Until recently, the exact cause of tying up (exertional rhabdomyolysis) in horses was unclear, although many potential causes were considered including vitamin/mineral deficiency, electrolyte imbalance, and poor thyroid function. In humans, more than half of the cases of exertional rhabdomyolysis are caused by an error in the metabolism of muscle cells. These defects in metabolism are inherited and specific. It only makes sense that we would eventually identify a primary defect in muscle metabolism in horses with this clinical condition.
Polysaccharide storage myopathy (PSSM) has been identified to be the most common cause of tying up in Quarter Horses and Draft breeds (some specify the Draft horse disease EPSM). It is a primary muscle disorder characterized by the accumulation of unusable carbohydrate the muscle cells. The cause is a defect in muscle metabolism resulting in inappropriately rapid processing of carbohydrate material and accumulation of abnormal polysaccharide in muscle cells. Many breeds have been identified with PSSM including Quarter Horses, Paints, warmbloods, Appaloosas, and Draft horses. It is more common in females than males and horses with PSSM often have a calm, sedate demeanor. A familial linkage has been identified in Quarter Horse-related breeds and is suspected to be inherited as a recessive gene.
Poor exercise tolerance is the most common problem reported by owners of horses with PSSM. Severely affected horses have numerous tying-up episodes recorded from the beginning of their training, and mildly affected horses may have 1 episode per year or less. The severity of an episode can range from mild exercise intolerance to severe, stiffness and pain. During a mild tying up episode, horses will develop a tucked-up abdomen, muscle twitching in the flank region, and a camped-out stance after exercise. Painful back muscles and shortened stride length may be the only signs of disease.
Diagnosis of PSSM is determined by evaluation of muscle enzymes (at rest or after exercise) and an abnormal muscle biopsy. A 15-minute exercise test at a trot on a lunge line can detect PSSM. Approximately 80% of horses with PSSM will fail the exercise test, defined by an increase in muscle enzymes measured from a blood sample obtained 4 hours after exercise. Draft horses suffering from EPSM commonly have normal muscle enzyme levels, even after an exercise test.
A change in nutrition to a high-fat, low-carbohydrate diet is very effective in eliminating clinical signs of PSSM. Sweet feed (high in carbohydrate) should be eliminated from the diet. A high-fat diet is achieved by supplementing fat or providing a specialty ration designed to provide high-fat, low carbohydrate. Regardless of the source of fat, the diet should provide 20 to 25% of total daily calories as fat, which is 2.5 to 3 times the approximately 200 gm of fat provided in typical equine diets. Rice bran, corn oil, or spray dried fat can be used to supplement fat in the diet. Corn oil can be fed at 480 ml/day mixed with soaked alfalfa cubes. Introduction of corn oil should be gradual, beginning with 60 ml/day and addition 60 ml every 2 to 3 days until 480 mls is attained. Sugars and starches (nonstructural carbohydrates) should constitute less than 15% of the total daily calories. Corn consists of 71% nonstructural carbohydrates and should be avoided. Most commercial feed labels do not provide information regarding nonstructural carbohydrate content. Concentrate feeds with less than 33% nonstructural carbohydrates are effective diets when combined with good-quality grass hay or pasture roughage. Sweet feeds contain approximately 47% nonstructural carbohydrate, whereas commercial high fat diets contain 28 to 39%, and alfalfa pellets contain approximately 2%. A high-protein diet (12 to 17%) is recommended to help build muscle. Vitamin E and selenium supplementation are recommended. This diet successfully prevents post-exercise elevation in muscle enzymes and clinical episodes of tying up within 3 to 6 months. Improvement in attitude, stride, energy, exercise tolerance, and muscling are also noted by most owners.
Stall rest for more than 12 hours per day increases the incidence of tying up. Dietary changes without alteration in training are insufficient to prevent episodes. Daily lunging or riding and pasture access are essential to successful management. A 15 minute exercise test is used to determine the amount of exercise necessary in the initial training stages. If the horse does not pass, 2 weeks of pasture turn-out with dietary change are recommended. If horses successfully pass the exercise test, 15 minutes of lunging per day is recommended. Exercise should be increased by a few minutes every few days. Once horses are capable of trotting for 30 minutes on the lunge without difficulty, work under saddle can begin.