Hyperkalemic periodic paralysis (HYPP, HyperKPP) is a genetic disorder that occurs in horses and humans, where it is also known as Impressive syndrome. It is aninherited autosomal dominant disorder that affects sodium channels in muscle cells and the ability to regulate potassium levels in the blood. It is most commonly associated with horses, but occurs in humans, where it may be called Gamstorp episodic adynamy. It is characterized by muscle hyperexcitability or weakness which, exacerbated by potassiumor cold, can lead to uncontrolled shaking followed by paralysis. Onset in humans usually occurs in early childhood.
The mutation which causes this disorder is dominant on SCN4A with linkage to thesodium 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 inactivate properly.