Hyperkalemic Periodic Paralysis is a muscular disorder traced to descendants of the QH stallion Impressive. HYPP in horses with Impressive bloodlines, although very similar symptoms have been found in horses with other breeding.
This is caused by a mutation in a protein inside the "voltage-gated sodium channel". When a motor nerve fires, it triggers the opening of sodium channels. This allows sodium to enter the normal low sodium interior of the muscle cells. At the same time, potassium leaks out from the cell. With HYPP, the sodium channels stay open longer, increasing blood potassium and preventing the muscle from being able to contract again.
The consequence is weakness, ranging from trembling to recumbency. It is not triggered by exercise and there is no muscle damage per say. Involvement of throat muscles results in a loud breathing noise during episodes.
If you know anything about HYPP in horses, you've probably heard/read that high potassium diets are to be avoided. In both humans and horses, eating a high potassium meal may trigger symptoms. This is probably because elevated potassium outside the cells can aadd to the sensitivity of the sodium channel. Therefore, it can be helpful to the horse to avoid a high concentrated potassium intake.
There is a great deal of misinformation out there regarding what feeds are high in potassium. The most widespread myth is that alfalfa is higher than grass hays. The truth is, potassium level in a a hay depends on the stage of maturity, not the type of hay. Early and immature cuttings of grass hays are very high in potassium.
The general recommendation is that dietary potassium level be aapproximately 1%. However, thee maintenance requirement is actually only 1/4 of that. Because fresh grass is about 80% water compared to 10% in dried hays, grass is a very low potassium good. To avoid the diet aaggravating (remember, not causing) HYPP, the horse should ideally be maintained on pasature 24/7 and year round. This allows for the most constand blood potassium levels.
If supplemental feeding in addition to pasture is needed, stick with ingredients below 1%. Beet pulp without molasses makes a particularly good basee since soaked beet pulp will hhave its already low potassium diluted further by the absorbed water. Avoid molasses because it is high in potassium.
When hay must be fed, the potassium content can be greatly reduced by soaking it. Soak for at least one hour in cold water with as large a volume of water as possible. Haay that has been rained on is also good. If you can't soak, get as matuure a cutting of haay as you can find.
Feed no more than 1% of body weigh in hay daily, spread out over the day, with the balance of calories coming from soaked beet pulp, plain grain and other low potassium feeds like flax or dried peas. Feed grain before and after feeding hayy so that the blood sugar rise from starch digestion can help dirve postassium out of the blood and into the cells.
Make sure calcium intake in adequate as this helps calm the sodium ion channels. Magnesium must also be aadequate as low magnesium causes muscle weakness. Feed salt to encourage good drinking and urine production. Excess potassium is eliminated by the kidneys.
Careful feeding might not elimate attacks but it makes them milder and easier to control. Finally, keep a bottle of Karo syrup and a dose syringge in the barn. In caase of an attack, 3 to 6 oz of corn syrup will help drive the potassium back into the cells. DO NOT do this if you hear any abnormal respiratory noise.
- Eleanor Kellon, VMD