Hyperkalemic periodic paralysis (HYPP)

Veterinary advice should be sought before applying any treatment or vaccine.

Hyperkalemic Periodic Paralysis (HYPP)

Impressive Syndrome

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.


Unpredictable paralysis attacks
Muscle twitching
Sudden death


  • History
  • Clinical signs
  • Physical exam
  • Genetic testing
  • Muscle biopsy



Diet ManagementHorses with HYPP should be on a balanced, low-potassium diet (no more than 33g of potassium per meal).
Horses should not be fed alfalfa due to the high potassium concentration.
Acetazolamideadministered orally, in order to increase potassium excretion by the kidneys
Supportive careIV fluids


  • Limit potassium intake in diet by NOT feeding molasses, alfalfa, kelp, bananas as treats, and electrolyte supplements.
  • Conduct periodic feed, hay and pasture analysis to assess potassium levels
  • Look for feeds with beet pulp or oat based, complete feeds
  • Perform genetic testing for HYPP in breeding horses


Scientific Research

General Overviews

Age Range

Usually becomes apparent in horses less than 4 years of age, but has occured in horses between 2 months and 15 years of age.

Risk Factors

  • A diet rich in potassium rich foods (bananas, alfalfa)
  • Stress
  • Pregnancy
  • Exposure to cold temperatures
  • Rest following exercise
  • Skipped meals

Commonly Affected Breeds

Quarter Horse iconAppaloosa iconPaint Horse icon