Hyperkalemic periodic paralysis (equine) facts for kids

Hyperkalemic periodic paralysis (HYPP), also called Impressive syndrome, is a genetic disorder that affects horses. It's named after a famous horse called Impressive, who was the first known case. This inherited condition affects how muscle cells work and how a horse's body controls potassium levels in the blood.

Horses with HYPP can have muscle problems. They might experience uncontrolled shaking or weakness. Things like high potassium in their diet, heat, or cold can make these problems worse. Sometimes, it can even lead to paralysis.

This problem happens because of a small change, called a mutation, in a specific gene. This gene helps create "sodium channels" in muscle cells. These channels are like tiny gates that control what goes in and out of muscle cells. In horses with HYPP, these gates don't close properly, especially when there's too much potassium.

HYPP is quite common in Quarter Horses, affecting about 1 in 50. All these cases can be traced back to that one famous horse, Impressive.

What are the signs of HYPP in horses?

This inherited disease causes horses to have sudden attacks. During an attack, you might see violent muscle twitching or a lot of muscle weakness. Sometimes, it can even lead to paralysis.

HYPP is a dominant genetic disorder. This means if a horse has just one copy of the changed gene, it can show symptoms and pass the disease on. If a horse with HYPP breeds with a normal horse, there's a 50% chance their foal will also have the condition.

There's no cure for HYPP, but treatments can help reduce the signs. How much treatment helps can be different for each horse. Horses often lose muscle control during an attack.

Some horses are more affected than others. Even in the same horse, some attacks can be worse than others. Here are some signs of an HYPP attack:

• Muscle trembling or shaking.
• The third eyelid flickering or covering more of the eye than usual.
• General weakness all over the body.
• Weakness in the back legs, making the horse look like it's "dog-sitting."
• Complete collapse to the ground.
• An unusual whinny because the voicebox muscles are affected.
• In very serious attacks, the horse might even die if the breathing muscles become paralyzed.

HYPP attacks can happen at any time. A horse standing quietly in a stable can have an attack, just like a horse during exercise. After an attack, the horse usually looks normal and isn't in pain. This helps tell it apart from other muscle problems like "tying up" (Equine Exertional Rhabdomyolysis). Tying up usually happens after exercise and causes muscle damage and pain.

Horses with HYPP are fully awake and aware during an attack. They are not like horses having a seizure. However, they can suffocate if their breathing muscles become paralyzed. If a horse collapses, it will try hard to get back up. This can be dangerous for anyone handling or riding the horse.

How is HYPP inherited?

In 1994, scientists at the University of Pittsburgh found the gene change that causes HYPP. They also created a blood test for it. This test helps identify horses as:

• H/H: These horses have two copies of the changed gene. They will always pass the disease on to their offspring.
• N/H: These horses have one copy of the changed gene. They are affected by the disease, but usually less severely. They pass the disease on about half the time.
• N/N: These horses do not have the changed gene. They cannot get HYPP or pass it on, even if they are related to Impressive.

The famous horse Impressive had muscles that were always contracting a little. This made him look very muscular, which was popular in horse shows at the time.

How HYPP spread and where it's found

The disease is linked to the famous American Quarter Horse stallion Impressive. As of 2003, he had over 55,000 living descendants. HYPP is mostly found in Quarter Horses and related breeds like American Paint Horses and Appaloosas. However, because of cross-breeding, it's now starting to appear in other types of horses too.

For a long time, horses with HYPP were often favored in halter competitions. This is because a side effect of the condition is heavy, bulky muscles, which judges liked. This trend started with Impressive, even before people fully understood the disease. This helped the disease spread. Some horse breeds with Quarter Horse bloodlines still haven't put rules in place to stop the spread of HYPP.

Rules to control HYPP

Some horse organizations have made rules to try and stop this disease from spreading.

• The American Quarter Horse Association (AQHA) now requires testing for foals related to Impressive if both parents weren't completely free of the gene (N/N). Since 2007, they no longer register foals that have two copies of the gene (H/H).
• Since 2007, the Appaloosa Horse Club (ApHC) has also required foals related to Impressive to be tested. The results are then noted on the foal's certificate.
• The American Paint Horse Association (APHA) decided that after 2017, all stallions must be tested for HYPP. This helps mare owners make smart choices when picking a stallion for breeding.