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Hyperkalemic periodic paralysis
From Citizendium, the Citizens' Compendium
Hyperkalemic periodic paralysis is a genetic disorder which occurs in both humans and horses. It is characterized by muscle hyperexcitability or weakness which, exacerbated by potassium or cold, can lead to uncontrolled shaking followed by paralysis. Onset in humans usually occurs in the twenties.
In humans, it is "an autosomal dominant familial disorder which presents in infancy or childhood and is characterized by episodes of weakness associated with hyperkalemia. During attacks, muscles of the lower extremities are initially affected, followed by the lower trunk and arms. Episodes last from 15-60 minutes and typically occur after a period of rest following exercise. A defect in skeletal muscle sodium channels has been identified as the cause of this condition. Normokalemic periodic paralysis is a closely related disorder marked by a lack of alterations in potassium levels during attacks of weakness."
The mutation which causes this disorder is dominant with linkage to the sodium channel expressed in muscle (called SCN4A). In the presence of high potassium levels, sodium channels fail to activate properly. The mutation causes single amino acid changes in parts of the channel which important for inactivation.
Equine hyperkalaemic periodic paralysis occurs in 1 in 50 quarter horses and can be traced to a single ancestor (A stallion named Impressive). Episodes in these horses are commonly brought on by alfalfa, which contains high levels of potassium.