HypoPP is congenital. This means it is present at birth. In most cases, it is passed down through families (inherited) as an autosomal dominant disorder. In other words, only one parent needs to pass the gene related to this condition on to their child in order for the child to be affected.

In some cases, the condition may be the result of a genetic problem that is not inherited.

Unlike other forms of periodic paralysis, people with hypoPP have normal thyroid function. But they have a very low blood level of potassium during episodes of weakness. This results from potassium moving from the blood into muscle cells in an abnormal way.

Risk factors include having other family members with periodic paralysis. The risk is slightly higher in Asian men who also have thyroid disorders.

Symptoms include attacks of muscle weakness or loss of muscle movement (paralysis) that come and go. There is normal muscle strength between attacks.

Attacks usually begin in the teen years, but they can occur before age 10. How often the attacks occur varies. Some people have attacks every day. Others have them once a year. During attacks the person remains alert.

The weakness or paralysis:

• Most commonly occurs at the shoulders and hips
• May also affect the arms, legs, muscles of the eyes, and muscles that help with breathing and swallowing
• Occurs off and on
• Most commonly occurs on awakening or after sleep or rest
• Is rare during exercise, but may be triggered by resting after exercise
• May be triggered by high-carbohydrate, high-salt meals, stress, pregnancy, heavy exercise, and cold
• An attack usually lasts for several hours up to a day

Another symptom may include eyelid myotonia (a condition in which after opening and closing the eyes, they cannot be opened for a short time).

The health care provider may suspect hypoPP based on a family history of the disorder. Other clues to the disorder are muscle weakness symptoms that come and go with normal or low results of a blood potassium test.

Between attacks, a physical examination shows nothing abnormal. Before an attack, there may be leg stiffness or heaviness in the legs.

During an attack of muscle weakness, the blood potassium level is low. This confirms the diagnosis. There is no decrease in total body potassium. The blood potassium level is normal between attacks.

During an attack, muscle reflexes are decreased or absent. And muscles go limp rather than staying stiff. Muscle groups near the body, such as the shoulders and hips, are involved more often than the arms and legs.

Tests that may be done include:

• Electrocardiogram (ECG), which may be abnormal during attacks
• Electromyography (EMG), which is usually normal between attacks and abnormal during attacks
• Muscle biopsy, which may show abnormalities

Other tests may be ordered to rule out other causes.

The goals of treatment are relief of symptoms and prevention of further attacks.

Muscle weakness that involves the breathing or swallowing muscles is an emergency situation. Dangerous irregular heartbeats (heart arrhythmias) may also occur during attacks. Any of these must be treated right away.

Potassium given during an attack may stop the attack. Potassium can be taken by mouth. But if weakness is severe, potassium may need to be given through a vein (IV).

Taking potassium supplements may help prevent muscle weakness.

Eating a low-carbohydrate diet may help decrease symptoms.

A medicine called acetazolamide may be prescribed to prevent attacks. Your provider may tell you to also take potassium supplements because acetazolamide may cause your body to lose potassium.

If acetazolamide does not work for you, other medicines may be prescribed.

HypoPP responds well to treatment. Treatment may prevent, and even reverse, progressive muscle weakness. Although muscle strength starts out normal between attacks, repeated attacks may eventually cause worsening and permanent muscle weakness between attacks.

Health problems that may be due to this condition include:

• Kidney stones (a side effect of acetazolamide)
• Irregular heartbeat during attacks
• Difficulty breathing, speaking, or swallowing during attacks (rare)
• Muscle weakness that worsens over time

Contact your provider if you or your child has muscle weakness that comes and goes, especially if you have family members who have periodic paralysis.

Go to the emergency room or call the local emergency number (such as 911) if you or your child faints, has trouble breathing, speaking, or swallowing.

HypoPP cannot be prevented. Because it can be inherited, genetic counseling may be advised for couples at risk of the disorder.

Treatment prevents attacks of weakness. Before an attack, there may be leg stiffness or heaviness in the legs. Doing mild exercise when these symptoms start may help prevent a full-blown attack.

Periodic paralysis - hypokalemic; Familial hypokalemic periodic paralysis; HOKPP; HypoKPP; HypoPP

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Updated by: Walead Latif, MD, Nephrologist and Clinical Associate Professor, Rutgers Medical School, Newark, NJ. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.