Feeding and Managing to Reduce Recurrent Exertional Rhabdomyolysis in Horses

A number of horses, predominantly fillies, have recurrent episodes of rhabdomyolysis (muscle cramping) with even light exercise. Recurrent exertional rhabdomyolysis (RER) is seen in many breeds of horses including Quarter Horses, Paints, Appaloosas, Thoroughbreds, Arabians, Standardbreds, and Morgans. A wide variety of causes for RER have been proposed including electrolyte imbalances, hormonal imbalances, lactic acidosis, and vitamin E and selenium deficiencies. Specific causes of RER have been identified in the horse including a disorder of muscle contractility or excitation contraction-coupling.

RER is a common occurrence in Arabian, Standardbred and Thoroughbred horses. It most frequently occurs in young fillies with a nervous disposition. About 5% of Thoroughbred racehorses develop RER during the racing season, often when they are trained at a gallop but held back from full racing speeds. Some highly susceptible individuals have repeated episodes resulting in persistent elevations in serum aspartate aminotransferase (AST), creatine kinase (CK), and poor performance. Many of these fillies are retired to broodmare careers. In other susceptible horses, episodes may be very intermittent, and as a result the term chronic intermittent rhabdomyolysis has been used to describe this syndrome. Stress and a period of stall rest preceding exercise appear to trigger RER in susceptible horses. Studies of equine lymphocyte antigens provide some support for a familial basis for RER in Standardbred horses. A diagnosis of chronic intermittent rhabdomyolysis is based on the history and clinical signs as well as documented elevations in serum AST and CK. Muscle biopsy findings in affected horses include varying stages of muscle necrosis and regeneration with centrally located myonuclei.

Lactic acidosis was previously believed to cause RER, and many treatments still used today (lactinase, DMG, sodium bicarbonate) are directed at resolving a lactic acidosis. Research has shown, however, that RER occurs most commonly with aerobic exercise, and that during an episode, affected horses have low muscle lactate concentrations and metabolic alkalosis. Research conducted in England suggested that a dietary deficiency of sodium or a low calcium:phosphorus ratio based on urine creatinine clearance ratios might contribute to RER. Subsequent studies showed that many of the Thoroughbred racehorses with chronic intermittent rhabdomyolysis had normal electrolyte ratios. Most recently, an abnormality in excitation-contraction coupling has been identified in Standardbreds and Thoroughbreds with RER. The altered relaxation of muscle following a contractile twitch in affected horses suggests that abnormal intracellular calcium regulation is the cause of RER. In addition, a recent study showed elevated myoplasmic calcium concentrations in horses with acute RER.

Prevention of further episodes of RER in susceptible horses should include standardized daily routines and an environment that minimizes stress. The diet should be adjusted to include a balanced vitamin and mineral supplement, high-quality hay, and a minimum of carbohydrates such as grain and sweet feed. Dietary fat supplements may help to maintain weight in nervous fillies without providing excessive carbohydrates. The use of low doses of acepromazine before exercise is believed to help some excitable horses. Daily exercise is essential, whether in the form of turnout, longeing, or riding. In the past, horses have been stall-rested for several weeks following an episode of RER. This is actually counterproductive and increases the likelihood that the horses will develop RER when put back into training. The initial muscle pain usually subsides within 24 hours of acute RER and daily turnout in a small paddock can be provided at this time. Subsequently, a gradual return to performance is recommended once serum CK is within normal range. Dantrolene (2mg/kg orally) given an hour before exercise is believed to be effective in preventing RER in some horses. Dantrolene is used to prevent malignant hyperthermia in humans and swine by decreasing the release of calcium into muscle. Phenytoin (1.4-2.7 mg/kg by mouth twice a day) has also been advocated as a treatment for horses with RER. Therapeutic levels vary, so oral doses are adjusted by monitoring serum levels to achieve 8 ug/ml and not exceed 12 ug/ml. Phenytoin acts on a number of ion channels within muscle and nerves including sodium and calcium channels. Unfortunately, long-term treatment with dantrolene or phenytoin is expensive.

Some horses with recurrent rhabdomyolysis have been found to have polysaccharide storage myopathy, or PSSM. This is a glycogen storage disorder characterized by the accumulation of an abnormal polysaccharide in the muscle. To date Quarter Horses, Paints, Appaloosas, drafts, draft crossbreds, warmbloods, and a few Thoroughbreds have been identified with PSSM. Horses with PSSM often have a calm and sedate demeanor. Most horses have a history of numerous episodes of tying up beginning with the commencement of training; however, mildly affected horses could have only one or two episodes per year. Exercise intolerance, muscle atrophy, renal failure, and respiratory distress are less common presenting complaints. Elevations in muscle enzymes are usually found if blood samples are obtained and muscle enzymes may remain elevated for long periods even when rested. The severity of episodes of rhabdomyolysis can range from mild stiffness to severe pain resembling colic. Several horses have been euthanized due to the severity of muscle damage. A diagnosis is based on examination of muscle biopsies or genetic testing. The distinctive features of these muscle biopsies are subsarcolemmal vacuoles, glycogen storage, and abnormal PAS positive inclusions in fast-twitch fibers. Muscle glycogen concentrations are often 1.5 to 4 times normal. Serum CK activities are often increased by 1,000 U/L or more 4 hours after 15 minutes of exercise at a trot.

Treatment of horses with PSSM is based on increasing the oxidative capacity of skeletal muscle through gradual training and providing a high fat diet. Most PSSM horses have competed successfully as pleasure and hunter horses when their diets are switched to good-quality grass hay, no grain or sweet feed, and a fat supplement. Rice bran, corn oil, or spray dried fat supplements can be used. Daily longeing or riding as well as pasture access are essential. Stall rest for more than 12 hours per day appears to increase the incidence of rhabdomyolysis.

A familial basis for this disorder has been identified in Quarter Horse-related breeds. A few young Quarter Horses (8 months to 2 years) have recently been identified with PSSM that developed moderate to severe rhabdomyolysis without any associated exercise. These young halter horses were on a high-grain diet, and serum CK normalized when switched to a lower carbohydrate ration.

Information in this article was taken from Advances in Equine Nutrition.