What are Hyperkalemic Periodic Paralysis?
Hyperkalemic periodic paralysis (HYPP) is a relatively new dominant genetic disease that has been traced to the bloodline of just one stallion. Impressive, an American Quarter Horse, was born in 1969 and went on to become the 1974 World Champion Open Aged halter stallion, the first time this title had been awarded to an American Quarter horse. Impressive sired 2,250 foals, 30 of them world champions in their own rights. Along with his extraordinary musculature and refined form, many of his descendants also acquired HYPP, a dominant genetic disease which causes intermittent weakness, trembling, and sudden collapse. In severe cases, this disorder can cause sudden death due to the paralysis of the diaphragm.
Hyperkalemic periodic paralysis (HYPP) is a dominant genetic disease most common in Quarter Horses, and which causes intermittent weakness, trembling, and sudden collapse.
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Symptoms of Hyperkalemic Periodic Paralysis in Horses
This hereditary disease seems to strike at random, and typically episodes last somewhere around fifteen to sixty minutes. Some animals will have more severe and frequent attacks than others on average, and even in the same horse, the intensity of the attack can vary from episode to episode. Common symptoms include:
- Muscle weakness (either generalized or hind end)
- Sudden death
- Third eyelid prolapse
- Vocal changes
Horses generally recover completely and without pain directly after the attack.
Quarter Horses are prone to several genetic disorders. Other dominant disorders common to the American Quarter Horse include:
- Polysaccharide storage myopathy (PSSM) - Like HYPP, this disease is a dominant genetic disorder whereby horses with this disorder may exhibit muscle stiffness and cramping that can interfere with regular exercise; PSSM is managed through dietary modification.
- Malignant hyperthermia (MH) - This genetic trait is also dominant, however, it is often asymptomatic unless it is triggered by extreme stress, the anesthetic halothane, or the muscle relaxant succinylcholine; when triggered, the horse's temperature rises dangerously, and abnormal heart rhythms, high blood pressure, and the breakdown of muscle tissues often follow and in the most extreme cases, death may occur
Causes of Hyperkalemic Periodic Paralysis in Horses
This disorder can be passed down from either or both parents and affects not only Quarter Horses, but Quarter Horse crossbreeds and the closely related breeds of Appaloosa and Palomino horses may also test positive for this mutation.This genetic defect causes a periodic disruption of the sodium ion channels in the membrane of the muscle cells. This means that the cells are occasionally flooded with sodium, temporarily changing the current in the muscle cells themselves. When the current shifts, it causes a loss of control of the muscles. High levels of potassium may instigate episodes of disruption, as can illness and exercise restriction. General anesthesia is also known to cause attacks, so it is important that your veterinarian is made aware of the disorder if any sort of surgery is ever required.
Diagnosis of Hyperkalemic Periodic Paralysis in Horses
Tests are available to determine if a horse has the genetic defect responsible for this disorder. Results of this testing can reveal one of the following classifications:
- H/H (homozygous) - This horse is affected by the disease and carries a copy from both sire and dam. 100% of this horse’s offspring will inherit HYPP
- n/H (heterozygous) - This animal is also affected, but usually not as severely; only one copy of the gene is present giving this horse a 50% chance to pass it on
- n/n - This is the result if the horse does not have HYPP; symptoms of this disorder with a result of n/n would require further testing to determine what the correct underlying condition is
The symptoms vary from most seizures as the horse generally remains lucid throughout. The signs of HYPP are also similar to another genetic disorder, equine exertional rhabdomyolysis (ER). ER can be differentiated from HYPP by both timing and recovery. ER generally only occurs after exertion or exercise, whereas HYPP can happen at any time, and animals are usually in pain during the attack and for quite some time after when experiencing an attack of rhabdomyolysis.
Treatment of Hyperkalemic Periodic Paralysis in Horses
Once diagnosed, this disease is manageable in most cases, although to what degree is variable from horse to horse. Medical interventions that can help during an attack can include:
- Administration of diuretics
- Administration of glucose or carbohydrates
- Intravenous calcium supplementation
- Intravenous glucose and insulin
Dietary changes that alter the levels of potassium or sodium in the blood may trigger an episode, as can fasting, particularly if followed by a high potassium meal like alfalfa, so adjustments to the feed should be made carefully and with an aim to lower the potassium levels in the daily diet. Drugs like acetazolamide and hydrochlorothiazide have been made available that can help with preventing episodes of paralysis.
Recovery of Hyperkalemic Periodic Paralysis in Horses
In order to reduce the number of foals born with this disorder, most Quarter Horse breeders required testing to determine if the animals that they are breeding carry the faulty gene. As of 2007, the American Quarter Horse Association requires testing for the gene and no longer allows horses that test positive for the homozygous version of this disorder to be added to the registry. Also in 2007, the American Palomino registry began to disallow registry of animals that tested positive for either homozygous or heterozygous. The Appaloosa Association jumped on board in 2008, denying registry to horses that test positive for the homozygous trait.