Muscle Tremor Disorder in Horses

2 min read

Hyperkalemic Periodic Paralysis

Hyperkalemic Periodic Paralysis (HYPP) is a type of muscle disorder that is normally found in the American Quarter Horse breed. Initially, the disease seems has similar symptoms to other muscular disorders, but it is actually very different and is caused by a variety of factors. Anyone who owns a horse of the American Quarter breed -- or a horse that has been crossbred with the American Quarter -- should know what HYPP is and how to recognize it in order to seek immediate veterinary care.


Symptoms and Types

Because HYPP affects the muscles, a horse will generally display stiffness in its muscles or suffer from muscle tremors. These “attacks” may subside quickly or may spread as the disease worsens. Other common signs of HYPP include:

  • Contraction of facial muscles, sometimes causing the animal to “smile”
  • Strange body posturing (e.g., swaying on feet, stumbling)
  • Frequent standing or laying down
  • Flaccid muscles



Transferred genetically, Hyperkalemic Periodic Paralysis' effects are due to the way the horse's body handles sodium and potassium ions. As sodium ions leak into the cells of the horse's muscles, vital potassium ions are pushed out of the cells.


HYPP is only found in a very small percentage of the world's equine population, so, needless to say, it is not a diagnosis that is often made. Your veterinarian may conduct a thorough medical history on the horse and ask you a great deal of questions about its health and diet.

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