The disorder involves 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 childhood. How often the attacks occur varies. Some people have several attacks a day. Attacks typically last only 1 to 2 hours, but can sometimes last as long as a day. They are usually not severe enough to need therapy. Some people have associated myotonia, in which they cannot immediately relax their muscles after use.

The weakness or paralysis:

• Most commonly occurs at the shoulders and hips
• May also involve the arms and legs but does not affect muscles of the eyes and those that help you breathe and swallow
• Most commonly occurs while resting after activity
• May occur on awakening
• Comes and goes
• Usually lasts 1 - 2 hours

Triggers may include:

• Resting after exercise
• Not eating (fasting)
• Exposure to cold
• Eating small amounts of potassium-rich foods or taking medicines that contain potassium

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

Between attacks, a physical examination shows nothing abnormal. During an attack, muscle reflexes are 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.

The health care provider may run additional tests to rule out other causes.