H.Y.P.P. stands for Hyperkalaemic Periodic Paralysis. It may also be called Hyperkalaemic Polymyopathy. Hyperkalaemia means too much Potassium in the blood and this condition is a disorder of Potassium metabolism.
H.Y.P.P. is a genetic disorder caused by a defect in the Sodium channel in muscle cells of affected horses which facilitates the passage of Potassium into the cell and Sodium out of the cell. The mutation causing the defect originated in a quarter horse stallion called Impressive and affected horses are related to that stallion or his descendants. Around 4% of quarter horses are now affected.
The disease causes abnormal flows of sodium ions into muscle cells precipitating waves of electrical activity and contraction of those cells. Potassium is the main electrolyte ion inside cells whereas sodium is mainly in the extracellular fluid. Symptoms of H.Y.P.P. include trembling, cramps, muscle fasciculation (twitching) often seen first over the flank and ribs but possibly affecting other muscle groups as well. Some horses become lethargic and have very low heart rates. Signs may mimic colic and the third eyelid may become everted. Horses can become recumbent or assume a sitting posture. Foals may show signs similar to respiratory obstruction. In the severest cases paralysis of respiratory and heart muscles can be fatal.
Immediate veterinary treatment is by administration of intravenous calcium and glucose and possibly a specific diuretic which increases potassium excretion in the urine. Nutritional management involves reducing potassium intake by reducing forages, especially legumes like Lucerne and selecting feeds based on their potassium concentration under veterinary supervision.