Hyperkalemic periodic paralysis (HYPP) is a muscular disorder caused by an inherited genetic mutation. It affects Quarter Horses, Paints, Appaloosas and crossbred Quarter horses. The gene responsible has been traced back to one horse named Impressive and has the alternative name, Impressive Syndrome, named after this horse. HYPP is a dominant disorder, which means both homozygous positive (HH) and heterozygous (nH) horses are affected. Only homozygous negative (nn) horses are not at risk of developing HYPP.

HYPP resembles hyperkalemic periodic paralysis in humans, and is characterized by sporadic episodes of muscle tremors and weakness, often resulting in the horse collapsing. Abnormal loud breathing noises are sometimes made by horses undergoing an HYPP episode, caused by paralysis of the upper airway muscles.

HYPP episodes are brought on by a number of factors, including being fed potassium-rich foods (bananas, alfalfa) or diet, stress, pregnancy, exposure to cold temperatures, certain medications, rest following exercise, and skipped meals. Because HYPP episodes often resemble many other conditions, horse owners often confuse them with colic, tying up, laziness, misbehaving, and sometimes neurological disorders.