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is characterised by painful muscle cramping when the horse is exercising, it can range from a reluctance to move to an acute episode with the horse completely seized up.

Equine Polysaccharide Storage Myopathy (EPSM) is a genetically programmed cause of tying up (Exertional rhabdomyolysis). The muscle pigment myoglobin causes red brown discolouration of the urine from muscle breakdown. Tying up is a symptom rather than a specific disorder. There are two categories with tying up, horses that have sporadic/isolated episodes and those that have repeated episodes. Isolated episodes are more likely to be due to management/dietary factors. Common causes include electrolyte imbalances to plain overworking of a horse to heat stroke. EPSM on the other hand is a genetic cause.

EPSM was first described by Dr. Beth Valentine (Oregon State University) to describe the condition in Draft horses and Polysaccharide Storage Myopathy (PSSM) was coined by Dr. Stephanie Valberg (University of Minnesota) when she first described the condition in Quarter Horses. Since then EPSM or PSSM have been found to occur in other breeds. One study documented PSSM in Warmbloods, Anglo Arabs and Andalusians.




Horses with EPSM or PSSM have abnormal glycogen in their muscles. Glycogen is the storage form of glucose. In one form, the glycogen is resistant to breakdown. This is the ‘classical’ form of EPSM/PSSM. In another form of the disease, the glycogen is normal but found in abnormal locations. Drs. McCue and Valberg of Minnesota recently found a genetic mutation in horses with the classical form. This causes them to produce glycogen at a faster than normal rate. Since the enzyme systems in muscle cells prevent them from burning and forming glycogen at the same time, they likely have less glucose available for energy. It is still unknown what the root of the problem is in the other form of EPSM/PSSM, but we know that Quarter Horses with PSSM and all Draft horses have high uptake of glucose into their muscle cells, or ‘glucose hunger’. Bottom line is that there are many possible causes, end result is an energy shortage in the muscle cell that prevents it from relaxing.

Over time it is understandable that the terms EPSM and PSSM have become interchangeable but there are notable differences depending on breed, the studies just mentioned show Quarter Horses with PSSM are very insulin sensitive compared to other Quarter Horses whereas in Draft horses, both effected EPSM horses and non EPSM have the same high insulin sensitivity. The proportions of the types of muscle fibres matters. Draft horses have a higher percentage of type 2A muscle fibres and Quarter Horses have a higher concentration of type 2B muscle fibres. Both of these fibre types are red, ‘fast twitch’ fibres but the 2A fibres of Drafts are geared to be able to sustain a contraction for a longer period of time while 2B fibres are geared for speed. This gives the draft horse the ability to haul heavy loads and the Quarter Horse fast speed over short distances.

Fat can only be burned aerobically, meaning in presence of oxygen by mitochondria, the ‘powerhouses’ or energy factories of cells. No matter how much fat is available, the 2B ‘fatigable’ muscle fibres are limited in how much they can use due to the low density of mitochondria, the preferred fuel is glucose from glycogen. When glycogen runs low, the 2B muscle fibres are in big trouble, for this reason PSSM Quarter Horses are always associated with muscle damage, tying up and elevated muscle enzymes. ‘Fatigue resistant’ 2A muscle fibres have a higher capacity for aerobic metabolism than the mainly anaerobic 2B muscles. There is a higher density of mitochondria in 2A muscle fibres, the greater the number of mitochondria, the greater ability to burn fat. Draft horses are more likely to show symptoms of weakness and muscle loss and may do so without marked elevations of muscle enzymes or myoglobin, meaning it’s not necessarily a tying up issue.

Diagnosis and symptoms

The symptoms of acute tying up can be very dramatic but acute back pain from poor saddle fit, colic and pedal bone fractures can present with similar symptoms. Horses can have darkish coloured urine which looks likes it’s stained by myoglobin but are simply dehydrated. Symptoms such as tripping can be caused poorly balanced hooves with toes too long. There is a concern that it’s being over diagnosed.

Horses may display a number of the following signs:

  • Tying up


  • Reluctance to ‘move out’ or abnormal gaits


  • Stiff or hard muscles


  • Muscle twitching or sensitivity


  • Symptoms of ‘shivers’ or muscle quivering with abnormal hind leg action


  • Unhappy attitude


  • Kicking at nonexistent flies (muscle cramps)


  • Resistance to holding up the hind feet


  • Excessive sweating (muscle pain)


  • Excessive tail swishing (muscle pain)


  • Tires easily


  • Subtle lameness episodes


  • Saddle issues (sore back)


  • Tripping


  • A a preference for rubbing or rolling a lot


  • Looking at belly or flanks as if colicky (muscle pain)


  • Weakness or muscle loss


  • Difficulty rising


  • Bucking


Muscle enzyme elevations can be measured but in Draft horses and similar breeds, there may not be any. Previously, the only diagnostic tool was a muscle biopsy to see if there is increased glycogen storage and to look at the structure of glycogen. Horses with PSSM have lower than normal numbers of mitochondria. Since mid 2008, a genetic test has been developed by Geneticist Dr. Molly McCue at the University of Minnesota, in conjunction with Dr. James Mickelson and Dr. Stephanie Valberg.

In Drafts, 87% of PPSM/EPSM cases have a genetic mutation for an enzyme involved in the manufacturing of glycogen. The same mutation is found in 72% of Quarter Horses. Quarter Horses and related Quarter Horse breeds may have a second mutation called the modifying gene, a positive for both mutations will have more severe signs of the disease.


All breeds of horse with PSSM benefit from regular exercise to improve their capacity to burn fuels with oxygen and a high fibre, grain-restricted or grain-free diet which will limit the amount of glycogen in the muscle cells. Regular exercise can increase the number of mitochondria and can even cause some muscle fibre types to switch over to aerobic capacity. The greater the number of mitochondria, the more fat can be used as a fuel.

Many horses diagnosed with EPSM/PSSM have been helped by changing their diet to include at least 15% fat. Supplemental fat is likely to be of more benefit to Draft than Quarter Horses because of their higher percentage of muscle fibre types that can efficiently burn fat.

The high fat feeding proponents say the fat gives the muscle an alternate energy source, and high fat feeding ‘trains’ muscle to rely more highly on fat. This may be so but it may be slowing down carbohydrate metabolism.

Horses evolved on high fermentable fibre derived from plant vegetation with less than 6% of the diet as fat. Digestion of fibre starts in the upper portion of the stomach where bacteria begin the job of fermenting sugars and starches into products that can be burned directly in the mitochondria to produce energy. The efficiency of fat digestion in horses is amazingly high, about 80 to 90%+. Since the level of fat is so low in vegetation perhaps that’s why horses need to be so efficient with absorbing fat for the essential omega-3 and omega-6 fatty acids in plants.

Feeding high levels of fat can come at a cost. One 2002 study found a 15.8% fat diet depressed fibre fermentation by almost 15% but the main problem for horse owners is insulin resistance(IR), especially for horses known to be ‘good doers’. Horses can develop insulin resistance and be more susceptible to laminitis (founder). Insulin resistance affects blood sugar regulation and is similar to type 2 diabetes mellitus in people. One of the triggers can be a high fat diet; fat has been used by researchers to induce a state of insulin resistance in horses. One 2005 study by Dr Nicholas Frank and others used the equivalent of just under a cup of rice bran oil and induced insulin resistance in Thoroughbreds.

Dr Eleanor Kellon VMD, an equine nutrition specialist recommends a different approach “I’ve lost track of how many horses suspected to have PSSM/EPSM and put on high fat diets end up with me because they get insulin resistance and laminitic. We don’t know the safe dose of fat for an insulin resistant horse, but we sure don’t want to be feeding as much fat as is already known to induce insulin resistance in a normal horse!”

Her approach is to limit carbohydrates and supplement with L-carnitine to assist in getting fat from body stores into the mitochondria, in conjunction with regular exercise and a mineral balanced diet. L-carnitine is a type of amino acid, necessary for moving long chain fatty acids into the mitochondria to be burned. Studies have shown that availability of free carnitine in the muscle cell influences the relative rate of burning of fat or carbohydrate. Even normal horses in training benefit from L-carnitine supplementation, showing higher metabolic efficiency during exercise. With L-carnitine supplementation, it’s been possible to greatly reduce or eliminate high fat feeding, with better results. Inside the body, L-carnitine exists either in the free, L-carnitine, form or as acetyl-L-carnitine. More recently, Dr. Kellon has been using the acetyl-L-carnitine form (ALCar) because a laboratory study has found acetyl-L-carnitine can turn on a pivotal enzyme which directs glucose into the mitochondria to be burned rather than into pathways that produce glycogen.

Dr. Kellon says, “Until we know more details of the biochemistry of EPSM/PSSM, it’s important to treat each case as an individual. How a horse responds to treatment depends both on the disorder and other underlying features of their metabolism. Some horses respond quite well to the high fat diet, others relapse or have a partial response. Some have complications. The L-carnitine and acetyl-l-carnitine has helped many horses improve further.”

If you would like to find out more about how to help your EPSM/PSSM horse or a horse that is prone to tying up, go to the EPSM-PSSM IO discussion group, run by Dr Eleanor Kellon VMD. See link below.

Article originally published in the April – May 2011 (Vol 32 No 6) issue of Hoofbeats magazine, updated since

Further reading:

Links may change over time. If a link doesn’t work, search the title in your search engine.

EPSM-PSSM IO discussion group

Dr Eleanor Kellon offers equine nutrition courses, NRCPlus and Nutrition and Therapy: Genetic Basis of EPSM

NRC Nutrient Requirements of Horses (2007)

Frank N, Andrews FM, Elliott SB, Lew J and Boston RC (2005)
Effects of rice bran oil on plasma lipid concentrations, lipoprotein composition, and glucose dynamics in mares J. Anim. Sci. 83:2509-2518

Dr Beth Valentine DVM Ph.D
EPSM—Muscle Disease in Draft Horses

Thumbnail EC Stock Photo of Quarter Horse

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