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Equine Polysaccharide Storage Myopathy (also known as EPSM or PSSM) is a heritable muscle disorder that is seen in Quarter Horses, Paints, Appaloosas, draft breeds, draft crosses, Warmbloods, and occasionally Thoroughbreds. It results from an increased sensitivity of skeletal muscle to insulin. This results in an increased uptake of glucose, increased production of glycogen in muscle, and production of abnormal polysaccharides in muscle cells.
Significant muscle pain and back soreness can be seen with this disease, but the most common presentation is poor performance with an inconsistent degree of stiffness and muscle contracture. The syndrome is often more serious and crippling in Belgians and Percherons. These horses sometimes have elevated muscle enzymes even at rest, and may have a significant rise in enzyme levels 4-6 hours after 15 minutes of trotting. However, a definitive diagnosis is best achieved with a muscle biopsy.
A muscle biopsy is a minor surgical procedure that can be done on the farm. It is done under standing sedation with local anesthesia which involves removing a 2cm cube of muscle; usually the Semimembranosus (hamstring) is used. The incision heals rapidly with stall confinement, and complication rates are very low. Sutures are removed in 12-14 days and activity can resume at that point.
Dietary management is the cornerstone of EPSM management. A high fat, low carbohydrate (low starch), high fiber diet is the key for successful management of affected horses. This means eliminating sweet feed, oats, corn, other grains, molasses, and wheat bran from the diet. Alfalfa tends to be low in carbohydrates, but grass hay is variable.
Fortunately many of the major feed companies have now formulated feeds that are high in fat and low in carbohydrate content, so it is no longer difficult to manage the diet. It may take a few weeks or several months for the dietary changes to take effect and for the muscle soreness to resolve. Dietary management should be combined with a regimented exercise plan that includes as much turnout as possible and frequent, regular exercise.
If your horse has shown chronic or recurring back soreness, muscle cramping, or stiffness, a muscle biopsy may be in order. However, a trial diet as described above may provide a presumptive diagnosis and a solution at the same time.
Do not hesitate to contact your Veterinarian if your horse is exhibiting muscle soreness, episodes of severe muscle cramping, or just poor performance. Your Veterinarian should be able to help sort through all of the conditions, including muscle disorders, which may be holding your horse back.
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