Hyperkalemic periodic paralysis (HYPP) is a genetic condition in horses that affects how their muscles function and respond to signals from the nervous system. These horses have high levels of potassium in the blood and should be fed low potassium diets.

Horses affected by HYPP have pronounced musculature which is sometimes perceived as a desirable appearance. They may also perform better in halter classes. [1]

This aesthetic advantage could have contributed to the increase in affected horses through breeding practices that selected for these traits.

Horses that carry the genetic variant that results in HYPP typically begin to show signs around 2-3 years of age. Some horses may never show signs of the condition but are still genetic carriers that can have affected offspring.

The most common sign of HYPP is periodic muscle tremors and general muscle weakness. These horses have defects in how their muscle cells handle sodium and potassium resulting in high levels of potassium in the blood.

Horses with HYPP should be fed a low potassium diet and should avoid feeds such as alfalfa hay or soybean meal.

For help with formulating an appropriate diet for a horses with HYPP, submit your horse’s information online. Our nutritionists can help you design a feeding plan to minimize HYPP symptoms for free.

Hyperkalemic Periodic Paralysis

HYPP is a muscle disorder resulting from a gene mutation that affects ion transport in muscle tissue. Specifically, it impairs the normal function of voltage-gated sodium channels in muscle cells. [9]

HYPP is most commonly seen in Quarter Horses, Paints, Appaloosas and crossbreeds. It is estimated that 4% of Quarter Horses alive today have HYPP. [2][3]

This genetic condition originates from a popular Quarter Horse sire named Impressive. He was born in 1969 and is known to have sired over 350,000 Quarter Horse offspring.

It is now mandatory to do genetic testing of foals born to descendants of Impressive to determine whether they carry the known genetic mutation that causes this condition and to discourage breeding of affected horses. [2]

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Effects of HYPP in Horses

For movement to occur, your horse’s muscles receive signals from nerves to initiate the contraction of the muscle.

This results in brief activation of voltage-gated sodium channels in the muscle cell membrane that pump sodium into the cell and move potassium out into the blood.

In non-HYPP horses, these channels are only briefly open and move the appropriate amount of ions needed to stimulate muscle contraction.

In horses with HYPP these channels stay open for longer, resulting in excessive sodium moving into the cell and potassium moving out.

This causes prolonged depolarization of muscles, meaning they may be unable to respond to the next neural signal which causes temporary paralysis and muscle weakness. [4]

The resulting high levels of potassium in the blood can cause cardiac dysfunction leading to a potentially fatal heart attack.

Luckily this prolonged opening of sodium channels doesn’t happen all the time. Most HYPP horses will show signs of the condition intermittently with no obvious issues between episodes.


Definitive diagnosis of HYPP is done by performing a genetic test that looks for the specific gene mutation.

Horses that are descendants of Impressive and show periodic muscle trembling, weakness or collapse are very likely to have HYPP and should be assessed by a veterinarian to get a firm diagnosis.

Horses that have two copies of the mutation, from both the mother and father (homozygous), will be more severely affected.

Those that only have one copy of the mutation from either the mother or father (heterozygous), will likely be less severely affected and may show no symptoms. [5]

Knowing whether your horse has one or two copies of the gene can help you understand the potential severity of the condition. Horses known to carry this genetic mutation (even without showing symptoms) should not be bred to avoid passing on this condition.

Signs of HYPP

Signs of HYPP can vary greatly between individual horses. Some horses that carry the gene mutation never show signs of the condition, while others experience daily muscle tremors and weakness.

Affected horses often go long periods without showing symptoms between episoides.

They may experience temporary episodes of muscle paralysis that can range from mild muscle tremors and cramping to seve