The periodic paralyses are a group of rare inherited disorders that cause temporary episodes of muscle weakness or paralysis. Periodic Paralysis is found in all races and in both sexes. Some patients have their first attack within minutes of birth, but a few don't have symptoms until they are in their 60’s or 70’s. Attacks can last only a few moments or go on for days, depending on the type of periodic paralysis the person has. Some forms of periodic paralysis include muscle stiffness or rigidity as part of the attacks.