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Hyperkalemic periodic paralysis

Hyperkalemic periodic paralysis (HYPP), also known as Impressive Syndrome, is an inherited autosomal dominant disorder which affects sodium channels in muscle cells and the ability to regulate potassium levels in the blood of horses. This inherited disease is characterized by uncontrollable muscle twitching and substantial muscle weakness or paralysis among affected horses. HYPP is a dominant disorder; therefore heterozygotes bred to genotypically normal horses will still likely produce clinically affected offspring 50% of the time.

The disease is contained to the bloodline of the famous Appendix American Quarter Horse stallion Impressive, who has over 55,000 living descendants as of 2003. Although the disease is primarily limited to the American Quarter Horse breed and closely related breeds such as American Paint Horses and Appaloosas at this time, cross-breeding has begun to extend it to grade horses and ponies. The spread of the disease is perpetuated by the favorable judgings given to diseased horses in showing, due in part to involuntary muscle twitching which helps to build large, bulky muscles that judges favor.

In 1994, researchers at the University of Pittsburgh, with a grant from various horse organizations, isolated the gene responsible for the problem and developed a blood test for it. Using this test, horses may be identified as:

• H/H, meaning they have the gene and it is homozygous. These horses always pass on the disease.
• N/H, meaning they have the gene and it is heterozygous. These horses are affected to a lesser degree, and pass on the disease 50% of the time.
• N/N, meaning they do not have the disease and cannot pass it on, even if they are a descendant of Impressive.

Recently, horse organizations have begun instituting rules to attempt to eliminate this widespread disease. The American Quarter Horse Association (AQHA) now mandates testing for the disease and will no longer register homozygous (H/H) foals as of 2007, with discussion of heterozygous (N/H) foals pending. The Appaloosa Association will no longer accept homozygous foals as of 2008. It is believed that both primary palomino registries will exclude any HYPP-carrying foal as of 2007. The main organization affected by HYPP that has not yet taken action is the American Paint Horse Association (APHA), although many other smaller organizations are also affected.

Although much rarer, hyperkalemic periodic paralysis has also been observed in humans. The most common underlying cause is one of several possible point mutations in the genes synthesising calcium or sodium ion channels in skeletal muscle; this mutation can either be inherited in an autosomally dominant or recessive manner.