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Hyperkalemic Periodic Paralysis (HYPP Horse)

Hyperkalemic Periodic Paralysis (HYPP) is caused by a malformed channel protein in skeletal muscle cells and results in temporary paralysis due to malfunctioning signal transmission from the brain to the muscles/organs.

The disease occurs in the Quarter Horse and related breeds. The inheritance is incomplete autosomal dominant.


  • General weakness
  • Periodic tremors and paralysis
  • Respiratory noise
  • Heavily muscled

General Information

  • Hyperkalemic periodic paralysis (HYPP) is caused by a mutation in the gene for the sodium channel-(voltage-gated)-type IV-protein (SCN4A).
  • Sodium channels are ion channels (conduct ions in and out of the cell) in the cell membrane and have, for example, the function of signal conduction in skeletal muscle.
  • This is important for the transmission of nerve signals to the muscle, i.e. for the contraction of the muscle cells.
  • Due to the mutation, the ion channels are impaired in their function and thus the muscle movement.
  • Symptoms vary in severity.
  • Homozygous animals usually show more pronounced symptoms than heterozygous animals.
  • Life-threatening if cardiac arrhythmias or respiratory paralysis occur.
  • Can be traced back to the sire "Impressive".
  • Episodes occur more frequently after feed changes, stress or transport.
  • Symptoms can be reduced by a low-potassium diet (e.g. avoidance of luzern/alfalfa in the hay).

Breeds affected

Quarter Horse and related breeds like Paint Horse and Appaloosa

Test Information

This muatation test shows the change of a single base pair (c.4248C>G) in the SCN4A gene.

Genotype and Lab Report

Inheritance: autosomal-dominant

→ Horses with one or two copies of the variant (n/HYPP oder HYPP/HYPP) are affected. Homozygous horses usually show more pronounced symptoms than heterozygous horses.




n/n = normal

The horse has no copies of the genetic variant causitive for HYPP and therefore cannot pass it on to its offspring.


n/HYPP = affected

The horse is affected. It has one copy of the genetic variant causitive for HYPP, which will be passed on to its offspring with a probability of 50%. These foals will be affected.


HYPP/HYPP = affected

The horse is affected and has two copies of the genetic variant causitive for HYPP.  The variant will be passed on to its offspring with a probability of 100%. All offspring will be affected.


Affected animals (n/HYPP, HYPP/HYPP) should not be used for breeding.


Rudolph, J. A., Spier, S. J., Byrns, G., Rojas, C. V., Bernoco, D., & Hoffman, E. P. (1992). Periodic paralysis in quarter horses: a sodium channel mutation disseminated by selective breeding. Nature Genetics, 2(2), 144-147. doi: 10.1038/ng1092-144.


Further information is available at Online Mendelian Inheritance in Animals.

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