Periodic Paralysis: Sudden Attacks of Muscle Weakness

How periodic paralysis works

Muscle fibers contract when electrical signals travel along their surface membranes. These signals rely on channels that move sodium, calcium and potassium in and out of the cell. In periodic paralysis, faulty channels or hormone effects make the membrane lose its electrical charge, so the muscle temporarily stops responding to nerve signals. At the same time potassium moves into muscle cells (low blood potassium) or out of them (high or normal blood potassium).

Between attacks, nerves and muscles test largely normal, and the body's total potassium is usually normal. The problem is where potassium sits, not how much there is.

Types of periodic paralysis

Inherited forms usually pass down as an autosomal dominant trait, meaning a parent with the gene has a 50% chance of passing it on. New gene changes also occur. More detail on the inherited forms is on the familial periodic paralysis page.

Thyrotoxic periodic paralysis

Thyrotoxic periodic paralysis (TPP) is the most common form in some populations. It affects about 2% of people with hyperthyroidism of East Asian descent and is much rarer in other groups. It is far more common in men, even though hyperthyroidism is more common in women. Most cases are linked to Graves' disease.

Thyroid hormone boosts the pumps that move potassium into cells. A large carbohydrate meal (which raises insulin), alcohol, or rest after strenuous exercise pushes the system further, and blood potassium drops sharply. Thyroid symptoms can be mild or unnoticed at the time of the first attack.

Treating the hyperthyroidism stops attacks permanently. Until the thyroid is controlled, a beta-blocker such as propranolol can prevent episodes.

Secondary hypokalemic paralysis

Severe potassium loss can also cause paralysis that looks similar. Causes include diuretics, severe diarrhea or vomiting, renal tubular acidosis, high aldosterone levels and heavy licorice intake. In these cases total body potassium is truly low, and treatment needs larger amounts of potassium. See hypokalemia and hyperkalemia.

Symptoms of an attack

• weakness that usually starts in the thighs and hips, then spreads to the arms
• inability to stand, climb stairs or lift the arms
• sometimes complete paralysis of the limbs
• normal feeling and alertness, which helps tell it apart from many nerve and brain problems
• warning signs such as aching, heaviness, tingling or stiffness before an attack
• in hyperkalemic forms, muscle stiffness and short, frequent attacks

Eye, face, swallowing and breathing muscles are usually spared, but severe attacks can affect breathing. Very low or high potassium can disturb heart rhythm.

Telling it apart from other causes of sudden weakness

Doctors consider Guillain-Barré syndrome (weakness rising over days, with lost reflexes), myasthenia gravis (droopy eyelids, double vision, fatigable weakness), stroke (one-sided weakness), spinal cord problems and low phosphate or magnesium. A blood potassium level taken during the attack is the most useful single test.

How it is diagnosed

1. Blood tests during an attack: potassium, plus magnesium, phosphate, kidney function and CK (a muscle enzyme).
2. Thyroid tests: TSH, free T4 and T3 in everyone with a first attack.
3. ECG: looks for rhythm effects of potassium and the distinctive patterns in Andersen-Tawil syndrome.
4. Urine potassium: helps separate a shift into cells from true potassium loss.
5. Long exercise test: a nerve conduction study done before and after several minutes of exercise, which shows a characteristic drop in muscle response.
6. Genetic testing: confirms inherited forms.

Treatment

During an attack

• Hypokalemic and thyrotoxic attacks: potassium, usually by mouth, given in small amounts with frequent blood checks. Too much can cause dangerously high potassium as it moves back out of the cells when the attack ends. Glucose-containing IV fluids are avoided, since they can worsen the attack.
• Hyperkalemic attacks: mild exercise, eating carbohydrates, and sometimes an inhaled asthma medicine (salbutamol) or other treatments that move potassium into cells.
• Heart monitoring during significant attacks.

Preventing attacks

• Avoid triggers: for hypokalemic forms, large carbohydrate or salty meals, alcohol and sudden rest after heavy exercise. For hyperkalemic forms, fasting and potassium-rich foods or supplements.
• Medicines: carbonic anhydrase inhibitors such as acetazolamide or dichlorphenamide reduce attacks in many people. Potassium-sparing diuretics help some hypokalemic forms, and thiazide diuretics help hyperkalemic forms.
• Treat hyperthyroidism with antithyroid drugs, radioactive iodine or surgery in thyrotoxic paralysis.

Surgery and anesthesia

Fasting, IV glucose, cold and some anesthetic drugs can trigger attacks. Tell your surgical team in advance so they can monitor potassium and adjust fluids.

Outlook

Attacks are rarely life-threatening when managed well. In inherited hypokalemic periodic paralysis, attacks often become less frequent after middle age, but many people gradually develop permanent weakness of the hip and shoulder muscles. Preventive treatment may reduce this. Thyrotoxic periodic paralysis disappears once thyroid levels are normal.

Frequently asked questions

What triggers a periodic paralysis attack?

Common triggers are resting after strenuous exercise, large carbohydrate-rich meals, alcohol, stress, cold and illness in hypokalemic forms. In hyperkalemic forms, fasting, potassium-rich foods and cold are typical triggers. Many attacks begin in the early morning, on waking.

What is thyrotoxic periodic paralysis?

It is sudden muscle weakness with low blood potassium caused by an overactive thyroid. It occurs mostly in young men, particularly of East Asian descent, often after a large carbohydrate meal, alcohol or rest after exercise. Treating the thyroid problem cures it.

How long does a periodic paralysis attack last?

Hyperkalemic attacks usually last from minutes to a few hours. Hypokalemic and thyrotoxic attacks typically last several hours and can last a day or more. Strength usually returns fully between attacks, though some people develop lasting weakness over many years.

Is periodic paralysis dangerous?

Most attacks are not dangerous, but rarely weakness affects breathing, and abnormal potassium levels can cause serious heart rhythm problems. Over-correcting potassium during an attack is also a risk. A first attack always needs emergency assessment.

Is periodic paralysis inherited?

Most forms are inherited, usually in an autosomal dominant pattern, so each child of an affected parent has a 50% chance of inheriting the gene. Thyrotoxic periodic paralysis and paralysis from potassium loss are acquired, although genetic factors may increase susceptibility to thyrotoxic attacks.