The story of Hyperkalemic Periodic Paralysis (HYPP) begins with the Quarter Horse Impressive. Born in 1968 an Appendix American Quarter Horse, he earned his full AQHA registration in 1971. He was a big good looking horse with well developed musculature and the 1974 World Champion Open Aged halter stallion. He passed this along to many of his foals siring over 2000 foals and dozens of World Champions before dying in 1995. Unfortunately all these good looks came at a price. He was also passing along a genetic defect that apparently originated with him as a autosomal point mutation. The resulting disease was given the descriptive name of Hyperkalemic Periodic Paralysis (HYPP). Looking at the terms one at a time:

• "Hyperkalemic" means there is a high level of potassium ion (K+) in the blood.
• "Periodic" suggests there are periods of no symptoms between periods of symptoms.
• "Paralysis" refers to the weakness that can lead to recumbency and a inability to rise.

HYPP is characterized by recurring episodes of muscle weakness that appear as muscular trembling, weakness, and/or collapse. Episodes can be severe enough in some individuals to cause collapse and death. Though there is no cure, many affected horses with this disease have good and useful lives with good management and medication. in spite of the disease.

This article discusses the symptoms, diagnosis, physiology, genetics, management, treatment, and how to prevent having foals from being affected by HYPP.

HYPP horses vary in the severity of their symptoms. While some horses have little in the way of problems others can be crippled by the disease. In severely affected horses weakness may begin as a foal often first expressed as respiratory stridor, a honking or rattling sound made during breathing. For most HYPP individuals problems start in young horses beginning training. Often there is a history of recurring attacks though single episodes do occur. These attacks will last a few minutes to a few hours and usually resolve on their own but treatment can greatly curtail the symptoms.

Muscle tremors are the first signs of a attack and may progress to buckling knees, swaying, and staggering. There may be prolapsing of the third eyelid.

If the episode is severe the horse may show signs of difficult breathing, sweating, and collapse with a inability to rise for several hours. Death can occur. Other than the sweating, the horse does not seem painful. Horses with more severe signs are more likely to be homozygous for this condition, for more on this see Genetics topic below.