Familial periodic paralysis

Alternative names:

hypokalemic periodic paralysis; periodic paralysis - familial; periodic paralysis - hypokalemic

Definition:

A disorder that occurs within certain families and causes intermittent episodes of muscle weakness.

Causes, incidence, and risk factors:

Familial periodic paralysis is a condition of intermittent episodes of paralysis or muscle weakness that occurs in multiple members of a family group. Many (but not all) cases of familial periodic paralysis are termed "hypokalemic" periodic paralysis.

Hypokalemic periodic paralysis occurs as an inherited condition. In most cases, it is inherited as an autosomal dominant disorder (only one parent must transmit the gene). In other cases, the disorder appears sporadically in a family group.

The disorder involves attacks of muscle weakness or paralysis alternating with periods of normal muscle function. Attacks usually begin in adolescence but can occur before age 10. Attacks of intermittent weakness that do not begin until adulthood are rare and usually caused by other disorders. The frequency of attacks varies from daily to yearly. Episodes of muscle weakness may last for a few hours or persist for several days.

During an attack of muscle weakness, there is a low level of potassium in the bloodstream (serum). Serum potassium levels are normal between attacks. There is no decrease in total body potassium, however. Potassium flows from the bloodstream into muscle cells. Insulin levels may affect the disorder in some people because insulin increases the flow of potassium into cells.

Weakness most commonly affects the muscles of the arms and legs but may occasionally affect the eye muscles or the muscles involved in breathing and swallowing (which can be fatal). 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 ingestion of high carbohydrate meals. Attacks most commonly occur after sleep or rest and are rare during exercise, but rest after an exercise period may trigger an attack. The risk is slightly higher in Asian men who also have thyroid disorders (thyrotoxic periodic paralysis).

Disorders that cause intermittent episodes of paralysis as their primary effect are uncommon. Familial periodic paralysis occurs in approximately 1 out of 100,000 people. Men are affected more often than women and usually have more severe symptoms.