Familial periodic paralysis

Periodic paralysis is a general term applied to three distinct, though possibly related, non-lethal disorders of voluntary muscle. The three types of periodic paralysis are hypokalemic, hyperkalemic and normokalemic. Hypokalemic, hyperkalemic and normokalemic periodic paralysis differ from one another in age of onset, precipitating factors, clinical characteristics and response to therapy. All three are characterized by intermittent attacks of weakness, during which affected muscles become slack, weak and unable to contract. However, between attacks, the affected muscles usually work normally.

Intervals between attacks vary from days to months and even years. Does periodic paralysis become worse with age? Periodic paralysis can cause progressive weakness, but the number of attacks and their severity tend to decrease with age and the attacks may disappear altogether, especially in women.

The names of the three forms of the disease are derived from the measure of potassium (kalemic) levels in the blood during an attack. Potassium may fall, rise or remain constant during an attack. Defects in a gene on chromosome 17 appear to give rise to hyperkalemic periodic paralysis. A number of different defects in this same gene have been found to be associated with hyperkalemic periodic paralysis, at least one form of myotonia congenita and paramyotonia congenita.

Hypokalemic periodic paralysis appears during adolescence to young adulthood. It was the first of the three types of periodic paralysis to be identified and studied and is probably the most common form of periodic paralysis. It is characterized by a fall in potassium levels in the blood. The administration of potassium during an attack may relieve symptoms. However, long-term usage of potassium to prevent attacks often has no effect. Attacks of hypokalemic periodic paralysis are usually triggered by exercising too strenuously, eating too many carbohydrates or taking drugs such as insulin. These attacks can last two to three hours or as long as a week, and they often begin at night. Initially they affect the back, shoulder and thigh muscles and then spread to the arms, neck and lower legs. Speech and breathing are also affected, although generally not drastically.

Hyperkalemic periodic paralysis, also called, Adynamia episodica hereditaria, begins in infancy to early childhood. Individuals with hyperkalemic periodic paralysis display a rise in potassium levels in the blood during episodes. They often experience myotonia, the inability to immediately relax a contracted muscle. However, this myotonia is neither muscle-wasting nor progressive, as it is in myotonic dystrophy, nor does it affect all muscle groups as does myotonia congenita. Attacks are usually triggered by strenuous exercise, administration of potassium or by exposure to cold. Such attacks may induce myotonia, paralysis or a mixture of both. They frequently begin with pain, numbness and a tingling sensation in the legs. While the consequent weakness may be felt in all parts of the body, the muscles of the arms and the legs are primarily involved. Attacks are typically more frequent but shorter in duration than those of hypokalemic periodic paralysis.

Normokalemic periodic paralysis also has its onset during infancy to childhood. Blood potassium does not rise above or fall below normal levels during an attack of normokalemic periodic paralysis. However, attacks are more severe and generally last longer than those of either hypokalemic or hyperkalemic periodic paralysis. Exercise or a number of different drugs may trigger these attacks. Normokalemic periodic paralysis is often similar to hyperkalemic periodic paralysis.