Exertional rhabdomyolysis and polysaccharide storage myopathy in horses

Polysaccharide storage myopathy (PSSM) is one of many causes of exertional rhabdomyolysis in horses. PSSM is a glycogen storage disorder in Quarter Horse-related breeds, Warm-bloods, and draft horses that exhibit clinical signs of exertional rhabdomyolysis. It is not currently known whether the biochemical basis for the disorder is the same in all affected breeds. The clinical signs include exercise intolerance, muscle stiffness, stretching out, reluctance to move, and sometimes severe pain similar to that associated with colic. Serum aspartate transaminase and creatine kinase are often persistently elevated in association with clinical signs. On the basis of clinical, histologic, and biochemical features, PSSM can be classified as a glycogenosis; the precise nature of the metabolic defect has not yet been identified. A diagnosis is made via muscle biopsy and identification of polysaccharide inclusions that are periodic acid-Schiff positive in muscle fibers. Treatment involves changing the diet to high-quality hay with no grain and adding a fat supplement, such as rice bran. In addition, the most important factors in preventing rhabdomyolysis apparently include pasture turn-out and daily exercise. This article considers the clinical signs, occurrence, pathophysiology, diagnosis, treatment, prevention, and prognosis associated with PSSM as a cause of exertional rhabdomyolysis in horses.