PSSM1 and PSSM2

Polysaccharide Storage Myopathy (PSSM) is a muscle disorder in which glycogen, stored within the muscle cells, accumulates abnormally¹. This build-up of sugar molecules in the muscle cells prevents the stored energy source, in the form of glycogen, from being available to the muscles. As a result, problems can occur during exercise, potentially leading to episodes of tying-up.

Muscles require energy to function. Horses obtain this energy from nutrients such as sugars, starch, fibre and fats. When there is an excess of sugar and starch in the diet, part of it is converted into glycogen and stored in the muscle cells². When blood glucose levels drop, for example during exercise, glycogen can be converted back into glucose to supply energy to the body and muscles. In horses with PSSM, the process of glycogen storage is abnormal.

There are two different forms: PSSM type 1 and PSSM type 2. The clinical signs of both forms are similar, but the difference is that PSSM1 is hereditary³. Type 1 is caused by a mutation in the glycogen synthase gene.

PSSM1 occurs in several horse breeds but has a higher prevalence in breeds such as the Belgian Draft horses⁴. PSSM2 is found across a wide range of breeds.​

Horses with PSSM may show various symptoms, including: low energy during exercise, reluctance to move forward, muscle stiffness, excessive sweating, reduced muscle mass, and muscle tremors.

A diagnosis can be made through a muscle biopsy, which can determine whether glycogen accumulation is present in the muscles. PSSM1 can also be identified through a genetic test detecting the specific mutation associated with the condition³. But, this is currently not possible for PSSM2. Genetic testing for PSSM2 is currently not considered as reliable.

At present, there is no cure for PSSM. For horses with PSSM, a carefully adapted diet and exercise programme are essential to help reduce episodes and provide optimal support.

Due to the abnormal glycogen accumulation, it is important to keep sugar and starch levels in the diet of horses with PSSM significantly low⁵. Forage should form the basis of the ration, but it is important to ensure that the forage is low in sugars and starch. A forage analysis can be carried out to determine sugar and starch levels. As forage alone does not provide sufficient vitamins and minerals to meet requirements, a balancer or vitamin and mineral supplement can be used. Horses with PSSM also have an increased requirement for vitamin E, and supplementation is therefore recommended⁶. It is important to keep the horse in regular work. But, training intensity should be carefully managed, and the horse should not be worked too intensively⁷.​

Recurrent Exertional Rhabdomyolysis

Recurrent Exertional Rhabdomyolysis (RER) is a recurring muscle disorder characterised by a disruption in the regulation of calcium within the muscle cells⁸. Calcium is essential for muscle contraction. In horses with RER, calcium regulation inside the muscle cells is disrupted (not due to dietary deficiencies), which ultimately leads to muscle cramping and pain because the muscles continue to contract abnormally⁹. In addition, this disruption affects muscle energy metabolism, damages the mitochondria, and causes injury to muscle fibres¹⁰. Because RER is a complex condition, it is suspected that genetic factors may play a role. Episodes appear to be influenced by factors such as stress and excessive training. However, there is currently insufficient research to determine the exact underlying cause.

Symptoms of RER include muscle stiffness and painful muscles as a result of muscle damage, increased respiratory rate, excessive sweating, and reluctance or refusal to move⁹.

For horses with RER, forage also forms the basis of the ration. However, compared with horses with PSSM, sugar and starch levels in forage appear to be less critical⁶. It is still advisable to keep sugar and starch levels moderate and avoid excessive amounts. This is mainly recommended because high sugar and starch diets can influence a horse’s temperament. As stress can trigger RER episodes, avoiding excessive starch and sugar intake is advised. For horses with RER, a ration containing no more than 20% combined sugar and starch is recommended⁶. To meet higher energy requirements, fats from vegetable oils such as linseed oil can be used. A balancer or vitamin and mineral supplement is also recommended to meet the horses nutrient requirements.

Myofibrillar Myopathy (MFM)

Muscle fibres are made up of myofibrils, structures responsible for muscle contraction. These myofibrils run along the entire length of the muscle fibre and are normally aligned in an orderly manner. The protein desmin plays an important role in maintaining this alignment.

In Myofibrillar Myopathy (MFM), there is an abnormal accumulation of the protein desmin within the muscle¹¹. As a result, the myofibrils clump together and their normal, organised structure becomes disrupted¹². This leads to clinical signs consistent with tying-up, such as muscle stiffness and reluctance to move.​

MFM is a relatively recently recognised muscle disease in horses¹³. It is seen primarily in Warmbloods and Arabians. In the past, horses with MFM were often diagnosed with PSSM2 because glycogen accumulates between the clumped myofibrils¹¹ ¹⁴, which resembled PSSM2. It is now understood that the underlying cause differs, and that this condition should be classified as MFM rather than PSSM2.​

MFM is diagnosed by muscle biopsy. As research is still ongoing, it is currently unclear whether the condition is genetic, and genetic testing is not recommended as it does not provide a reliable diagnosis.​

Voor paarden met MFM is het belangrijk dat het rantsoen voldoende hoogwaardige eiwitten bevat van een hoge kwaliteit 11. Om het lichaam te ondersteunen en omdat hoogstwaarschijnlijk oxidatieve stress en rol speelt bij deze aandoening is het goed om antioxidanten aan het rantsoen van het paard toe te voegen 11. Nutriënten die antioxidante functies hebben in het lichaam zijn onder andere Vitamine E, Vitamine C en MSM.

Hyperkalaemic Periodic Paralysis (HYPP)

Hyperkalaemic Periodic Paralysis (HYPP) is a genetic muscle disorder that primarily occurs in breeds such as the American Quarter Horse, American Paint Horse, and Appaloosa¹⁵. In HYPP, sodium flow in and out of the muscle cells is disrupted. This results in elevated potassium levels in the blood, causing muscles to contract more than normal¹⁶. This leads to uncontrolled muscle contractions and tremors¹⁶. Clinical signs vary depending on the individual horse and the severity of the condition, but common symptoms include muscle tremors, heavy sweating, generalised weakness, yawning, and weakness of the hindquarters. In severe cases, respiratory function may be affected, leading to heavy breathing¹⁵. Episodes may be triggered by stress or by feeding a diet high in potassium.​

Because HYPP is a genetic disorder, diagnosis can be confirmed through genetic testing to determine whether the specific mutation responsible for HYPP is present. Testing also establishes whether the horse is heterozygous (one copy of the gene) or homozygous (two copies) for the condition¹⁵.

For horses with HYPP, it is important that the total dietary potassium content does not exceed 1%. Forage remains the basis of the ration, but it is essential to select forage with a low potassium content⁶. A balancer or vitamin and mineral supplement can be used to complete the ration. Vegetable oils, such as linseed oil, may be added to increase energy intake.

Atypical Myopathy

Atypical Myopathy (AM) is a life-threatening muscle disease caused by ingestion of the toxin hypoglycin A (HGA). Horses are exposed to this toxin by consuming leaves, seeds, or seedlings from maple trees. Read more about AM in the blog: Atypical Myopathy: Cause, Treatment and Prevention.

References

1.           Naylor, R.J. (2015) Polysaccharide storage myopathy – the story so far. Equine Vet Educaction, 27(8):414-419.

2.           Hollands, T., Drury, L. (2023) Chapter 12 - Feeding the Performance Horse. In: Hollands, T., Drury, L. Evidence Based Equine Nutrition A Practical Approach For Professionals. CAB International.

3.           Finno, C.J. (2025) Genetics of Muscle Disease. Veterinary Equine Clinic, 41:17-29.

4.           McCue, M.E., Valberg, S.J., Lucio, M., 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.

5.           Pagan, J.D., Valberg, S.J. (2020) Feeding Performance Horses with Myopathies. AAEP Proceedings, 66:66-74.

6.           Urschel, K.L., McKenzie, E.C. (2021) Nutritional Influences on Skeletal Muscle and Muscular Disease. Veterinary Equine Clinic, 37:139-175.

7.           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):1-17.

8.           McKenzie, E.C., Valberg, S.J., Godden, M,G., 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. J Vet Intern Med, 17:693-701.

9.           Valberg, S.J. (2025) Sporadic and Recurrent Exertional Rhabdomyolysis. Veterinary Clinics of North America: Equine Practice, 41(1):111-124.

10.         Barrey, E., Jayr, L., Mucher, E., Gospodnetic, S., Joly, F. (2011) Transcriptome analysis of muscle in horses suffering from recurrent exertional rhabdomyolysis revealed energetic pathway alterations and disruption in the cytosolic calcium regulation. Animal Genetics, 43: 271-281.

11.         Pratt-Philips S. (2025) Nutritional considerations for equine myopathies. UK-VET Equine, 9(5):198-203.

12.         Williams, Z.J., Velez-Irizarry, D., Petersen, J.L., Ochala, J., Finno ,C.J., Valberg, S.J. (2020) Candidate gene expression and coding sequence variants in Warmblood horses with myofibrillar myopath. Equine Veterinary Journal, 53(2):306-315.

13.         Valberg, S.J., Nicholson, A.M., Lewis, S.S., Reardon, R.A., Finno, C.J. (2017) Clinical and histopathological features of myofibrillar myopathy in Warmblood horses. Equine Veterinary Journal, 49(6).

14.         Valberg, S.J., McKenzie, E.C., Eyrich, L.V., Shivers, J., Barnes, N.E., Finno, C.J. (2016) Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis, 48(5): 548-556.

15.         Finno, C.J., Spier, S.J., Valberg, S.J. (2009) Equine diseases caused by known genetic mutations. Veterinary Journal, 179(3):336-347.

16.         Naylor, J.M. (1994) Equine hyperkalemic periodic paralysis: Review and implications. The Canadian Veterinary Journal, 35(5):279-285.