The disorder involves attacks of muscle weakness or loss of muscle movement (paralysis) that come and go. Initially, there is normal muscle strength between attacks.

Attacks usually begin in adolescence, but they can occur before age 10. Attacks that do not begin until adulthood are rare in people with hypokalemic periodic paralysis and are usually caused by other disorders.

How often the attacks occur varies. Some people have attacks every day, while others have them once a year. Episodes of muscle weakness usually last between a few hours and a day.

The weakness or paralysis:

• Most commonly is located at the shoulders and hips
• May also affect the arms, legs, muscles of the eyes, and muscles that help you breathe and swallow
• Occurs intermittently
• Most commonly occurs on awakening
• Most commonly occurs after sleep or rest
• Is rare during exercise, but may be triggered by rest after exercise
• May be triggered by heavy, high-carbohydrate, high-salt meals or alcohol consumption
• Usually lasts 3 - 24 hours

Other symptoms may include:

• Eyelid myotonia (a condition in which after opening and closing the eyes, the patient cannot open them for a short time)

Note: The patient's thinking remains alert during attacks.

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, and blood potassium levels are normal between attacks.

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

• An ECG or heart tracing may be abnormal during attacks.
• An EMG or muscle tracing is usually normal between attacks and abnormal during attacks.
• A muscle biopsy may show abnormalities.