Hyperkalemic Periodic Paralysis (HYPP)

Description

Equine Hyperkalemic Periodic Paralysis (HYPP) is a muscular disease caused by an inherited genetic variant. HYPP has been traced back to a horse named Impressive, and the condition is also known as Impressive Syndrome. Signs may include muscle twitching, respiratory noises, and unpredictable paralysis attacks, often triggered by changes in diet, fasting, stress, or feed high in potassium. Severity varies from unnoticeable to collapse or sudden death, with death usually resulting from respiratory failure, cardiac arrest, or both.

The condition is caused by a missense variant in the skeletal muscle sodium channel gene SCN4A (c.4248C>G), which replaces phenylalanine with leucine in the channel’s alpha subunit. The altered channel becomes leaky as blood potassium levels fluctuate, making the muscle overly excitable and causing it to contract involuntarily.

How it is inherited

HYPP is autosomal dominant, so a horse needs only one copy of the variant to show signs. Both homozygous (H/H) and heterozygous (n/H) horses can be affected, with H/H horses generally affected more severely, and severity varying widely between individuals. Only n/n horses are unaffected.

If an affected horse is bred to an unaffected one, each foal has a 50% chance of inheriting the variant. If two n/H horses are bred, 75% of foals are expected to inherit at least one copy. An H/H horse passes a copy to every foal. Because the variant is dominant, it moves into other breeds whenever Quarter Horse bloodlines are crossed in, which is why testing breeding stock matters beyond the breed it started in.

Registry rules

Several registries have adopted rules around HYPP. The American Quarter Horse Association designates it a genetic defect, requires descendants of Impressive to be tested, and since the 2007 foal crop has made H/H horses ineligible for registration; n/H horses remain registerable, with results recorded on the registration certificate. Other registries accepting Quarter Horse bloodlines, including the American Paint Horse Association and the Appaloosa Horse Club, have testing requirements of their own. Confirm current rules with the relevant registry before making breeding decisions.

Possible results

Breeds Affected

American Indian, Appaloosa, Appaloosa Cross, Appaloosa/Friesian, Appaloosa/Quarter Horse, Appaloosa/Thoroughbred, Appendix, AraAppaloosa, Arabian/Paint, Arabian/Quarter Horse, Australian Stock Horse, Azteca, Baroque Pinto, Canadian, Cleveland Bay, Clydesdale/Paint Cross, Colonial Spanish Horse, Crossbred, Curly Horse, Grade Horse, Grade Pony, Miniature Appaloosa, Mixed Breed (Horse), Morgan Horse, Origine Constatée, Paint Cross, Paint Horse, Paint Horse/Thoroughbred, Paint/Warmblood, Ponies of America, Pura Raza Espanola, Quarter Cross, Quarter Horse, Quarter Pony, Quarter/Paint Horse, Quarter/Stock Horse, Quarter/Thoroughbred, Quarter/Warmblood, Spanish Barb, Spanish Mustang, Stock Horse, Stonewall Sporthorse, Sugarbush Harlequin Draft, Thoroughbred

Reference

Periodic paralysis in quarter horses: a sodium channel mutation disseminated by selective breeding.

Related Tests

• Polysaccharide Storage Myopathy (PSSM1) — another muscle disorder of Quarter Horses, often confused with HYPP
• HERDA — a recessive skin disorder of cutting-bred Quarter Horses
• GBED — a recessive disorder causing aborted and stillborn Quarter Horse foals

Breeding Quarter Horses? The Quarter Horse Disorder Panel screens for HYPP along with PSSM1, HERDA, GBED, MH, IMM and EJSCA.