Polysaccharide storage myopathy (EPSM or EPPSM) is blamed for a wide variety of clinical symptoms in horses, from the classical tying-up picture to skin twitching, tail wringing, reluctance to back, gait abnormalities and other very nonspecific symptoms.
Because the disorder gets a lot of ”press,” it is frequently self-diagnosed by owners who then put their horses on high-fat diets, which are not without risks (e.g. obesity, laminitis).
Many owners of Quarter Horses hear so much about EPSM they think their animal is a case just waiting to happen.
A study was done by the University of Minnesota, where 164 clinically normal (no symptoms) Quarter Horses from five states were studied by blood samples and muscle biopsy. They were looking at the glycogen content of muscles, muscle enzymes in the blood and for the appearance of the diagnostic polysaccharide inclusions on muscle biopsies.
While the number of clinically abnormal horses with diagnostic biopsies was as high as 40% on a farm known to have an EPSM problem (likely for genetic reasons), they estimated that for the Quarter Horse population at large, the risk of having EPSM in a horse showing no symptoms was between 6 and 12%. They also found a strong correlation between higher muscle enzymes in the blood and biopsy results.
Bottom Line: If your Quarter Horse is showing hard to pinpoint symptoms or behavior issues, but has normal muscle enzymes, the odds of it being EPSM are relatively small. Before assuming it’s EPSM and making major diet changes, have your horse biopsied to make sure.