• Briony Witherow BSc MSc RNutr. PGCERT FHEA

Feeding the horse with polysaccharide storage myopathy

Briony Witherow explains the importance of correct nutrition when managing a horse that 'ties up'.

Polysaccharide Storage Myopathy (PSSM) or, more commonly referred to as ‘tying up’, is a clinical issue affecting the muscle function in horses. While the term ‘tying up’ does a good job of describing the key symptoms of PSSM and many other muscle disorders, research is moving away from this generic term, with concerns it may over-simplify and distract from the fact that while many key signs of muscle disorders are shared, often the underlying causes are different ¹ ² ³. Making this distinction early on is critical to actioning the most appropriate management regime.

How does it affect the horse?

In research, muscle disorders in the horse are typically described under the umbrella term of Exertional Rhabdomyolysis Syndrome (ERS). These are then categorised into either sporadic episodes of muscular dysfunction, which are typically infrequent and brought on by exercise, or intermittent episodes, which usually relate to inherited muscle problems. With the latter, the horse would typically be seen to have repeated episodes of exertional rhabdomyolysis, sometimes with only mild exertion. Polysaccharide Storage Myopathy would tend to sit under this category.

Sweating and a reluctance to move is a typical sign of PSSM

The condition of PSSM is thought to prevent ‘normal’ metabolism and storage of glycogen (the storage form of glucose or sugar) resulting in an accumulation of glycogen (stored sugar) within the muscles,5 which causes energy deficit, stiffness, sweating and a reluctance to move³ ⁴ ⁵. These symptoms are reported to occur either at the onset or immediately post-exercise, particularly in the unfit or intermittently exercised horse ³ ⁴ ⁵.

While diagnosis of PSSM typically hinges on either genetic testing and/or muscle biopsy, further research into PSSM has highlighted significant breed predisposition, making this a fundamental factor in the diagnosis of muscle disease ⁵ ⁷ ⁸. Under a microscope, a muscle biopsy of a horse with PSSM would typically show a clumping and accumulation⁶ of stored sugar inside muscle cells².

Research has enabled PSSM to be further categorised into either type 1 or type 2 ² ⁴ ⁶ . Type 1 is characterised by a genetic mutation in the glycogen synthase-1 gene (GYS1). Type 2, upon investigation, lacks this mutation. Instead, a muscle biopsy would show abnormal glycogen accumulation, indicating abnormal sugar storage ² ⁴ ⁶.

Are some horses more susceptible than others?

Breed type may not only provide an indication of the risk for PSSM, but also the likelihood of type 1 or 2, and can therefore help to signpost management ⁵ ⁷ ⁸. Of the two types, PSSM type 1 is more prevalent and holds a strong association with halter bred quarter horses (as many as 6-10% of the population being affected ⁹ ¹⁰) and related breeds (such as warmbloods and draught horses¹¹), although it should be noted that in the latter, symptoms may be less consistent. PSSM type 2 is thought to be more common in reining, racing and cutting quarter horses² and warmbloods ². At this point in time, genetic testing can be used to identify PSSM type 1 only. No scientifically validated test is currently available for type 2.

Quarter horses are more susceptible to PSSM

There is still much more to learn about PSSM. The categorisation of type 1 and 2, while consistent in quarter horses, does not appear to be so in other breeds. While in warmbloods PSSM type 1 causes clinical signs of tying up, type 2 appears not to – while clumping of sugar within muscle cells is apparent and seemingly similar (albeit with lower levels of sugar accumulation), classical clinical signs are absent (exercise intolerance often being the reason behind investigation)⁵ . This leads researchers to consider the possibility of different causes for PSSM type 2 across different breeds⁵, possibly suggesting that, in this subset of horses, nutritional management alone (as would be recommended for PSSM type 1) may not result in improved performance and that alternative strategies may be required.

Why is feed important?

PSSM and nutrition are intrinsically linked. Nutrition therefore forms a significant part of PSSM’s management. The following nutritional management is recommended for horses diagnosed with PSSM type 1. For PSSM type 2 and other muscle myopathies, nutritional management should be based on veterinary findings and response to management. Evidence to support specific nutritional intervention for PSSM type 2 is currently lacking and would therefore, need to be considered on a case-by-case basis.

  • The fundamentals upon which the diet should be based are as follows:

  • The foundation of the diet should be an appropriate forage ¹² ¹³ source.

  • Starch and sugar levels in the diet should be kept low (<10% starch and sugar combined¹⁰)

  • Where more calories or energy are required, this should be sourced from fats and oils ¹² ¹⁴ ¹⁵

  • Ensure the diet is fully balanced ¹²

  • Employ feed management tactics to promote movement and reduce stress.

Feed plenty of forage

While fibre should be the foundation of all horses’ diets, the selection of appropriate forage for a horse with PSSM is essential¹². Not only can management and provision of appropriate pasture and forage help to reduce sugar intake but, the management of this can play a key role in promoting movement and exercise. As horses with PSSM are often good doers, ensuring ample and appropriate forage can be challenging.

The current recommendations for what constitutes an appropriate forage is that it should have less than 12% Non-Structural Carbohydrates (NSC – which is starch and sugar combined) on a dry matter basis¹² ¹³ As such, recommendations for forage are very similar to what we might recommend for a laminitic, or, for those with metabolic issues requiring a low starch and sugar ration. As such, management practices like soaking hay are often also recommended¹⁶.

While hay or haylage may fit the NSC brief, the same challenges that present when approaching forage choice with laminitics also apply here: while haylage is typically lower in sugar, it is typically more palatable and digestible, therefore it can be a less appropriate choice for better doers. In addition to this, haylage lacks the flexibility of hay, where soaking can be employed to manipulate suitability.

When it comes to pasture access, the recommendations (again) are much the same as for those requiring a low sugar ration for other clinical reasons (like laminitis or EMS). Tactical grazing management to reduce sugar intake by turning out overnight, avoiding lush pasture, utilising shady paddocks and restriction through muzzling or other grazing management techniques are typically encouraged¹² (for more information see our recent piece on managing the obese horse https://www.thehorsehub.co.uk/post/grassland-management-for-the-obese-horse.) As turnout and movement go hand in hand, regular access to pasture can be beneficial. However, careful management is required, so as to reduce the risk of over-exertion and, at the opposite extreme, of muscles getting too cold.

Ensure the diet is balanced and low in starch and sugar

Where the ration is forage-based, typically this reduces the reliance on concentrate feeds for calories/energy, the majority of the calorie/energy requirement being met by forage alone. In these cases, a workload-appropriate balancer as the sole concentrate feed is often sufficient¹². Provision of the recommended amount of an appropriate supplement or hard feed for bodyweight and workload is essential to ensure that the diet is balanced. As many micronutrients play a vital role in muscle function, this is essential. In addition to this, ensuring adequate salt provision (30-50g per day)⁵ should be considered a fundamental step in ensuring a balanced ration. If workload warrants it, an electrolyte supplement should also be provided ⁵ ¹². Additional antioxidant supplementation, such as vitamin E, may be justified in many cases, but the quantity required would largely depend on the content within the hard feed ration, workload and access to pasture¹².

While a low starch and sugar, forage-based ration is sufficient to reduce the severity of symptoms in some horses, it has been noted that the addition of even a small amount of vegetable oil (definitive recommendations not available) can be beneficial and see clinical improvement ¹⁴ ¹⁵ ¹⁷. It should be noted that the addition of oil should be approached on an individual basis and with consideration of the horse’s bodyweight and condition.

For those requiring more energy/calories, there are now many low starch and sugar rations available for performance horses that would be appropriate. Products that source calories from fat and fibre, as opposed to cereals, are now commonplace; products aimed at other clinical issues such as gastric ulcers are often suitable options – look out for the BETA Food Approval Mark for products suitable for horses prone to Equine Gastric Ulcer Syndrome.

Feed management

Feed management can be employed to further tailor the ration to the horse with PSSM. For those on a restricted forage ration (for weight management), and even those that are not, employing tactics to promote movement when stabled or in a paddock, such as the use of hay balls or provision of multiple feeding stations, can be beneficial in extending chewing time and promoting movement¹⁸.

As previously mentioned, many of the management recommendations for limiting NSC intake for laminitics, and for good doers more generally, can be applied to those with PSSM. Whether it be grazing muzzles, slow feeders, soaked hay or strip grazing, be sure to introduce any new management technique gradually and monitor use and progress regularly.

The bigger picture

It is important to note that for successful management of PSSM, dietary changes must be combined with other changes in management, particularly exercise. For this purpose, it can be useful to keep a diary of workload and management, and reflect upon these regularly in order to ensure that any changes are mirrored in all aspects of the horse’s care. For example, ensuring that the amount fed closely correlates with workload and feeding for work done, not in anticipation of work¹². Planning ahead and proactive management, wherever possible, can be invaluable in the routine management of horses with PSSM.


Ease & Excel for Healthy Muscle Function

As a low starch performance feed, Baileys Ease & Excel is ideal for those horses with moderate to high calorie requirements and whose starch intake needs minimising. Containing no whole cereal grains, its slow release calories are supplied by high levels of digestible fibres and oil and will not exacerbate the excitable temperament.

Quality protein sources support the repair and rebuild of muscle tissue and help promote top line, while a boosted antioxidant package helps neutralise harmful free radicals and support healthy muscle function. Performance levels of vitamins and minerals, including calcium and magnesium, which are involved in neuro transmission, also support well-being and recovery.

Ease & Excel is nutrient and energy-dense so feed volumes and meal sizes are manageable. Baileys Ease & Excel Cubes offer many of the same nutritional benefits and are particularly palatable for the fussiest feeders. For good-doers and those who maintain condition on forage/pasture alone, low starch Baileys Performance Balancer supplies essential nutrients, without unwanted calories.

For more information follow the link below:



  1. Valberg, S.J., Mickelson, J.R., Gallant, E.M., MacLeay, J.M., Lentz, L. and De La Corte, F,D. (1999) Exertional rhabdomyolysis in Quarter Horses and Thoroughbreds; one syndrome, multiple etiologies. Equine Veterinary Journal Supplement, 30: 588-538.

  2. McCue, M.E., Armièn, A.G., Lucio, M., Mickelson, J.R., Valberg, S.J. (2009) Comparative skeletal muscle histopathologic and ultrastructural features in two forms of polysaccharide storage myopathy in horses. Veterinary Pathology, 46(6):1281-91.

  3. Valberg, S.J., McCue, M.E., Mickelson, J.R. (2011) The interplay of genetics, exercise and nutrition in polysaccharide storage myopathy. Journal of Equine Veterinary Science, 31 (5-6): 205-210.

  4. Lewis, S.S., Nicholson, A.M., Williams, Z.J., Valberg S.J. (2017) Clinical characteristics and muscle glycogen concentrations in warmblood horses with polysaccharide storage myopathy. American Journal of Veterinary Research, 78 (11): 1305-1312.

  5. Valberg, S.J. (2012) Muscling in on the Cause of Tying Up. Milne Lecture: Key note address American Association of Equine Practitioners: 85-123.

  6. Williams, Z.J., Bertels, M., Valberg. S.J. (2018) Muscle glycogen concentrations and response to diet and exercise regimes in Warmblood horses with type 2 Polysaccharide Storage Myopathy. PLOS ONE, 13 (9): e0203467. doi: 10.1371/journal.pone.0203467.

  7. Valberg, S.J., Geyer, C.J., Sorum, S. and Cardinet III, G.H. (1996) Familial basis of polysaccharide storage myopathy and exertional rhabdomyolysis in Quarter Horses and related breeds. American Journal of Veterinary Research, 57:286-290.

  8. McCue, M.E., Valberg, S.J., Lucio, M. and Mickelson, J.R. (2008) Glycogen Synthase 1 (GYS1) Mutation in Diverse Breeds with Polysaccharide Storage Myopathy. Journal of Veterinary Internal Medicine, 22 (5):1228-1233.

  9. McCue, M.E., Valberg, S.J. (2007) Estimated prevalence of polysaccharide storage myopathy among overtly healthy Quarter Horses in the United States. Journal of the American Veterinary Medical Association, 231(5):746-50.

  10. Ribeiro, W.P., Valberg, S.J., Pagan, J.D., Essen Gustavsson, B. (2004) The effect of varying dietary starch and fat content on serum creatine kinase activity and substrate availability in equine polysaccharide storage myopathy. Journal of Veterinary Internal Medicine, 18 (6): 887-894.

  11. McCue, M.E., Ribeiro, W.P., Valberg, S.J. (2006) Prevalence of polysaccharide storage myopathy in horses with neuromuscular disorders. Equine Veterinary Journal Supplement, 36:340–344.

  12. Harris, P.A., and Rivero, J.L.L. (2015) Nutritional considerations for equine rhabdomyolysis syndrome, Equine Veterinary Education, 29 (8): 459-465.

  13. Borgia, L.A., Valberg, S.J., McCue, M.E., Watts, K., Pagan, J.D. (2011) Glycemic and insulinemic responses to feeding hay with different nonstructural carbohydrate content in control and Polysaccharide Storage Myopathy-affected horses. Journal of Animal Physiolology and Animal Nutrition, 95(6):798-807.

  14. McKenzie, E.C., Valberg, S.J., Godden, S., Pagan, J.D., MacLeay, J.M., Geor, R.J., Carlson, G.P. (2003) Effect of dietary starch, fat and bicarbonate content on exercise responses and serum creatine kinase activity in equine recurrent exertional rhabdomyolysis. Journal of Veterinary International Medicine, 17: 693-701.

  15. Borgia, L.A., Valberg, S.J., McCue, M.E., Pagan, J.D., Roe, C.R. (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. American Journal of Veterinary Research, 71 (3): 326-336

  16. .Longland, A., Barfoot, C. and Harris, P. (2014) Effect of water temperature and agitation on loss of water-soluble carbohydrates and protein from grass hay: implications for equine feeding management. The Veterinary Record, 174 (3): 68.

  17. De La Corte, F.D., Valberg, S.J., Macleay, J.M., Mickelson, J.R. (2002) Developmental onset of polysaccharide storage myopathy in Quarter horse foals. Journal of Veterinary Internal Medicine, 16: 581-587.

  18. Goodwin, D., Davison, H.P., and Harris, P. (2002) Foraging enrichment for stabled horses: effects on behaviour and selection, Equine Veterinary Journal, 34 (7): 686-691.


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