Definition of Hyperkalemic periodic paralysis:

Hyperkalemic periodic paralysis is associated with elevated levels of potassium in the bloodstream. The disorder causes occasional episodes of muscle weakness.

Causes, incidence, and risk factors:

The cause of hyperkalemic periodic paralysis is believed to be a genetic muscle disease. In most cases it is inherited directly (from parent to child); in other cases, it occurs randomly in a family group.

The disorder involves attacks of muscle weakness or paralysis, alternating with periods of normal muscle function. Attacks usually begin in early childhood. Multiple daily attacks are possible.

Attacks typically last only 1 to 2 hours, but can sometimes last as long as a day. They tend to occur while resting after exercise or exertion. Attacks may also be triggered by exposure to cold, or by eating small amounts of potassium.

Unlike other forms of periodic paralysis (such as familial periodic paralysis and thyrotoxic periodic paralysis), hyperkalemic periodic paralysis is not associated with low potassium in the bloodstream (serum potassium). In fact, there may be normal or high levels of potassium in the bloodstream during and between attacks. The condition may be termed "normokalemic periodic paralysis" if potassium is normal during attacks.

Glucose and insulin administration, which trigger attacks of other forms of periodic paralysis, do not trigger attacks of hyperkalemic periodic paralysis. Giving potassium, which is used to treat weakness associated with the other disorders, worsens the weakness in this case.

The serum potassium levels are usually not at the level that would be expected to cause paralysis. The potassium level may remain normal or only very slightly elevated. It is thought that the disorder is caused by problems with the muscle cell wall and the way the body controls electrolyte levels in the cell (electrolytes are molecules, such as sodium and potassium, that are capable of carrying an electrical current.

Weakness most commonly affects the muscles of the arms and legs. Heart arrhythmias may occasionally develop during attacks. Although muscle strength is initially normal between attacks, repeated attacks may eventually cause progressive and persistent muscle weakness between attacks.

Risks include a family history of periodic paralysis. Attacks may be triggered by fasting. Attacks seldom occur during exercise but may be triggered by rest following exercise.

Disorders such as hyperkalemic periodic paralysis, which cause intermittent episodes of paralysis as their primary effect, are uncommon. More commonly, an intermittent episode of paralysis or weakness is a symptom of another disorder.

Hyperkalemic periodic paralysis occurs in approximately 1 in every 100,000 people. Men are affected more often than women and usually have more severe symptoms.