Gypsy, the Equine Encyclopedia, has something to say about Hyperkalemic Periodic Paralysis. Like all shetlands, she is always watching, listening and learning.
Hyperkalemic Periodic Paralysis, also known as HYPP, occurs mainly in Quarter Horses and other stock breeds such as Appaloosas and Paints. It has been traced back to one Quarter Horse stallion named Impressive.
Horses affected with this genetic disorder experience muscle tremors, stiffness and paralysis during which the horse can become unable to breathe. Episodes are not generally associated with work or exercise; HYPP can be triggered by diets high in potassium.
Being able to see the third eyelid is a warning sign. Horses with Impressive in their pedigree should be tested for HYPP although not all of those horses are affected.