Polysaccharide Storage Myopathy

Stephanie Valberg, DVM, PhD, professor of large animal medicine and director of the University of Minnesota's Equine Center discussed PSSM at the AAEP Convention 2006. The disorder causes muscle pain, stiffness, and cramping, and some horses demonstrate exercise intolerance, poor performance, muscle weakness, atrophy, or related behavioral changes. This syndrome is caused by an inherited enzyme defect that allows accumulations of glycogen in the muscle.

Gene expression is markedly influenced by the horse's environment, particularly as related to diet and exercise. The most prevalent breeds affected include Quarter Horse-related breeds, drafts, and Warmbloods. Individuals within other breeds have been recognized with this syndrome.

The typical horse with PSSM is calm--even sedate--and well-muscled, but not very fit. Signs usually show up at the beginning of a training session. An affected horse is not necessarily being fed a high-grain diet, although high-carbohydrate and high-starch diets amplify signs in many instances. Many affected horses stop to posture as if to urinate; it is thought that this relieves cramping in the abdominal muscles. In draft breeds, it is typical to see muscle atrophy, abnormal gait, and high-stepping action in the rear limbs. Eventually there is progressive weakness and muscle atrophy. Warmblood horses often demonstrate signs of a sore back, sore muscles, reluctance to collect, and overall poor performance issues.

Although there is a genetic marker test potentially available in the near future, the most definitive way to diagnose this problem is with a muscle biopsy, which is sent to an appropriate lab where it is frozen for analysis. The results are graded as mild, moderate, or severe.

Not all horses with PSSM demonstrate clinical signs, nor do they all have elevations in muscle enzymes. It tends to be first recognized in Quarter Horses at four to five years of age and Warmbloods and draft horses aged eight to 11 years. After exercise, a normal horse will slowly absorb glucose from the bowel and slowly replenish metabolized glycogen to normal resting levels. In a PSSM horse following exercise, there is overcompensation, so glycogen is packed into the muscles, reaching twice- normal levels.

A PSSM horse that eats grain seems to produce less of an insulin response compared to a normal horse, yet that insulin has a greater impact to transport more glucose into the cells as free, non- phosphorylated (not yet activated into a useful high-energy compound) glucose. If the glucose is not immediately used, it is synthesized and stored as excess glycogen. These glycogen packets are visible under the microscope through staining of the biopsy sample. There seems to be some limitation on how well PSSM horses can use this excess glycogen for energy generation.

Once a diagnosis is obtained, treatment relies on management with strict diet and exercise protocols. These horses will always be at risk of some underlying muscle soreness, but at least 80% can improve with diet and exercise.

It is best to turn a PSSM horse out on pasture and resist the urge to confine the horse to a stall or paddock. Those PSSM horses confined to stalls or paddocks will experience CK elevations, particularly within the first five minutes of exercise. Those horses turned out to exercise at will and can walk around to forage do not experience CK elevations. Free exercise is key to keeping a PSSM horse comfortable over the long term. Exercise should be applied through careful conditioning programs that steadily increase demand and allow the horse to adapt to the demand.

Dietary modifications essential to managing PSSM include feeding a low-sugar, low-starch diet with fat to serve as an alternative energy source. This helps the muscle cells take up less glucose while providing free fatty acids for muscle use during aerobic exercise. Figure out how much energy each individual horse really needs. Small, frequent meals work best to eliminate generation of high blood glucose surges from a meal. Caution should be taken in feeding a high-fat diet to an already fat horse. These horses might need hay restricted to no more than 1% of body weight and also to be fed low-starch, fat-supplemented feed that is high in fiber, combined with daily exercise to get weight under control.

The worse the clinical signs experienced by the horse, the lower in starch (non-structural carbohydrates or NSC) the diet should be. Less than 10% of a horse's daily energy should be fed as sugar or starch, and fat should be added at 10-15%. Commercial feeds are available to provide low starch and high fat. The horse's diet might need to be tweaked to find one that is most palatable and low enough in starch, yet one that provides adequate fat.

One cannot solely change the diet to achieve results. Increased daily exercise is key, especially in Quarter Horses that have low oxidative enzymes, so they must exercise and train to efficiently burn fat. Regardless of breed, a horse must be in regular exercise--working gradually up to 30 minutes daily for five days a week. To achieve the best results possible, it is essential to follow dietary recommendations along with providing routine exercise and regular turnout. Results from diet and exercise modifications should be evident in as few as three weeks, or it might take as long as four months.

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse's AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads.

About the Author

Nancy S. Loving, DVM

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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