Hyperkalemic periodic paralysis (HYPP, HyperKPP) is a genetic disorder which occurs in both humans and horses, where it is also known as Impressive Syndrome. It is an inherited autosomal dominant disorder which affects sodium channels in muscle cells and the ability to regulate potassium levels in the blood. It is most commonly associated with horses, but also occurs in humans, where it may be called Gamstorp episodic adynamy. 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.
The mutation which causes this disorder is dominant on SCN4A with linkage to the sodium channel expressed in muscle. The mutation causes single amino acid changes in parts of the channel which are important for inactivation. In the presence of high potassium levels, including those induced by diet, sodium channels fail to activate properly.
Equine hyperkalaemic periodic paralysis occurs in 1 in 50 quarter horses and can be traced to a single ancestor, a stallion named Impressive.
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