Hyperkalemic periodic paralysis, generally known as HYPP or Impressive Syndrome, has been wide-spread for several years now in Appaloosas, Quarter horses, Paints, and Palominos. Affected horses show well-developed muscles and are usually winning many halter classes. But, HYPP is not limited to just these stock breeds. It can cross over to any other breed if an affected horse is bred to one of a different breed which has yet to be affected. Impressive Syndrome was first identified in 1985 as a heritable disease of the muscle, which is caused by a genetic defect. Some humans have also been known to carry a rare disease very similar to this, called Eulenberg's disease. HYPP causes muscle weakness or paralysis and is related to high potassium levels in the bloodstream.
According to the Swedish National Board of Health and Welfare, "the cause of hyperkalemic periodic paralysis [in humans] is a change in a gene that regulates the production of a protein in the sodium ion channel of the skeletal muscle." These channels are affected by the defect in a way that they do not open and close properly; therefore sodium fluctuations cannot be controlled. When sodium fluxes are uncontrollable, the voltage current of the muscle cells changes, causing excess potassium to go to the bloodstream and the horse experiences periodic twitches and intense weakening of the muscles.
The oldest origin of this disease has yet to be found, but there is one horse from which we presently know it came from: the top-producing, top-winning Quarter horse stud, Impressive. This stallion was heavily muscled and won every halter class he was ever in. His owners soon found out after breeding him that he had another valuable trait: the ability to pass on his great conformation and musculature to his offspring. Horse breeders from all over wanted him as a stud, and that is why Impressive's name shows up on so many of today's papers for Quarters, Paints, and Appaloosas. Little did the breeders know that Impressive carried the dominant, defective gene that causes the rare hyperkalemic periodic paralysis. Because all of Impressive's ancestors are dead, no genetic information can be tested to see which relative had the disease before him. Any descendants of Impressive should be tested for HYPP before they are be bred to reduce the risk of spreading the disease. Horses that are affected with HYPP will have these symptoms from birth: "mild muscle twitching that is undetectable to the human eye; noticeable muscle twitching; crawling' skin, ranging from slight to very noticeable and usually from the back flank area forward; hind quarter paralysis; excessive yawning; and paralysis of the muscles surrounding the heart and/or lungs, causing death due to heart attack or suffocation." Some horses, though, may not show these signs until they are in their teen years.
As mentioned earlier, HYPP occurs due to a genetic defect in the transport system that regulates the passing of potassium and sodium across muscle cell membranes. Horses with HYPP have less potassium content in the muscles than normal horses because of a dysfunctional muscle membrane that allows potassium to build up in the bloodstream. This then causes leakage of sodium across the cells, resulting in a hyper-excitable muscle membrane and muscle twitches and spasms. As the muscle contractions or spasms continue, potassium leaves the cells to add to high potassium levels in the blood. In several minutes, the muscle membranes can no longer fire; the horse's muscles become paralyzed, and the horse weakens and collapses. HYPP can be fatal because of the varied effects of high blood potassium levels on the heart and the possibility for diaphragm muscles to become paralyzed. An attack is brought on by too much potassium in the diet, as occurs with alfalfa hay. Sudden dietary changes, irregular feeding schedules, with-holding food or stress from training or transport can lead to an attack.
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