Equine polysaccharide storage myopathy (PSSM or EPSM) is a genetic condition in horses that affects how muscle cells store sugars. This can lead to exercise intolerance, stiffness, and an abnormal gait in your horse.
Horses with PSSM are at higher risk of tying-up episodes, known as exertional rhabdomyolysis. These episodes cause significant pain and are characterized by stiff, firm muscles along with profuse sweating and reluctance to move.
In severe cases, the breakdown of muscle tissue can affect kidney function and can be fatal.
PSSM is most common in Quarter Horses and large draft breeds. It is estimated that up to 10% of Quarter Horses and 36% of draft horses have PSSM. 
Horses with this condition fall into two categories: PSSM1 and PSSM2. PSSM1 horses have a specific genetic mutation in the gene for glycogen synthase 1. PSSM2 horses show the clinical features of PSSM without this specific genetic mutation. 
The best way to manage PSSM horses is to provide a forage-first low-sugar and starch diet and meet additional energy requirements with dietary fat. A consistent exercise routine can also help promote glycogen breakdown in the muscle.
Wondering how to best feed your PSSM horse? Our equine nutritionists can look at your horse’s diet and help you come up with an appropriate feeding plan for free.
Equine Polysaccharide Storage Myopathy
PSSM is a muscle disorder characterized by abnormal accumulation of glycogen (the storage form of glucose/sugar) in skeletal muscle cells.
Horses with PSSM have more glycogen in their muscles. They also form unusual glycogen molecules that are not as easily metabolized.
Types of PSSM
There are two main types of PSSM: PSSM1 and PSSM2.  PSSM1 is known to be caused by a mutation in the glycogen synthase 1 (GYS1) gene. 
PSSM2 refers to horses with abnormal glycogen storage that do not have a mutation in the GYS1 gene. In these horses, the underlying cause has not yet been identified.
PSSM1 was first identified in 1992 in Quarter Horses that were affected by recurrent exertional rhabdomyolysis. Researchers analyzed muscle samples from these horses and found higher amounts of glycogen than normal and abnormally structured glycogen.
Further research identified the GYS1 mutation in Quarter Horses, related breeds (Paints, Appaloosas) and some draft breeds.
In all horses, the glycogen synthase 1 enzyme is important for making glycogen in muscle cells. In horses with the GYS1 mutation, this enzyme is more active than normal resulting in more glycogen being made. 
Diagnosis of PSSM1
An exercise tolerance test is the first step to determine whether your horse has a muscle issue. However, this test will not provide a definitive answer as to whether they have PSSM1.
Your veterinarian will take a blood sample 4 – 6 hours after a light exercise session of no more than 15 minutes. Levels of muscle enzymes, such as creatinine kinase (CK), are typically higher after exercise in PSSM horses than normal horses.
The exercise tolerance test will indicate whether the horse’s tying up episode can be explained by an issue in their muscles. If the test does not show elevated levels of muscle enzymes, the tying-up episode was likely due to their diet or exercise management at the time and not an underlying genetic condition.
Genetic tests are a relatively non-invasive way to definitively diagnose PSSM1. These are available to specifically detect the R309H-GYS1 variant that causes PSSM1.
A blood or hair root sample can be taken to isolate DNA and analyze the sequence of the GYS1 gene. There are two copies of this gene (one from the mother, one from the father).
PSSM1 is an autosomal dominant condition, meaning a horse with just one copy of the variant gene from either the mother or father can show signs of the disorder. In addition, foals that have one parent with PSSM have a 50% chance of carrying the variant and developing PSSM.
The genetic test will give one of three results:
- PSSM1 n/n: two normal copies of the GYS1 gene (negative for PSSM1)
- PSSM1 n/P1: one normal copy of the GYS1 gene and one variant copy (positive for PSSM1)
- PSSM1 P1/P1: two variant copies of the GYS1 gene (positive for PSSM1)
A PSSM1 diagnosis can also be based on evaluation of muscle biopsies in which the veterinarian will look for: 
- Presence of amylase-resistant, periodic acid-Schiff (PAS)-positive, abnormal polysaccharide inclusions
- Glycogen levels that are 1.5 – 2 times higher than normal
- Higher amount of damaged or necrotic (dead) muscle fibers
PSSM2 horses show the same clinical signs as PSSM1 horses, including muscle stiffness, pain, and reluctance to work. Muscle biopsies taken from these horses also show abnormal accumulation of glycogen.
However, these horses do not have the GYS1 mutation and the precise genetic cause is currently unknown. It is possible that several abnormal genes can contribute to this trait.
PSSM2 is more commonly found in warmblood and Arabian horses.
Diagnosis of PSSM2
There are commercially available genetic tests that claim to identify PSSM2 genetic variants, however, research showed no difference in frequency of those variants in horses with or without PSSM2.
Healthy control horses were just as likely to have the P2, P3 or P4 variants as horses with PSSM2 or myofibrillar myopathy (MFM), another muscle disorder.
This means that the currently available genetic tests for PSSM2 are inaccurate. Note that there are no regulatory requirements for commercial genetic tests in horses. 
Instead of relying on genetic tests, horses should be diagnosed with PSSM2 if they show the features of PSSM1 without the GYS1 mutation.
Diagnosis is typically based on chronic tying-up with or without elevated muscle enzyme levels and abnormal findings in muscle biopsies.
Signs of PSSM
Your veterinarian might suspect PSSM if your horse is showing one or more of the following signs: 
- Chronic exertional rhabdomyolysis (tying-up)
- Poor performance and lack of energy
- Muscle wasting and weakness
- A stiff or stringhalt-like gait
- Intermittent or chronic locking stifle
- May show signs of back pain or whole body pain
- Behavioural issues when saddled, reluctance to stand for the farrier
- Episodes of spasmodic colic
These symptoms can range from mild muscle cramping and stretching out to severe muscle pain with tissue breakdown. In the most severe cases, the horse is unable to rise to stand.
If you suspect your horse has PSSM, consult with your veterinarian for an accurate diagnosis and treatment plan.
Nutritional and Exercise Management
There is currently no cure for PSSM1 or PSSM2. However, adopting appropriate nutrition and exercise programs can lower the incidence of severe symptoms such as severe tying-up.
Feeding and exercise guidelines have primarily been researched in horses with PSSM1. However, a recent study in warmbloods with PSSM2 showed that the same strategies outlined below are effective for PSSM2. 
The following is a list of 8 important tips for managing horses with PSSM.
1) Provide a low starch and sugar diet
It is important to minimize sugar and starch in your horse’s diet to prevent excessive accumulation of glycogen in muscle tissue.
Sugars in the diet trigger insulin release from the pancreas which stimulates glucose uptake into muscle and glycogen synthesis.
Horses with PSSM are highly insulin sensitive and have greater glucose uptake into muscle than other horses. 
Providing a diet that is low in starch and sugar will limit insulin release and stimulation of glycogen synthesis.
It is recommended that the total diet provide less than 10% of the energy from non-structural carbohydrates (NSC). The NSC value is calculated from the combination of ethanol-soluble carbohydrates (ESC) and starch.
Low NSC Diet
There are a few things to consider for lowering the NSC content of the total diet:
- Eliminate concentrate feeds: These often contain processed grains that are high in starch and sugars
- Choose a low-NSC hay: If possible, select a hay with an NSC value of 12% or less
- Soak the hay: Soaking hay helps reduce the level of soluble carbohydrates. Aim for 30 minutes (with warm water) or 60 minutes (with cold water) and allow it to drain for 10 minutes afterwards
- Use a slow feeder hay net: This helps extend the foraging time so that any sugars they are consuming enters the body over a longer period of time and results in lower insulin release
- Use a grazing muzzle: If fresh, lush pasture cannot be avoided, consider using a grazing muzzle to limit their intake
- Limit pasture access to the early morning: Another option for lowering sugar intake from pasture is to limit pasture access to the early morning hours when the plants naturally have lower sugar levels
Note that you should never try to lower the NSC content of your horse’s diet by underfeeding forages. Not providing adequate forage can lead to other issues such as gastric ulcers, behavioural problems and hindgut dysfunction.
Always aim to feed 1.5 – 2% of your horse’s ideal body weight as forage. This is equivalent to 15 – 20 lbs (7 – 9 kg) of hay per day for a 500 kg (1100 lb) horse.
2) Meet additional caloric needs with fat
Fat does not trigger insulin release and will not be stored as glycogen in muscle. Therefore, if your horse requires additional energy beyond what they are getting from their forage, fat is the preferred choice.
Horses with PSSM can have up to 20% of their caloric needs met by fat. This is best achieved with fat sources that are high in triglycerides which are easily digested and absorbed in the small intestine.
If the horse requires additional calories, a common recommendation is to add 1 lb (0.5 kg) of dietary fat for a 500 kg (1100 lb) horse. This can be accomplished by adding 2 cups of oil to their feed.
It is important to accurately assess your horse’s current body condition to determine whether they need additional calories. A body condition score of 4 or less would suggest that they could benefit from added calories.
Even if your horse does not need to gain weight, lower levels of fat may help improve PSSM symptoms.
In a small study of 4 horses, exercise tolerance improved in horses fed diets providing 12% of calories from fat, instead of 7%. This diet had lower starch content (3% compared to 21% of calories from starch). 
Best Fat Sources for PSSM Horses
Some options for low-NSC fat sources include:
- Oils: Look for omega-3 fatty acid rich oils such as fish oil, flax seed oil or w-3 oil which is high in microalgal DHA
- Rice bran: Consists of approximately 20% fat. However, it also contains high levels of phosphorus which needs to be appropriately balanced with calcium in the diet
- Ground Flax: Consists of approximately 40% fat
Fat sources need to be introduced slowly to avoid digestive upset. For oils, start with 30 ml (1 oz) and increase it every 3-4 days to reach the desired amount over a 2-3 week period.
Feeding a low-NSC, high-fat diet can have additional benefits including reducing the risk of colic, laminitis and gastric ulcers.
3) Provide adequate vitamin E and selenium
Vitamin E and selenium are important antioxidants that support healthy muscle function and recovery from exercise. PSSM horses with low vitamin E and selenium intake may be more prone to muscle cramping and stiffness.
The National Research Council’s Nutrient Requirements of Horses recommends a vitamin E intake of 500 – 1000 IU for a mature horse at maintenance.
To avoid selenium deficiency, the NRC recommends at least 1 mg selenium per day. Optimal intake is closer to 2 – 3 mg per day.
For horses prone to exertional rhabdomyolysis, it is recommended to provide 1500 – 2500 IU of vitamin E and 3 mg of selenium in the total diet. 
4) Ensure adequate amino acid supply
Muscle atrophy is a common symptom of PSSM that can be partially mitigated by providing adequate levels of key amino acids in the diet.
For PSSM horses, the most appropriate strategy for providing adequate amino acid supply is to add protein or amino acid sources on their own, rather than as part of a complete feed. This helps minimize unwanted starch and sugars in the diet.
Other protein sources that provide a good balance of essential amino acids to support topline include:
- Alfalfa hay or cubes
- Soybean meal
- Canola meal
- Hempseed meal
- Flaxseed meal
- Whey protein
- Well-balanced amino acid blends
The quantity of additional protein or amino acids that may be required will depend on the protein content of your hay. For horses in light to moderate exercise, the protein content of moderate-quality hay is typically sufficient to meet their needs.
5) Meet their vitamin and mineral requirements
If you eliminate complete feeds and concentrates from your horse’s feeding plan to lower the overall NSC content, you may unintentionally create a nutrient deficiency in your horse’s diet.
Horses almost never obtain all of the vitamins and minerals they require from forage alone.
In addition to selenium and vitamin E, some of the vitamins and minerals that are important for supporting metabolic health and muscle function include:
It is recommended to provide a low-inclusion, comprehensive vitamin and mineral supplement to ensure your horse’s needs are met without oversupplying calories. Choose a supplement that does not contain high-NSC fillers.
Mad Barn’s AminoTrace+ is a complete vitamin and mineral supplement designed with the needs of PSSM horses in mind.
AminoTrace+ provides 100% organic trace minerals, including a full profile of B-vitamins, high levels of natural vitamin E and selenium in the form of organic selenized yeast.
6) Provide Regular Turnout
PSSM horses should be stabled as little as possible. Pasture turnout is highly beneficial for promoting movement and breakdown of glycogen stores.
As mentioned previously, grazing muzzles are a good option for reducing grass intake and should especially be used when turnout is on lush pasture.
If stress appears to be a triggering factor for your horse, pay particular attention to their social environment during turnout to ensure they are with compatible companions.
7) Avoid long rest periods
Long periods of rest after a tying-up episode can be detrimental for horses with PSSM.
It is common to rest horses when they tie up and wait muscle enzyme levels in the blood have returned to normal. However, this is not necessary and may worsen symptoms.
Once the horse is no longer reluctant to move, they can be reintroduced to daily turnout in a calm environment. Another option is to engage in slow hand-walking to re-introduce them to exercise and avoid extended stall rest.
8) Slowly introduce training programs
If your PSSM horse has had a recent tying-up episode, their return to training and exercise needs to be gradual.
Begin with light, uncollected work on a lunge-line or under saddle starting with just 3-5 minutes per day at a walk and trot. It is more important to restrict the duration of a single exercise than the intensity.
The duration of work can be increased by 2 minutes every day. Once they reach 15 minutes of exercise, provide a 5-minute walking break after each 15-minute interval of trotting. Continue with this for at least 3 weeks before introducing work at a canter.
Re-introduction to collected work should also be done very gradually, beginning with just 2-5 minute periods of collection under saddle followed by an opportunity to rest and stretch.
If more than 3-4 days elapse without adequate exercise, start back with just a small amount of exercise.
Sample Feeding Plan for PSSM Horses
Formulating an appropriate diet for a horse with PSSM requires the following information:
- Composition of their forage, including hay and pasture
- Current body condition
- Exercise level and physiological status
It is strongly recommended to get a hay analysis and work with a nutritionist to design an appropriate feeding plan.
Below are two examples of possible diets for a 500 kg (1100lb) mature horse in light exercise to support either weight maintenance or weight gain.
|Feed||Maintenance Diet||Weight Gain Diet|
|(Amount / Day)||(Amount / Day)|
|Hay||11 kg||10 kg|
|Ground Flax||0||0.5 kg|
|Canola Oil||0||300 ml|
|AminoTrace+||200 grams||200 grams|
|w-3 oil||100 ml||100 ml|
|Natural E / Organic Se||20 grams||20 grams|
|Digestible Energy (% Req)||102%||114%|
|NSC (% Diet)||8.7%||8.2%|
|Fat (% Diet)||3.75%||8.45%|
For assistance in formulating a diet appropriate for your horse with PSSM, contact our team of equine nutritionists to receive customized recommendations.
PSSM1 is a relatively common genetic condition that predominantly affects Quarter Horses and draft breeds. It leads to excessive and abnormal glycogen accumulation in muscle resulting in stiffness, muscle pain and increased risk of tying-up (exertional rhabdomyolysis).
PSSM2 is an inherited condition resulting in the same symptoms as PSSM1 but with an unknown genetic origin. It affects primarily warmbloods and Arabians.
Fortunately, PSSM1 and PSSM2 can be managed through diet and regular exercise. Provide a low NSC diet to limit glycogen accumulation in the muscle.
Use fat instead of starches and sugars if additional calories are required. Gradually introduce exercise in short intervals to support glycogen breakdown and decrease the risk of tying-up.
Dietary management can have a significant positive impact on the health and comfort of PSSM horses, and help to minimize the need for costly veterinary care.
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- McCue, M.E. et al. Estimated prevalence of the Type 1 Polysaccharide Storage Myopathy mutation in selected North American and European breeds. Anim Genet. 2010.
- McCue, M.E. et al. Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis. Genomics. 2008.
- Valentine, B.A. et al. Role of dietary carbohydrate and fat in horses with equine polysaccharide storage myopathy. J Am Vet Med Assoc. 2001.
- Williams, Z.J. et al. Muscle glycogen concentrations and response to diet and exercise regimes in Warmblood horses with type 2 Polysaccharide Storage Myopathy. PLoS One. 2018.
- Firshman, A.M. et al. Comparison of histopathologic criteria and skeletal muscle fixation techniques for the diagnosis of polysaccharide storage myopathy in horses. Vet Pathol. 2006.
- Valberg, S.J. et al. Commercial genetic testing for type 2 polysaccharide storage myopathy and myofibrillar myopathy does not correspond to a histopathological diagnosis. Equine Vet J. 2020.
- De La Corte, F.D. et al. Glucose uptake in horses with polysaccharide storage myopathy. Am J Vet Res. 1999.
- Harris, P.A. and Rivero, J.L.L. Nutritional considerations for equine rhabdomyolysis syndrome. Equine Vet Edu. 2015.
- Valberg, S.J. Muscle Conditions Affecting Sport Horses. Vet Clin North Am Equine Pract. 2018.
- Ribeiro, W.P. et al. The Effect of Varying Dietary Starch and Fat Content on Serum Creatine Kinase Activity and Substrate Availability in Equine Polysaccharide Storage Myopathy. J Vet Intern Med. 2004.
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