Equine polysaccharide storage myopathy

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Equine polysaccharide storage myopathy (EPSM) is a rare genetic disease of horses which causes lameness.

EPSM is a metabolic condition related to skeletal muscle dysfunction. The primary issue is an inability to properly metabolize carbohydrates from feed. Attempting to make dietary changes and maintaining a schedule of regular exercise seems to be the most effective methods to improve these horses. This is a lifelong condition and altering the carbohydrate to fat ratio will be a permanent change.

Potentially any type of horse could develop this condition, with the highest incidence involving Warmbloods, Draft-related breeds, Quarter Horses and Arabians. It is thought to be an inherited disorder, but due to variability in its onset this may not seem clear.

Clinical signs

There are a variety of signs including weakness, poor performance, an inability to move forward, back soreness, gait abnormalities with or without a lameness, attitude problems, poor muscling, decreased impulsion and general stiffness.

Lameness may develop from a slight stiffness to an actual lameness that involve one or both hind limbs. From behind the gait has been described as a "goose waddle". One of the more obvious horses I saw was a 4 year old Draft gelding that walked out of the stall stiffly and would trot, but appeared stiff and lame in both hind limbs. There was not muscle loss, but they were not developing well and they seemed remarkably tight and sore for the limited work the horse was doing. The lameness and sometimes vague signs make it difficult to distinguish EPSM from other causes of lameness, including Shivers and Stringhalt plus different neurologic conditions[1].


Attempting to diagnose EPSM with laboratory work is a worthwhile pursuit, but it still may elude an accurate diagnosis. The primary blood test involves looking at levels of muscle enzymes before and after exercise. Muscle enzymes will increase with exercise and the higher the rise the more likely that muscle inflammation is occurring. Unfortunately not every horse will exhibit the type of elevation necessary to diagnose this from bloodwork. A second method is by doing a muscle biopsy, which allows a pathologist to make the diagnosis based on a microscopic examination of the muscle tissue. The procedure is straightforward and well tolerated by the horse. Since this is somewhat of an invasive procedure most clients opt to treat rather than to biopsy. In order to confirm the diagnosis and to get a sense of the degree that EPSM is involved in the unsoundness, biopsying is essential.

A new genetic test has become available that evaluates DNA from hair or blood to determine if the horse carries the gene for what is being called Type 1 EPSM. The Type 2 - the more common form is only diagnosed presently through a muscle biopsy.


Quarter Horses with PSSM have enhanced sensitivity to insulin, resulting in high muscle glycogen concentrations. When fed a starch meal, these horses store a higher proportion of the absorbed glucose in their muscles compared with healthy horses. Thus, the ideal diet for PSSM is based on feeding forage at a rate of 1.5-2% of body weight, providing >15% of digestible energy as fat and limiting starch to <10% of daily digestible energy by limiting grain or replacing it with a fat supplement. Caloric needs should be assessed first to prevent horses becoming obese on a high-fat diet. Improvement in signs of exertional rhabdomyolysis for horses with PSSM requires both dietary changes and gradual increases in the amount of daily exercise and turn-out[2].

The goals of dietary therapy are to minimize dietary carbohydrates and maximize fat intake by providing 20-25% of the dietary caloric requirements from fat. All grain, sweet feed, and molasses should be eliminated from the diet and replaced with high quality forages such as alfalfa hay or grass-alfalfa hay mix diets. Additional calories are generally necessary due to the strict exercise regimen needed to control clinical signs; therefore, fat is added to the diet to provide an alternative energy source. Fat supplements of vegetable oil, powdered animal fat, or corn oil can be used. The recommended 1 lb. of fat/1000 pound horse can be accomplished with 2 cups of oil mixed with alfalfa cubes for palatability. These recommendations must be modified depending on the individual caloric needs of the patient[3].

Exercise therapy consists of daily turnout and as little stall rest as possible. The horse should be longed for 15 minutes a day and, if no increases in CK are evident, the workload can be gradually increased. When the horse can be worked for 30 minutes without difficulty, active riding can be initiated.

EPSM horses respond most favorably to both diet and exercise change. Typically, after 3-6 months of therapy, post-exercise serum concentrations of CK are within reference limits. Improvements in muscle function are proposed to be the result of segmental necrosis of fiber segments containing unmetabolized carbohydrates[4].

The prognosis varies among horses, but dietary and exercise changes almost completely control signs in most EPSM affected animals. The exception occurs in horses of the draft breeds that have become acutely recumbent; 50% do not survive despite aggressive treatment. Currently, affected horses are retired to breeding. Since ESPM may be a heritable condition, this practice should be discouraged.


  1. McKenzie EC & Firshman AM (2009) Optimal diet of horses with chronic exertional myopathies. Vet Clin North Am Equine Pract 25(1):121-135
  2. Merck Veterinary Manual
  3. McCue, ME et al (2009) Comparative skeletal muscle histopathologic and ultrastructural features in two forms of polysaccharide storage myopathy in horses. Vet Pathol 46(6):1281-1291
  4. Borgia, LA et al (2010) Effect of dietary fats with odd or even numbers of carbon atoms on metabolic response and muscle damage with exercise in Quarter Horse-type horses with type 1 polysaccharide storage myopathy. Am J Vet Res 71(3):326-36