Hyperkalemic periodic paralysis (HYPP) is a hereditary disease of the muscle sodium channel that is caused by a genetic defect. The condition is seen only in Quarter Horses and crosses that are descendants of the Quarter Horse sire, Impressive.

The sodium channel controls movement of ions in and out of the cell and is responsible for muscle contraction. In HYPP, the defective sodium channel is leaky. A result (and a cause) of this is hyperkalemia, a word meaning excessive potassium in the blood.

Usually, the abnormal sodium channel does not cause problems for the horse, but environmental factors and management factors can set off an episode. These factors include stress, other illness, feed changes, transport, high potassium diets, fasting, general anesthesia, and stall confinement, among many others.

Signs of HYPP are muscle tremors, weakness, wobbliness and collapse. Affected horses may fall down and stay down but seem aware of their surroundings and do not seem distressed or ill. An episode of HYPP is usually a sudden onset of muscle tremors (shaking or trembling), weakness and/or collapse. Attacks can also be accompanied by loud breathing noises resulting from paralysis of the muscles of the upper airway. Occasionally, sudden death can occur following a severe paralytic attack, presumably from heart failure or respiratory muscle paralysis.

Episodes of HYPP can take a variety of forms and can be confused with other conditions such as exertional rhabdomyolysis ("tying-up"), choke, colic, neurologic diseases, respiratory disease, botulism, and seizures. A key is that horses do not seem distressed and are alert during the episodes, and seem normal after the episodes. Any horse that is a known descendent of the horse Impressive should be suspected as having the condition unless the genetic test rules it out.

The diagnosis is suggested through history and physical exam and measurement of high potassium during an episode. Defiinitive diagnosis requires genetic testing, usually by the use of a hair or blood sample.

Treatment of an episode requires medications to lower the potassium level in the blood.

Once a horse has been diagnosed with HYPP, dietary management is extremely important. Avoid high potassium feeds such as alfalfa hay, brome hay, soybean meal, and sugar molasses and beet molasses. Instead make the staple feed a Timothy or Bermuda grass hay, grains such as oats, corn, wheat and barley, and beet pulp. Always avoid rapid feed changes, and especially going from grass to alfalfa hay, which is higher in potassium. Feed several times per day rather than once. Provide regular exercise and maximal turnout. Acetazolamide that reduces potassium levels in the blood and used for prevent episodes, however breed registries may have restrictions on the use of this drug during competition.