It is an inherited autosomal dominant disorder that affects sodium channels in muscle cells and the ability to regulate potassium levels in the blood.
Equine hyperkalemic periodic paralysis occurs in 1 in 50 Quarter Horses and can be traced to a single ancestor, a stallion named Impressive.
HYPP is a dominant genetic disorder; therefore, heterozygotes bred to genotypically normal horses have a statistic probability of producing clinically affected offspring 50% of the time.
Following an HYPP attack, the horse appears normal and is not in any pain which helps to distinguish it from Equine Exertional Rhabdomyolysis (ER), commonly known as "Azoturia," "Monday Morning Sickness" or "tying up."
A blood test will reveal elevations in certain muscle enzymes after an episode of ER and so the two diseases, while superficially similar, are easily distinguished from one another in the laboratory.
In 1994, researchers at the University of Pittsburgh, with a grant from horse organizations,[1] isolated the genetic mutation responsible for the problem and developed a blood test for it.