Hyperkalemic Periodic Paralysis is a muscular disorder traced to descendants of the QH stallion Impressive.
HYPP is caused by a mutation in a protein inside the “voltage gated sodium channel”. When a motor nerve fires, it triggers opening of sodium channels. This allows sodium to enter the normally 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 se. 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 add 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 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 approximately 1%. However, the maintenance requirement is actually only 25% of that. Because fresh grass is around 80% water compared to 10% in dried hays, grass is a very low potassium food. To avoid the diet aggravating (remember, not causing) HYPP, the horse should ideally be maintained on pasture 24/7 and year round. This allows for the most constant 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 base since soaked beet pulp will have its already low potassium diluted further by the absorbed water.
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. Hay that has been rained on is also good. If you can’t soak, get as mature a cutting of hay as you can find.
Feed no more than 1% of body weight in hay daily, spread out over the day, with the balance of calories coming from soaked beet pulp, grain and other low potassium feeds like flax or dried peas. Feed grain, with no or very limited molasses, before and after feeding hay so that the blood sugar rise can help drive potassium out of the blood and into the cells. Make sure calcium intake is adequate as this helps calm the sodium ion channels. Magnesium must also be adequate 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 eliminate attacks but it makes them milder and easier to control.
Eleanor Kellon, VMD