Muscle glycogen concentrations and response to diet and exercise regimes in Warmblood horses with type 2 Polysaccharide Storage Myopathy.
Authors: Williams Zoë J, Bertels Megan, Valberg Stephanie J
Journal: PloS one
Summary
# Editorial Summary: Managing Type 2 PSSM in Warmbloods Unlike Type 1 polysaccharide storage myopathy, which results from a known genetic defect affecting glycogen metabolism, Type 2 PSSM has an undetermined aetiology, yet current management recommendations treat both conditions identically using low-starch, fat-supplemented diets combined with structured exercise programmes. Williams and colleagues investigated whether these standard interventions actually benefit PSSM2-affected Warmbloods by analysing muscle glycogen concentrations and clinical responses in 36 affected horses and 23 unaffected controls, supplemented by owner questionnaires tracking performance changes over time. Whilst 80% of PSSM2 horses showed overall improvement with the recommended regime—including significant reductions in performance decline and rhabdomyolysis episodes—nearly half the cohort failed to progress as expected, with approximately one-third continuing to exhibit reluctance to go forward and collect. Notably, median muscle glycogen concentrations in PSSM2 horses did not differ from controls, though horses with the highest glycogen levels were significantly more likely to show performance decline, suggesting that glycogen accumulation alone may not drive PSSM2 pathology in the way it does in Type 1 disease. Practitioners should recognise that whilst low-starch/high-fat feeding and conditioning remain beneficial for PSSM2 cases, they represent an incomplete solution; horses failing to respond adequately may require additional investigative work to identify concurrent or alternative pathologies, and the absence of abnormal glycogen storage in PSSM2 warrants consideration of different aetiological mechanisms distinct from Type 1 disease.
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Practical Takeaways
- •Standard PSSM1 management (low-starch/high-fat diet plus exercise) helps 4 out of 5 PSSM2 horses but leaves about half still struggling with reluctance to go forward and collection—expect incomplete resolution and consider alternative diagnostics if poor response occurs.
- •PSSM2 typically emerges around age 6; if performance drops with exercise intolerance at this age in Warmbloods, PSSM2 should be investigated even though muscle glycogen levels alone won't confirm the diagnosis.
- •Horses with visibly elevated muscle glycogen on biopsy are at higher risk for performance problems, so glycogen analysis may help predict severity and guide prognosis when managing PSSM2.
Key Findings
- •Abnormal exercise responses in PSSM2 Warmblood horses typically began at approximately 6 years of age, including decline in performance and reluctance to go forward in over 50% of horses.
- •Low-starch/fat-supplemented diet and exercise regime improved 80% of PSSM2 horses, with significant decreases in performance decline and rhabdomyolysis, but 53% still showed incomplete recovery.
- •Median muscle glycogen concentrations did not differ between PSSM2 horses and control horses with no myopathy, but horses with highest glycogen concentrations were significantly more likely to show performance decline.
- •PSSM1-recommended management partially but not completely resolves PSSM2 clinical signs, suggesting different underlying pathophysiology between the two conditions.