Fit to be Tied: Part Three

In Parts 1 and 2 of this series, the muscular condition tying-up has been described thoroughly. Sporadic and chronic tying-up were discussed in Part 1, and exertional rhabdomyolysis (RER) was examined in Part 2. In this final part of the series, polysaccharide storage myopathy (PSSM) is explored.

PSSM is a glycogen (muscle sugar) storage disorder that is characterized by the accumulation of an abnormal polysaccharide in muscle. Two forms appear to exist, type 1 and type 2. The genetic cause for PSSM1 has now been identified but other forms of PSSM exist that are not due to this mutation.Horses with type 1 PSSM are able to quickly clear sugar from their blood and store 1.5-4 times the normal amount of muscle glycogen. An abnormal polysaccharide forms in muscle fibers because of increased activity of glycogen synthase the primary enzyme involved in making glycogen.  The genetic mutation responsible for PSSM is found in the gene (GYS1) encoding glycogen synthase. The mutation causes tying-up by disrupting regulation of energy metabolism in muscle during exercise.

Type 1 PSSM affects at least 20 breeds including Quarter Horse-related bloodlines, Belgians, Percherons, Morgans, Mustangs, Tennessee Walkers, and some warmblood breeds. It is found in 36-50% of Belgians and Percherons but is rare in Clydesdales and Shires. The prevalence is 8% in Quarter Horse-related breeds but is higher in halter horse (26%). Signs usually begin by 2 to 3 years of age but may occur in weanlings. Clinical signs seen include firm painful muscles, stiffness, skin twitching, sweating, weakness, and reluctance to move with light exercise. Sometimes gait abnormalities, mild colic, and muscle wasting are noted. Serum CK and AST levels are elevated except in draughts.

The genetic mutation in theGYS1 gene is inherited in an autosomal dominant fashion and genetic testing on mane or tail hair roots, or unclotted blood samples at the Neuromuscular Laboratory at the University of Minnesota is available. Muscle biopsy samples can be used for diagnosis as well.

Type 2 PSSM affects Quarter Horse-related breeds, warmbloods, some Thoroughbreds, some  Arabians, and other light breeds. These horses do not have the same genetic mutation as type 1 PSSM and research is currently focused on finding the genetic basis for this disease in Quarter Horses. Presently the only way to diagnose type 2 PSSM is by muscle biopsy. Horses with type 2 PSSM respond to similar management strategies as type 1 PSSM.

Horses with PSSM are different from horses with RER in that they have a calm, instead of a nervous, demeanor. Horses with PSSM typically have a history of tying-up problems associated with the onset of training, while the animal is still relatively unfit.

Treatment of horses with PSSM following a tying-up episode involves many of the same veterinary procedures as with RER or sporadic tying-up. Treatment protocols also attempt to minimize the occurrence of future episodes through management of exercise and dietary manipulation. For chronic cases, prolonged rest after an episode appears to be counterproductive and predisposes horses to further episodes of tying-up. Providing daily turnout with compatible companions can be very beneficial, as it enhances energy consumption in PSSM horses. But lush, high-sugar pasture must be avoided and grazing muzzles may be of benefit to PSSM horses for periods when grass is particularly lush.

Reintroduction of exercise to PSSM horses needs to be more gradual than approaches used for RER. Important principles include providing adequate time for adaptation to a new diet prior to commencing exercise and recognizing that it’s important not to overdo the duration of exercise each day. Exercise needs to be reintroduced gradually with some form of exercise daily. Confinement in box stalls for more than 12 hours per day appears to increase the incidence of clinical signs so should be avoided.

It is important to completely eliminate grain or sweet feed from the diet of horses with PSSM. For PSSM horses, the challenge can be to provide adequate fat for energy metabolism while preventing excessive weight gain. If horses are overweight, reducing caloric intake by using a grazing muzzle, using low- to medium-energy hay, restricting hay to 1.5% of body weight, and providing a vitamin/mineral ration balancer are recommended. Adding excessive calories in the form of fat to an obese horse is inadvisable. Until horses are of normal weight, fat metabolism can be enhanced by riding horses after a 5-8 hour fast. Once a horse has achieved the desired body weight, low-starch and low-sugar feeds combined with dietary fat can be introduced. The carbohydrate content of the diet of PSSM horses needs to be managed more stringently than for RER. Owners report that this type of diet improves clinical signs of muscle pain, stiffness, and exercise tolerance in draft horses, warmbloods, Quarter Horses, and horses of other breeds with PSSM when combined with the desired exercise program.

In Quarter Horses with PSSM,  a diet using KERx Releve providing less than 10% of daily digestible energy as dietary starch and 13% of daily digestible energy as dietary fat during a six-week trial resulted in normal serum CK activity four hours post exercise. The beneficial effect of the feed used in this study was believed to be the result of less glucose uptake into muscle cells and provision of more plasma free fatty acids for use in muscle fibers during aerobic exercise. Quarter Horses naturally have very little lipid stored within muscle fibers and provision of free fatty acids may overcome the disruption in energy metabolism that appears to occur during aerobic exercise. Recent research by KER and the University of Minnesota has shown that horses prone to PSSM should be fed  low-sugar hay and an NSC content of 12% or less is recommended. This can be established by lab testing of hay samples. Hay may also be washed in hot water to try and reduce sugar content but research has shown that this is not always effective.

The addition of fat alone is not beneficial and an exercise program must be instituted for PSSM horses to show improvement. In our experience, there is a great deal of variation in individual needs for added fat and this should be balanced with the horse’s weight and body condition. The combination of moderate-quality grass hay, a vitamin/mineral supplement, and judicious use of a fat supplement (rice bran and/or vegetable oil) allows many horses with PSSM to work successfully in pleasure activities. Daily activity along with pasture turnout is essential in minimizing the occurrence of PSSM tying-up episodes.

Conclusion

Managing muscle disorders in horses involves the identification of the specific cause of tying-up, selective use of rest, medications, diet regimes, rehabilitation therapy, and gradual reintroduction of exercise up to the intended performance. With careful attention to feeding and exercise programs, many horses with this condition can be managed to allow an active and useful life.

Many advances have been made over the last 30 years in identifying specific causes for tying-up, developing new diagnostic tests, and researching specific diet and exercise programs to decrease recurrence of muscle pain. In the future it is likely that there will be identification of new forms of tying-up, development of more genetic tests to diagnose specific causes, and continued development of dietary and exercise management tailored to individual cases.