic periodic paralysis
In this 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.
Thyrotoxic periodic paralysis
In this 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.
Andersen-Tawil syndrome
In this form, 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), webbed toes (syndactyly), abnormally positioned fingers (clinodactyly), a small chin (micrognathia), or low-set ears. 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 medications by vein (intravenously) that increase or decrease the level of potassium in the blood to see whether they provoke or cause an attack. 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.
Genetic testing can be done to identify mutations in the affected genes.
Treatment and Prevention
Depends on the form
Hypokalemic periodic paralysis
Hyperkalemic periodic paralysis
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.
Thyrotoxic periodic paralysis
Andersen-Tawil syndrome
People may need a heart pacemaker or implantable cardioverter-defibrillator to control heart symptoms and reduce the risk of sudden death.
More Information
The following English-language resource 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
