Hyperkalemic Periodic Paralysis (HYPP) has been recognised in Quarter Horses since the 1980s. Dr Sharon J. Spier takes an in-depth look at the syndrome since genetic testing has been available to breeders.
HYPP is an autosomal dominant trait affecting Quarter Horses, Paint Horses, Appaloosas, and Quarter Horse-crossbred animals worldwide. A “syndrome” in related horses was first recognised in the 1980s by breeders and veterinarians; it was first reported to be similar to HYPP in humans by JH Cox at the annual American Association of Equine Practitioners convention in 1985.
In December 1992, this genetic disease was publicly linked to a popular Quarter Horse sire named Impressive. This prolific sire, born in 1969, has 355,000 descendents registered with the American Quarter Horse Association (AQHA), and these offspring dominate the halter horse industry. Current estimates indicate that 4% of the Quarter Horse breed may be affected.
Unfortunately, the gene frequency has not decreased in the past 14 years since genetic testing has been available to breeders, and controversy continues among horse breeders whose stock carry this gene.
Affected horses seem to have been preferentially selected as breeding stock because of their pronounced muscle development, and there is evidence of selection of HYPP-affected horses as superior halter horses by show judges.
In 1996, AQHA officially recognized HYPP as a genetic defect or undesirable trait. To increase public awareness of this genetic defect, mandatory testing for HYPP with results designated on the Registration Certificate began for foals descending from Impressive born after January 1, 1998. In response to requests from the membership, the AQHA Stud Book and Registration Committee ruled in 2004 that foals born in 2007 or later that test homozygous affected for HYPP (H/H) will not be eligible for registration. Breeders opposed to restrictions argue that the disease can be controlled through diet and medication and that these horses are highly successful in the show ring.
Clinical signs among horses carrying the same mutation range from asymptomatic to daily muscle fasciculations and weakness. In the majority of horses, intermittent clinical signs begin by two to three years of age with no apparent abnormalities between episodes.
Ingestion of diets high in potassium (>1.1%), such as those containing alfalfa hay, molasses, electrolyte supplements, and kelp-based supplements, or sudden dietary changes commonly trigger episodes.
Fasting, anesthesia or heavy sedation, trailer rides, and stress may also precipitate clinical signs; however, the onset of signs is often unpredictable without a definable cause.
Other possible precipitating factors that have been noted in humans and horses are exposure to cold, fasting, pregnancy, and concurrent disease and rest after exercise. Exercise does not seem to stimulate clinical signs, and serum CK shows no change to very modest increases during episodic fasciculations and weakness.
In most cases, clinical episodes begin with a brief period of myotonia with some horses showing prolapse of the third eyelid. Sweating and muscular fasciculations are observed commonly in the flanks, neck, and shoulders. The muscle fasciculations become more generalized when additional muscle groups are involved.
Stimulation and attempts to move may exacerbate muscular tremors. Some horses may develop severe muscle cramping. Muscular weakness during episodes is a common characteristic of HYPP. Horses remain standing during mild attacks. In more severe attacks, clinical signs may progress to apparent weakness with swaying, staggering, dog sitting, or recumbency within a few minutes. Heart and respiratory rates may be elevated, and horses may show manifestations of stress (anxious appearance), yet remain relatively bright and alert during episodes. Affected horses usually respond to noise and painful stimuli during clinical manifestations of the disorder. Episodes last for variable periods, usually from 15 to 60 min.
Several horses have died during acute episodes.
Respiratory distress occurs in some animals as a result of paralysis of upper respiratory muscles, and a tracheostomy may be necessary. In addition, young horses that are homozygous for the HYPP trait have been observed to manifest a respiratory stridor and periodically may develop obstruction of the upper respiratory tract. Horses homozygous for HYPP may present for dysphagia or respiratory distress, and endoscopic findings include pharyngeal collapse and edema, laryngopalatal dislocation, and laryngeal paralysis.14
After the episode subsides, horses regain their feet and appear normal with no apparent or minimal gait abnormalities. Although HYPP horses appear normal between attacks, electromyographic examination of affected horses reveals abnormal fibrillation potentials, complex repetitive discharges with occasional myotonic potentials, and trains of doublets between episodes.
How it happens
HYPP results from a point mutation that causes a phenylalanine/leucine substitution in a key part of the voltage-dependent skeletal muscle sodium channel alpha subunit.
In horses with HYPP, the resting membrane potential is closer to firing than in normal horses.
Sodium channels are normally briefly activated during the initial phase of the muscle-action potential. The HYPP mutation results in a failure of a subpopulation of sodium channels to inactivate when serum-potassium concentrations are increased. As a result, an excessive inward flux of sodium and outward flux of potassium ensues, resulting in persistent depolarization of muscle cells and temporary weakness.
Descent from the stallion Impressive on the sire’s or dam’s side in a horse with episodic muscle tremors, weakness, or collapse is strongly suggestive of HYPP.
Quarter Horse foals born after 1998 that are offspring of an affected parent have a statement recommending DNA testing for HYPP on the Certificate of Registration. In most cases, hyperkalemia (6-9 mEq/l), hemoconcentration, and mild hyponatremia occur during clinical manifestations of the disease with normal acid-base balance.
Serum-potassium concentration returns to normal after the abatement of clinical signs. Some affected horses may have normal serum-potassium concentrations during minor episodes of muscle fasciculations.
Differentials for hyperkalemia include delay before sample centrifugation, hemolysis, acidosis, renal failure, severe rhabdomyolysis, and high-intensity exercise.
Because veterinarians may not be present during acute episodes, the definitive test for identifying HYPP is the demonstration of the base-pair sequence substitution in the abnormal segment of the DNA encoding for the alpha subunit of the sodium channel.
Submission of mane or tail hair should be made to a licensed laboratory such as the Veterinary Genetics Laboratory at the University of California at Davis.
If horses are just beginning to exhibit clinical signs, light exercise can sometimes abort an episode in mild cases. Feeding grain or corn syrup to stimulate insulin-mediated movement of potassium across cell membranes may also be helpful.
Other treatment options that may abort an episode include administration of epinephrine (3ml of 1:1000/500 kg, IM) and administration of acetazolamide (3 mg/kg every 8-12 h, orally). Many horses experience spontaneous recovery from episodes of paralysis and appear normal by the time a veterinarian arrives.
In severe cases, administration of calcium gluconate (0.2- 0.4 ml/kg of a 23% solution diluted in 1 litre of 5% dextrose) will often provide immediate improvement.
An increase in extracellular calcium concentration raises the muscle-membrane threshold potential, which decreases membrane hyperexcitability.
To reduce the serum potassium, IV dextrose (6 ml/kg of a 5% solution) alone or combined with sodium bicarbonate (1-2 mEq/kg) can be used to enhance intracellular movement of potassium.
With severe respiratory obstruction, a tracheostomy may be necessary.
Decreasing dietary potassium and increasing renal losses of potassium are the primary steps taken to prevent HYPP episodes. Feedstuffs to avoid include high potassium feeds such as alfalfa hay, brome hay, soybean meal, and sugar or beet molasses.
Optimally, later cuts of Timothy or Bermuda grass hay, grains such as oats, corn, wheat, and barley, and beet pulp should be fed in small meals several times a day.
Regular exercise and/or frequent access to a large paddock or yard are also beneficial. Pasture works well for horses with HYPP, because the high water content of pasture grass makes it unlikely that horses will consume large amounts of potassium in a short period of time.
Ideally, horses with recurrent episodes of HYPP should be fed a balanced diet containing between 0.6% and 1.1-1.5% total potassium concentration and meals containing .33 g of potassium.
Many horses may be asymptomatic on diets containing higher concentrations of potassium. It was shown that horses adapted to diets higher in potassium over a period of two weeks and experienced fewer fluctuations in potassium in blood with subsequent decreased frequency of clinical signs.
Because there is a wide variation in potassium concentration of forages depending on maturity and soils, it is advisable to have feeds analysed for potassium concentrations and other nutrient requirements.
Table 1 contains examples of feeds containing varying concentrations of potassium. Vitamin E, selenium, salt, and balanced minerals should be supplemented where indicated to meet nutritional requirements. Commercially available complete feeds with a guaranteed K. content may be more convenient for some HYPP horses, especially for owners with few horses.
For horses with recurrent episodes of muscle fasciculations, even after dietary alterations, acetazolamide (2-3 mg/kg every 8-12 h, orally) or hydrochlorothiazide (0.5-1 mg/kg every 12 h, orally) may be helpful. These agents exert their effects through different mechanisms; however, both cause increased renal potassium excretion. In addition, acetazolamide stabilizes blood glucose and potassium by stimulating insulin secretion. Breed registries and other associations may have restrictions on the use of these drugs during competitions, because diuretics may mask prohibited substances.
In most cases, HYPP is a manageable disorder; however, recurrent bouts may occur, and severe episodes can be fatal. Owners of affected horses should be strongly discouraged from breeding these animals for the long-term health of the Quarter Horse breed and other breeds. Because this is a dominant trait, breeding an affected horse to a normal horse results in a 50% chance of producing a foal with HYPP.
All affected horses share the same mutation, regardless of whether or not owners have witnessed symptoms in their horses.
Affected horses are not suitable for young or inexperienced riders. Owners of affected horses should advise veterinarians of HYPP status before anesthesia or procedures requiring heavy sedation.
References and Footnotes
- Cox JH. An episodic weakness in four horses associated with intermittent serum hyperkalemia and the similarity of the disease to hyperkalemic periodic paralysis in man, in Proceedings. 31st Annual American Association of Equine Practitioners Convention 1985;383-391.
- Spier SJ, Carlson GP, Holliday TA, et al. Hyperkalemic periodic paralysis in horses. J Am Vet Med Assoc 1990;197: 1009-1017.
- Rudolph JA, Spier SJ, Byrns G, et al. Periodic paralysis in quarter horses: a sodium channel mutation disseminated by selective breeding. Nat Genet 1992;7:141-147.
- Naylor J. Equine hyperkalemic periodic paralysis: review and implications. Can Vet J 1994;35:279-285.
- Meyer TS, Fedde MR, Cox JH, et al. Hyperkalemic periodic paralysis in horses: a review. Equine Vet J 1999;31:362- 367.
- Reed SM, Hegreberg GA, Bayly WM, et al. Progressive myotonia in foals resembling human dystrophia myotonica. Muscle Nerve 1988;11:291-296.
- Schooley EK, MacLeay JM, Cuddon P, et al. Myotonia congenita in a foal. J Equine Vet Sci 2004;24:483-488.
- Lehmann-Horn F, Jurkat-Rott K. Voltage-gated ion channels and hereditary disease. Physiol Rev 1999;79:1317-1372.
- Valberg SJ, Ward TL, Rush B, et al. Glycogen branching enzyme deficiency in Quarter Horse foals. J Vet Int Med 2001;15:572-580.
- Bowling AT, Byrns G, Spier SJ. Evidence for a single pedigree source of the hyperkalemic periodic paralysis susceptibility gene in Quarter Horses. Animal Genet 1996;27:279- 281.
- Naylor JM. Selection of quarter horses affected with hyperkalemic periodic paralysis by show judges. J Am Vet Med Assoc 1994;204:926-928.
- Groves L. HYPP: someone else’s problem? Quarter Horse J 1996;1:52-59.
- Reynolds JA, Potter GD, Greene LW, et al. Genetic-diet interactions in the hyperkalemic periodic paralysis syndrome in Quarter Horses fed varying amounts of potassium: III. The relationship between plasma potassium concentration and HYPP Symptoms. J Equine Vet Sci 1998;18:731-735.
- Carr EA, Spier SJ, Kortz GD, et al. Laryngeal and pharyngeal dysfunction in horses homozygous for hyperkalemic periodic paralysis. J Am Vet Med Assoc 1996;209:798-803.
- Pickar JG, Spier SJ, Snyder JR, et al. Altered ionic permeability in skeletal muscle from horses with hyperkalemic periodic paralysis. Am J Physiol 1991;260:C926-C933.
- Ott EA, et al., eds. National research council: nutrient requirements of horses, 5th ed. Washington, D.C.: National Academy Press, 1989.
- Reynolds JA. Equine hyperkalemic periodic paralysis (HYPP): overview and management strategies. Available online at TechBulletins/HYPP.htm.
- Zhou J, Spier SJ, Beech J, et al. Pathophysiology of sodium channelopathies: correlation of normal/mutant mRNA ratios with clinical phenotype in dominantly inherited periodic paralysis. Hum Mol Genet 1994;3:1599-1603. aGriffith G. Personal communication. 2005. bStatistics from University of California at Davis Veterinary Genetics Laboratory. Unpublished data on HYPP testing. 2006.