Hyperkalemic Periodic Paralysis in Horses

What Is Hyperkalemic Periodic Paralysis in Horses?

Hyperkalemic Periodic Paralysis or (HYPP) is a genetic disorder caused by a mutation in the gene responsible for sodium channel proteins, which help with muscle contraction. This mutation causes the sodium channels to “leak,” malfunction, and become hypersensitive, which causes over-stimulation and episodes of continuous, repeated contraction of the muscles and muscle stiffness, weakness, and collapse.

HYPP is most commonly found in Quarter horses and related breeds, and signs usually start to appear at around 2–3 years old. The heavily muscled stallion named Impressive was the start of an “impressive” lineage of bloodlines that acquired this condition. Because large musculature is desired in the halter horse world, these lines were used commonly in breeding before this disease was discovered to have a genetic component. Roughly 5% of Quarter horses, and as high as 50% of all halter horses inherit one HYPP positive mutated gene.

Symptoms of HYPP in Horses

An HYPP attack can be scary, especially if you’ve never witnessed one. Symptoms include:

• Muscle stiffness

• Pain, reluctance to walk

• Excessive sweating

• Rapid breathing and heart rate

• Potentially loud or strained breathing if muscles in the upper airway are affected

• Possible collapse and paralysis, or death due to cardiac or respiratory arrest

The severity, duration, and frequency of an HYPP episode can vary from horse to horse but can last anywhere from a few minutes to several hours. HYPP can resemble other diseases such as exertional rhabdomyolysis (tying up) or colic, so an exam by your veterinarian is recommended right away if your horse is exhibiting any of the above signs.

Causes of HYPP in Horses

HYPP is an autosomal dominant inherited condition; this means that a horse only needs to inherit one copy of the mutated gene from either parent to develop the disease. Horses that inherit a copy from both dam and sire are more severely affected and may experience more frequent and severe attacks.

Stress and increased potassium intake are both common triggers for HYPP episodes. Other factors can include diet restriction, strenuous exercise, illness, and general anesthesia.

HYPP can lead to other conditions in horses, such as:

• Arthritis

• Exercise intolerance

• Colic

• Wounds or traumatic brain injury if collapse has occurred during an acute episode

How Veterinarians Diagnose HYPP in Horses

Diagnosing HYPP in horses is typically a multifactorial process.

If your horse has a known breeding line that contains HYPP positive members, or if they have shown signs of a possible HYPP attack, have your veterinarian examine your horse. They will first begin with a physical exam and some questions for you about known breeding lines, history, diet, and what an “episode” may have looked like.

If a veterinarian arrives during an acute episode when your horse is experiencing muscle stiffness or tremors, they will likely pull a blood sample to check the potassium levels to check for elevation.

Definitive diagnosis of HYPP includes genetic testing, typically done with a hair sample. A clump of mane or tail hair is pulled (being sure to include the roots) and sent to the laboratory. There is a multi-test panel at the Veterinary Genetics Laboratory at the University of California Davis for Quarter Horses and related breeds to check for the most common genetically inherited diseases.

Treatment of HYPP in Horses

During an acute attack, there are a few things you can do for your horse, especially if it is mild or short. If the episode is severe, involving collapse or respiratory distress, you should call your veterinarian right away. In the meantime, you can:

• Administer glucose by wiping it on the gums. This will stimulate insulin release into the bloodstream, which drives potassium back into the cells and help with the hyperstimulation and overcontraction of muscles.

• Put your horse in a quiet or dark stall to minimize stimulation and excitement. Stress can increase the chances or prolong the length of an HYPP episode, so it is important to keep your horse as calm as possible.

When your veterinarian arrives, they’ll perform a quick physical examination, and depending on how your horse is doing at that time, may administer IV fluids containing glucose and/or calcium gluconate. Calcium helps counteract elevated potassium levels by stabilizing “leaky” cell membranes.

Recovery and Management of HYPP in Horses

HYPP is a genetic and lifelong condition. Once a diagnosis has been made, there will be daily maintenance routines you’ll need to follow with your horse, including:

• Medication: your veterinarian may prescribe medication such as acetazolamide or hydrochlorothiazide for administration two or three times per day to prevent the onset of attacks in horses with moderate to severe cases of HYPP. These medications work through different actions, although acetazolamide has shown to stabilize potassium and blood glucose through the release of insulin.

• Diet: horses with HYPP require a diet low in potassium, and benefit from multiple (at least two or three) feedings per day. This means avoiding alfalfa and brome hay, sugar and beet molasses, and many types of oils. Feeds that are commonly used for HYPP horses include:

  • Timothy or bermuda grass hay

  • Grains such as wheat, oats, and barley

  • Beet pulp

• Daily exercise: Daily, routine exercise or regular turnout in a large paddock is preferred over stall rest and can be critical in preventing an episode that can happen with too little movement followed by strain.

Prevention of HYPP in Horses

While prevention of this condition isn’t possible, preventing attacks in an HYPP-positive horse is a lifelong management practice. Along with the above daily routines and diet that will be implemented into your horse’s care, muscle support supplements or vitamin E antioxidants can also be beneficial. If you’re going to offer electrolyte supplementation, read the ingredients carefully as many contain potassium; your veterinarian may recommend plain salt as an alternative.