Muscle membrane excitability is affected in a group of disorders referred to as channelopathies. Onset is usually in childhood or adolescence. Episodes typically occur after rest or sleep, often following earlier exercise. May be due to genetic disorders of calcium (hypokalemic periodic paralysis [hypoKPP]), sodium (hyperkalemic periodic paralysis), chloride, or potassium channels.

• Attacks of hypoKPP are treated with potassium chloride (usually oral). Prophylaxis with acetazolamide (125-1000 mg/d in divided doses) is usually effective in hypoKPP type 1.
• Attacks of thyrotoxic periodic paralysis (usually in Asian men) resemble those of hypoKPP; attacks abate with treatment of the underlying thyroid condition.