Test Detail

Equine hyperkalaemic periodic paralysis (HYPP; the technically correct term is 'hyperkalaemic', due to elevated potassium, although in catalogues it appears as hypercalcaemic) is a hereditary myopathy of the Quarter Horse and related breeds, caused by a mutation in the SCN4A gene of the skeletal muscle sodium channel. The altered channel does not close properly when blood potassium rises, producing sustained muscle depolarisation: tremors, weakness and episodes of paralysis lasting from minutes to hours. The episodes recur throughout life and can be fatal. The test distinguishes N/N, H/N (heterozygous) and H/H (homozygous, far more severe) animals.

Linked to the descendants of the stallion Impressive, especially in halter lines of the Quarter Horse; also Paint and Appaloosa through crosses with that breed. Classic North American surveys described carrier frequencies of several percentage points in the breed and much higher in halter populations (limited data, variable depending on the period).

- Episodes of localised or generalised muscle tremors
- Sudden weakness with stumbling, flaccidity of the hindquarters and difficulty moving
- Intermittent prolapse of the third eyelid (nictitating membrane)
- Sweating, polypnoea and anxiety during the episode
- Recumbency with preserved consciousness and recovery within minutes or hours
- Death from respiratory or cardiac failure in severe episodes
- Triggers: potassium-rich feed (alfalfa), fasting, stress, transport, anaesthesia

The disease was recognised in the mid-1980s in halter-conformation Quarter Horses, and it was quickly seen that all cases traced back to the stallion Impressive. In the early 1990s, North American studies linked it by linkage to the skeletal muscle sodium channel gene (SCN4A), analogous to human hyperkalaemic periodic paralysis, and shortly afterwards the responsible mutation was characterised. The American breed association (AQHA) introduced the test and the annotation of status in the documentation of Impressive's descendants, and since then the incidence has declined markedly.

- Test every breeding animal with Impressive ancestry or from halter lines
- As it is dominant, a single copy of the allele (H/N) is enough to transmit the disease: the decision to breed with an H/N must always be justified and communicated
- Never mate H/N with H/N: 25% risk of severe H/H foals
- Ideally select N/N breeding animals; associations such as the AQHA restrict the registration of H/H animals
- The deliberate breeding of H/H foals is incompatible with welfare
- Always inform the veterinarian of the status before anaesthetising, castrating or transporting the animal

Differential diagnosis of the episode: heat stroke, rhabdomyolysis due to PSSM, atypical pasture myopathy, tetanus and painful colic. Between episodes the animal may be normal, and serum potassium is only elevated during the crisis. Management includes a low-potassium diet (limiting alfalfa and molasses, splitting meals), regular exercise, avoiding fasting and stress, and acetazolamide as supportive treatment prescribed by the veterinarian. Before anaesthesia in H/N or H/H animals, use non-depolarising agents and strict monitoring.

Rudolph JA et al. 1992. Periodic paralysis in quarter horses: a sodium channel mutation disseminated by selective breeding. Nat Genet. PMID: 1338908; Rudolph JA et al. 1992. Linkage of hyperkalaemic periodic paralysis in quarter horses to the horse adult skeletal muscle sodium channel gene. Anim Genet. PMID: 1323940; Naylor JM. 1997. Hyperkalemic periodic paralysis. Vet Clin North Am Equine Pract. PMID: 9106348; OMIA:000785-9796