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What is Polysaccharide Storage Myopathy?

Polysaccharide storage myopathy (PSSM) and equine polysaccharide myopathy (EPSM) are glycogen storage disease of horses that lead to exertional rhabdomyolysis, a condition in which exercise brings about muscle pain and cramping. The disease arises in horses of many different breeds. Generally called polysaccharide storage myopathy, or PSSM, the disease is called equine polysaccharide storage myopathy, or EPSM, when it occurs in draft horses. EPSM most commonly occurs in draft horses and Quarter Horses, but is also found in Warmbloods, Fjords, Friesians, Haflingers, Arabians and Icelandics. In Quarter Horses, EPSM has been proven to be an inherited disease, yet this has not yet been confirmed as such in all other breeds.  

At this time, there is not documented sex predilection to the disease. No matter the type, the mechanisms of the disease are complex. Glycogen is a polysaccharide molecule that is stored in animal cells along with water and then used as a source of energy. Glycogen is essential in animals as it provides energy for the benefit of skeletal muscles, as well as cardiac muscles.

Polysaccharide storage myopathy (PSSM or EPSM) is a glycogen storage disease of horses that leads to exertional rhabdomyolysis, a condition in which exercise brings about muscle pain and cramping.

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Symptoms of Polysaccharide Storage Myopathy in Horses

  • Poor performance 
  • Difficulty performing canter
  • Colic-like symptoms
  • Gait variation and abnormalities
  • Short strides
  • Depression
  • Stiffness
  • Sweating
  • Muscle cramping and spasms
  • Reluctance to move forward
  • Loss of muscle mass
  • Weakness 
  • “Shivers”
  • “Tying up”
  • Death


There are currently 2 types of the disease, known as Type 1 PSSM and Type 2 PSSM. Type 2 typically occurs in light horse breeds, while Type 1 occurs in heavy breeds.

Causes of Polysaccharide Storage Myopathy in Horses

Polysaccharide storage myopathy (PSSM) occurs when glycogen, the storage form of glucose, or glucose-6-phosphate, the form taken into cells, are present in muscles. The condition is clinically characterized differently from breed to breed, but frequently involves muscle pain, cramping, and cell damage during exercise. In worst case scenarios, the condition may progress to muscle atrophy, a condition defined by the partial or full wasting of the muscle leading to a decrease in muscle mass. Though PSSM and EPSM are lifelong diseases, the conditions can be well-managed through a careful regime of proper nutrition and exercise. To best control the frequency and severity of symptoms of PSSM, horses are often fed a diet low in sugar and starch, and high in fat. The hope is that the fat will be accessed as energy over glucose metabolism.

Diagnosis of Polysaccharide Storage Myopathy in Horses

Polysaccharide storage myopathy may present in a series of generalized symptoms that prohibit easy diagnosis; however, more severe cases may present with obvious signs of muscle weakness and exercise intolerance. Some of the hardier breeds will withhold signs of pain and weakness until the disease becomes moderate or severe. Often the disease is confused for other conditions, such as lyme disease.

A genetic test is available, but only in the case of Type 1 PSSM. Since it is a genetic test, it requires some sort of biological sample, typically blood or hair. The other way to obtain a certain diagnosis of Type 2 is through muscle biopsy. Muscle biopsy is taken from the hamstring muscles. Horses with Type 1 will likely have approximately 2 times the typical levels of glycogen in their skeletal muscle. The veterinarian will give a blood test in hope of tracking down increased serum levels of muscle enzyme.

Treatment of Polysaccharide Storage Myopathy in Horses

Horses with PSSM are treated through changes to the horse’s lifestyle, including diet and exercise. For pain and stiffness, medication may be given to make the horse more comfortable and willing to exercise.

Some horses may never return to peak performance, while others return, but at a very slow pace. Muscles take time to regenerate, and it is common for horses to still be recovering in a year’s time. Some horses may never recover a willingness to work or perform, likely due to lingering muscle pain, or a fear of pain.

Exercise is best introduced slowly over time. Veterinarians often recommend short, frequent bouts of maximum exercise rather than exercise followed by a few days of stall rest. Due to the rhabdomyolysis, horses need to avoid stiffening of the muscles.

Recovery of Polysaccharide Storage Myopathy in Horses

The primary way to maintain horses with PSSM at maximum health levels includes supplying a roughage-based diet of hay and pasture/greens, as well as a possible addition of supplements or replacement energy sources. 

It is suggested that horses with PSSM are kept on a low-starch diets, with the addition of fats over time. The fat may enable the extra fat to be accessed for energy. Oddly, those horses with the most advanced cases of PSSM begin to thrive the quickest and show the most drastic improvement after beginning the modified diet.