BY YANICE PAUWELS MVM/EQUINE SURGEON / EQUITOM
PHOTOGRAPHY: EQUITOM
A diagnosis of Polysaccharide Storage Myopathy (PSSM) can come as a shock to many horse owners, particularly when the horse is actively involved in sport. Fortunately, PSSM can be managed effectively, and with the right approach, many affected horses can return to a high level of performance.
This article explains what PSSM is, how to recognize it, how the diagnosis is made, and the key steps that can help maintain your horse’s health and wellbeing.
What is PSSM?
Polysaccharide Storage Myopathy (PSSM) is a muscle disorder that occurs in several horse breeds, including Quarter Horses, draft breeds, Warmbloods, and Arabians. The condition is characterized by abnormal storage and utilization of sugars within the muscles. Under normal circumstances, muscles store glucose in the form of glycogen, which serves as an important energy source during exercise. In horses affected by PSSM, excessive amounts of glycogen accumulate within the muscle tissue, leading to muscle dysfunction.
Two forms of PSSM are currently recognized:
• PSSM Type 1 is caused by a mutation in the GYS1 gene, which plays a key role in glycogen synthesis. This form is inherited and can be identified through genetic testing. It is particularly common in certain breeds with a known genetic predisposition.
• PSSM Type 2 is more complex, and its exact cause is not yet fully understood. Unlike Type 1, no specific genetic mutation has been consistently identified. Horses with PSSM Type 2 exhibit muscle abnormalities and exercise intolerance, but diagnosis generally requires further investigation through muscle biopsy and clinical evaluation.
Clinical signs can range from mild performance issues to severe episodes of muscle pain, commonly referred to as “tying-up.” Although PSSM cannot be cured, appropriate nutritional management and regular exercise can significantly reduce clinical signs and improve quality of life.
Pathophysiology: How PSSM affects muscle function
In horses with PSSM, abnormal glycogen accumulation interferes with normal muscle metabolism. During exercise, glycogen should be broken down into glucose to provide energy. In affected horses, this process becomes inefficient, resulting in several physiological consequences:
• Muscle stiffness: Excess glycogen can contribute to fluid accumulation within muscle cells, reducing flexibility.
• Energy deficiency: Despite abundant glycogen stores, muscles are unable to utilize this energy effectively during exercise.
• Cellular damage: Glycogen accumulation may disrupt muscle fiber architecture, leading to inflammation, degeneration, and muscle cell death.
Clinical signs
The clinical presentation of PSSM varies considerably between individuals and may be influenced by genetics, diet, management practices, and exercise levels.
Common clinical signs include:
• Muscle stiffness, particularly after periods of rest.
• Reduced athletic performance and early fatigue.
• Abnormal gait or difficulty moving freely.
• Muscle tremors or fasciculations.
• Excessive sweating, even during light exercise.
• Recurrent episodes of tying-up, characterized by severe muscle pain and reluctance or inability to move.
Clinical signs are often more pronounced in horses receiving high-energy diets while lacking sufficient daily exercise, highlighting the importance of balanced management...
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