Thyrotoxic periodic paralysis

Alternative names:

periodic paralysis - thyrotoxic

Definition:

A disorder characterized by intermittent episodes of muscle weakness that occurs in people with thyrotoxicosis, a disorder associated with high levels of thyroid hormones.

Causes, incidence, and risk factors:

Hypokalemic periodic paralysis (familial periodic paralysis) is an inherited condition, and in most cases, it is directly inherited. In other cases, the disorder appears sporadically in a family group. It can occur in people with thyrotoxicosis (high thyroid hormone levels), especially when the condition has persisted for several months. About 10% of people with hyperthyroidism will exhibit thyrotoxic periodic paralysis. The disorder is most common in men, particularly Latin American and Asian men.

The disorder involves attacks of muscle weakness or paralysis alternating with periods of normal muscle function. Attacks usually begin after symptoms of hyperthyroidism have developed. The frequency of attacks varies from daily to yearly. Episodes of muscle weakness may last for a few hours or may persist for several days.

During an attack, 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. It may occasionally affect the muscles of the eyes or the muscles involved in breathing and swallowing (which can be fatal). Heart arrhythmias may occur during attacks because of the drop in potassium levels. Although muscle strength is initially normal between attacks, repeated attacks may eventually cause progressive and persistent muscle weakness.

Risks include a family history of periodic paralysis and hyperthyroidism. Attacks may be triggered by eating high carbohydrate or high salt meals or by taking a thyroid hormone. Attacks most commonly occur after sleep or rest and are rare during exercise, but rest after an exercise period may trigger an attack.