Hyperkalemic Periodic Paralysis

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Hyperkalemic periodic paralysis, also known as HYPP, is a genetically-linked muscle disease that is passed from a parent horse to a foal offspring. This disease is caused by a mutation in the sodium-channel gene which regulates muscle contractions.

In horses with HYPP, the sodium channel is particularly sensitive to variations in serum potassium levels. This sensitivity can cause muscle excitability and excessive contraction of the muscle fibers leading to muscle twitching and weakness and unpredictable episodes of paralysis.

All cases of hyperkalemic periodic paralysis have been traced back to the American Quarter Horse stallion named Impressive. Research shows that approximately 4% of all American Quarter Horses develop this syndrome and American Quarter Horse Crossbreds, Appaloosas and American Paint Horses may also be affected.

A foal with hyperkalemic periodic paralysis can carry the mutated gene on one or both of the gene copies received from its dam and the stallion that sires the foal. Horses with only one copy of the gene are less severely affected than horses with copies of the gene from both parents, but both can experience clinical symptoms and both can pass the mutation to their offspring.


  • Muscle tremors
  • Shaking
  • Weakness
  • Collapse
  • Loud, labored breathing
  • Occasionally sudden death
  • Horses that survive HYPP attacks will often appear normal once the attack passes


While the HYPP mutation is the direct cause of disease, attacks are triggered by environmental factors, including dietary changes (particularly to high potassium feeds), stress, anesthesia, fasting, other illnesses, and exercise restriction.

Although HYPP is not associated with exercise, it can occur following any stressful event.


Dietary management is key to successfully preventing episodes of this disease. Owners of HYPP-affected horses should avoid feeding high potassium feeds such as alfalfa, molasses, brome hay, canola oil, and soybean meal or oil.

Feeding multiple times per day on a regular schedule and providing regular exercise can also help to prevent HYPP attacks. Horses should be provided fresh water at all times.

Some horses may require medication such as acetazolamide or hydrochlorthiazide to prevent attacks. Such medications should be used only under the prescription and supervision of a licensed veterinarian.

Horses suspected of being HYPP carriers should be tested for the gene. This includes all horses with an affected parent, and all known descendants of Impressive. Any horse that carries the HYPP mutation on one or both alleles can pass that mutation to its offspring.

Since the risk of producing an HYPP foal increases if both sire and dam carry the gene, genetic testing can be done at several laboratories in the United States to determine whether or not horses are carriers of the gene. Horses do not “outgrow” HYPP. An affected horse is affected for life.


Conservative, long-term treatment includes management of diet and exercise as noted above. However, acute attacks of HYPP should be considered life-threatening emergencies since both heart and respiratory function depend on proper muscle contraction. A veterinarian should be contacted immediately in the case of an HYPP attack.

Medical treatment may involve IV fluids spiked with dextrose (sugar) and/or calcium. The sugar will trigger an insulin release which may help drive potassium back into the cells; calcium can help counteract the effects of the potassium on the muscle.

Acetazolamide can be given to help the horse to rid its body of excess potassium through the kidneys and into the urine.

Further Reading:
American Quarter Horse Association
UC Davis Veterinary Genetics Laboratory 

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