We diagnosed the Impressive syndrome in the early 1980's but could not genetically test for it, nor did we know to treat the syndrome. Despite the HYPP concerns, Impressive and his offspring made substantial positive contributions to the Quarter Horse breed. And, in fairness, all breeds can have undesirable genetics. With line breeding, we can select for desirable traits and unknowingly concentrate defective genes. HYPP can show up any time.
Many horses carry the gene, but are normal.
To date, there is no method to predict the incidence of clinical episodes. For some reason still not understood, potassium leaks out of the muscle cell and sodium moves into the cell. One proposed therapy is to reduce dietary potassium. By far, the greatest amount of potassium ingested by all horses is in forage. Alfalfa hay is high in potassium, and horses suffering this problem should avoid it.