Polysaccharide Storage Myopathy

Polysaccharide Storage Myopathy is a disease that occurs when excess glycogen (storage form of glucose) or glucose-6-phosphate (form of glucose taken into cells) is present in muscles.

Often referred to as PSSM, this disease is called equine polysaccharide storage myopathy, or EPSM, when it occurs in draft horses.

A clinical diagnosis of PSSM can be made when a horse that suffers from chronic exertional rhabdomyolys (ER) is found to have amylase-resistant polysaccharide in his muscles. Amylase is the enzyme that breaks starches down to sugars. ER is a syndrome of muscle pain and cramping that is associated with exercise.

However, the build-up of amylase-resistant polysaccharide may take quite some time to build up before it can be detected. In one study, Quarter Horse foals that showed symptoms of chronic ER did not have the amylase-resistant polysaccharide apparent in their muscles until they were two years old!

The good news about PSSM is that its symptoms can be controlled by diet to an extent.

Signs and Symptoms

Most of the signs and symptoms of PSSM have to do with the muscles, which makes sense, as the disease affects the muscles.

Common signs and symptoms include:

  • Exertional Rhabdomyolysis
  • Exercise Intolerance
  • Muscle Stiffness
  • Shifting Lamenss
  • Back Pain
  • Camped-out stance - standing with legs behind or in front of body instead of directly under
  • Muscle atrophy
  • Colic-like symptoms

In draft horses and other related breeds such as Warmbloods, the symptoms of EPSM appear as muscle soreness in the back and hindquarters, stiffness, and sometimes ER.

In Belgains, EPSM may occur in conjunction with a disease known as "shivers", which shows up as muscle tremors and over-extension of the back legs.

Controlling Symptoms with Diet

To control the symptoms of PSSM with diet, horses must be fed a diet that is low in starch and sugar and high in fat.

This type of diet has been shown to reduce the frequency and severity of symptoms.

To be helpful, the total diet must contain less than 8% of the digestible energy from starch and sugar. In addition, it must contain more than 10% of digestible energy from fat.

Horses with PSSM must also be exercised regularly, as lack of exercise increases the frequency and severity of symptoms. In fact, even if the diet is changed, there will usually not be a reduction in symptoms unless the horse also receives regular exercise and turnout.

The exact recommendations for a PSSM diet are:

  • Feed a minimum of 1.5% body weight of forage per day
  • Remove all concentrates containing grain or molasses from diet
  • Use alternative energy sources if digestible energy needs are more what is provided by forage alone.

It is suggested that the forage provided be grass or oat hay, as these hays are lower in starch and sugar than legume hays.

Alternative energy sources include vegetable oil, rice bran, non-molassesed beet pulp, and black oil sunflower seeds.

Some horses may need a fat content higher than 10% of the diet, so you may have to do some experimentation with your individual horse to find the correct balance of starch and sugar content, fat content, and exercise.


Polysaccharide Storage Myopathy can be a scary disease until it is properly managed. Knowledge of the disease allows you to manage it through diet and exercise.

By following the recommendations of:

  • Adequate forage intake
  • No grains or molasses
  • Use of alternative energy sources

as well as ensuring adequate daily exercise you will be able to successfully manage almost every case of PSSM or EPSM.

Once PSSM is managed through diet and exercise, it becomes much less of a problem for both the horse and his owner.


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