Hyperkalemic periodic paralysis was given its name because the veterinarians initially noted that during attacks, horses had a very high level of potassium in their blood (hyperkalemia). It seemed that horses affected with HYPP had an intrinsic, but intermittent inability to regulate potassium (so it got the name periodic), which sometimes left the horse unable to rise (so it got the name paralysis). Tying up syndrome
Sodium is found in very high levels outside of cells, in the blood, ordinarily, whereas potassium is found in very high levels inside cells. The uneven distribution of the electrolytes, sodium and potassium, allow an electrical voltage to be built up, and without this voltage, muscles would be unable to contract, and nerves would be unable to transmit vital signals throughout the body. In the normal animal, the movement of these electrolytes is highly regulated, and, indeed, the movement of sodium is important in triggering the movement of potassium in and out of cells. If sodium and potassium levels are disturbed, the body suffers severe consequences, ranging from abnormal muscle and nerve function to death.
Small disturbances in blood potassium, that would easily be handled by normal horses, trigger the abnormal sodium channels in horses with HYPP. When the sodium channels open, and start to leak sodium, potassium is triggered to leak into the blood stream. Soon, this vicious cycle leads to the muscle being first hyperexciteable, that is, it contracts far more easily than normal muscle. Then, as time goes on, the muscle becomes flaccid.
As with any genetic disease, HYPP has different degrees of what is termed penetrance, which is the extent to which a genetic trait can be seen in a particular individual. Some horses will be severely affected, and this will be exacerbated if they are homozygous for the trait, and others may look normal for the majority of their lives. Signs that you may see during an attack of HYPP are:
When a horse is having an attack of HYPP, he usually remains very alert – this helps to distinguish HYPP from a seizure, in which the horse is not aware of who or where he is. A horse having an HYPP attack will have normal muscle enzymes. HYPP attacks can also occur at any time – even when a horse is standing quietly in his stall. After an HYPP attack is over, the horse appears normal and does not show signs of stiffness or muscle pain.
This helps to distinguish HYPP from tying up syndrome, which is characterized by painful, stiff muscles that may last for days to weeks after an episode. The horse often sweats profusely, and appears to be painful. Tying up syndrome is also accompanied by high muscle enzymes, such as CPK, because muscle cells are actually breaking down, and is usually precipitated by exercise.
Heart abnormalities severe enough to cause collapse are usually accompanied by other signs of cardiac failure, such as weakness, abnormal jugular pulses, or peripheral edema – even when an attack is not occurring. These horses will also often have murmurs or cardiac arrhythmias – this is not seen with HYPP unless the horse has more than one problem.
When HYPP was first seen in horses, it was often confused with: