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HYPP is a muscular disorder that causes muscle spasms and tremors

Full name

Hyperkalaemic periodic paralysis

Breeds known to be affected

Quarter Horse and related breeds

Inheritance pattern

Autosomal dominant


HYPP is a muscle disorder that causes delayed muscle relaxation. Symptoms range from muscle twitches and weakness to severe muscle spasms, paralysis, respiratory noises, collapse and occasionally death, usually from cardio-respiratory effects.

HYPP is a dominant trait, meaning a horse only needs 1 copy of the mutation (HYPP/n) to be affected. There is some evidence that homozygous horses (HYPP/HYPP) are more severely affected than heterozygotes. The severity and onset of symptoms can be managed with diet, particularly by avoiding high potassium feeds. HYPP is associated with Quarter Horses from the "Impressive" line and is caused by a mutation in the SCN4A gene.

Interpretation of results

n/n: Horse does not carry the mutation associated with HYPP.

HYPP/n: Horse has one copy of the mutation associated with HYPP.

HYPP/HYPP: Horse has two copies of the mutation associated with HYPP.

Reference Rudolph et al. Periodic paralysis in Quarter Horses: a sodium channel mutation disseminated by selective breeding. Nature 1992; 2:144-147 doi:10.1038/ng1092-144

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