Polysaccharide Storage Myopathy (PSSM) is a metabolic muscle disorder that affects thousands of horses worldwide. As a condition with genetic underpinnings and clear clinical signs, PSSM can be a significant concern for horse owners and veterinarians. Understanding the science behind PSSM, its symptoms, diagnosis, and management strategies can help owners provide the best care for affected horses.
I write this article from a place of frustration for other horse owners. I have two of my own and know the feeling when your horse is displaying odd symptoms with little to no reasoning. The worrying is unbearable. I have had many horses over the years that have come into the practice with varying "strange" symptoms. One horse, only a couple of years ago, was presented to me with the owner reporting frustration of the horse's stiffness when coming out of the box and muscle cramps despite this horse being bought as a successful jumper and a good level of fitness. After contacting the previous over, it was reported that the horse regularly needed veterinary intervention for a vitamin E deficiency....
A vitamin E deficiency.... I thought to myself, "well this is something I have seen in horses with PSSM in the past". The vets were then able to test for PSSM and confirmed type 1. This is a long story, shortened for the context of this blog post but I hope that by sharing this we can continue to be more informed horse owners and better understand some of these complex issue.
#What is PSSM?
PSSM is a muscle glycogen storage disorder characterised by an abnormal accumulation of glycogen (a form of stored glucose) and polysaccharide in the muscles. This accumulation leads to disruptions in normal muscle function, causing symptoms such as stiffness, tying-up episodes, and reluctance to move.
1. PSSM Type 1 (PSSM1):
- Caused by a mutation in the GYS1 gene, which encodes glycogen synthase 1.
- The mutation leads to excessive glycogen production and altered carbohydrate metabolism in muscle cells.
2. PSSM Type 2 (PSSM2):
- A broader category encompassing horses without the GYS1 mutation but showing similar clinical signs.
- Recent studies suggest associations with other genes such as RYR1 and potentially others, but PSSM2 remains less understood than PSSM1.
Key Breeds Affected:
PSSM1 is common in draft breeds (e.g., Belgian, Percheron), Quarter Horses, and Warmbloods. PSSM2 is frequently reported in Warmbloods and sport horse breeds.
#Clinical Signs
PSSM’s symptoms can vary widely, from mild to severe, and are often exacerbated by exercise.
Common Signs:
- Muscle stiffness or cramping, especially after light work.
- Reluctance to move or exercise intolerance.
- Sweating and elevated heart rate unrelated to workload.
- Tying-up episodes (exertional rhabdomyolysis ER).
- Gait abnormalities or shifting lameness.
- Difficulty holding up hind legs for farriery.
Chronic cases may develop muscle atrophy or changes in behavior due to pain.
Early diagnosis is critical for effective management. Veterinary diagnostics rely on a combination of clinical history, physical examination, and specialised tests.
#Genetic Testing
- #PSSM1:A DNA test for the GYS1 mutation is definitive for PSSM1.
- #PSSM2: While no single genetic marker exists, research is ongoing to identify associated variants. Genetic tests can suggest predispositions in PSSM2 cases.
2. Muscle Biopsy
- A muscle biopsy (often from the semimembranosus muscle) remains a key diagnostic tool for PSSM2.
- Histopathological examination reveals abnormal polysaccharide deposits and excess glycogen storage.
3. Exercise Challenge Test
- Involves measuring serum creatine kinase (CK) levels before and after moderate exercise. Elevated post-exercise CK levels suggest muscle damage.
#Management and Treatment
While PSSM cannot be cured, it can often be managed effectively through dietary and exercise modifications.
#Dietary Management
1. Low-Starch, High-Fat Diet:
- Reduces carbohydrate metabolism and reliance on glycogen.
- Feed low-starch grains (e.g., beet pulp, rice bran) and high-fat sources (e.g., vegetable oil, flaxseed).
- Aim for fat supplementation to provide 20–25% of daily calories.
2. Balanced Nutrition:
- Ensure adequate levels of vitamins (e.g., Vitamin E) and minerals (e.g., selenium) to support muscle health.
- Avoid high-starch feeds such as sweet feeds or traditional grain mixes.
#Exercise Management
1. Consistent Exercise Routine:
- Daily light-to-moderate exercise helps reduce muscle glycogen stores.
- Avoid prolonged periods of rest, which can exacerbate symptoms.
Gradual Conditioning:
- Introduce exercise programs slowly, allowing muscles to adapt.
#Medical Interventions
- Anti-inflammatory medications (e.g., NSAIDs) may be prescribed during tying-up episodes.
- In severe cases, muscle relaxants or other supportive therapies might be necessary.
#Recent Advances in Research
Emerging studies have enhanced our understanding of PSSM, especially PSSM2. New technologies, such as next-generation sequencing, are uncovering additional genetic mutations linked to myopathies in horses. Improved muscle biopsy techniques and molecular studies may soon provide clearer diagnostic criteria and management options.
#Preventive Measures for At-Risk Horses
- Perform genetic testing for breeding horses to avoid passing on GYS1 mutations.
- Manage at-risk horses with appropriate diet and exercise regimens from a young age.
- Monitor performance and adjust training to prevent overexertion.
Conclusion
PSSM, while a complex and sometimes challenging condition, can often be effectively managed with a tailored approach combining diet, exercise, and veterinary care. Advances in genetic research promise a brighter future for affected horses and their owners, paving the way for better diagnostics and therapies.
References
1. McCue, M. E., Valberg, S. J., & Mickelson, J. R. (2008). GYS1 mutation in horses with polysaccharide storage myopathy. Nature Genetics, 40(7), 876–877.
2. Valberg, S. J. (2009). Diagnostic evaluation of exertional myopathies in horses. Journal of Equine Veterinary Science, 29(5), 310–317.
3. Firshman, A. M., & Valberg, S. J. (2007). Factors affecting clinical outcome in polysaccharide storage myopathy. Journal of the American Veterinary Medical Association, 231(5), 746–754.
4. Valberg, S. J., et al. (2018). Genetic testing for PSSM1 and beyond: Current advances. Equine Veterinary Journal, 50(5), 593–600.
5. Harris, P. A., et al. (2006). Dietary management of polysaccharide storage myopathy in horses. The Veterinary Clinics of North America: Equine Practice, 22(3), 419–432.
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