Most horse owners are familiar with the term “tying up,” which describes the pain and cramping of a horse’s muscles with exercise. Veterinarians refer to this condition as exertional rhabdomyolysis, and now recognize a specific type known as Polysaccharide Storage Myopathy, or PSSM.

PSSM is an inherited condition in stock-type breeds such as Quarter Horses, Paints, and Appaloosas and is due to a dysfunction in the way muscles store glycogen. Signs of the condition can occur after only a few minutes of exercise and include:

• Stiffness, reluctance to move
  
• Posturing to urinate, stretching-out
  
• Muscle twitches
  
• Pawing, rolling

Horses that “tie up” might also have firm and painful muscles, increased heart and respiratory rates, and sweat excessively. Exercise should be stopped and a veterinarian called immediately if any of these signs occur. Diagnosis is confirmed with bloodwork and muscle biopsy and the condition is managed through diet and exercise.

The best diet for a horse with PSSM contains less non-structural carbohydrates and more fat (note: nutritionists are moving away from the term NSC toward the more specific water soluble carbohydrates (WSC) and ethanol soluble carbohydrates (ESC)–for more on this see www.TheHorse.com/ViewArticle.aspx?ID=9380). Replace grain with a multi-vitamin/mineral supplement or ration balancer and the lowest NSC forage available. Make sure the horse is receiving adequate levels of Vitamin E/Se and electrolytes. Add up to one pound of fat per 1000 pounds of body weight per day.

Most importantly, PSSM horses should be turned out as much as possible and exercised daily, whether by riding, longeing, driving, or other activity.

Article by Lydia F. Gray, DVM, MA, Medical Director/Staff Veterinarian at SmartPak. Gray will be host GetSmart Q&A discussions at the SmartPak Store in Natick, MA, on 6/20, 7/24 and 8/22. Topics include Lameness Issues, Ulcers and Colic, and Metabolic Syndrome and Insulin Resistance. Coffee and desserts will be served. For more information see www.smartpakequine.com/store.aspx