Polysaccharide Storage Myopathy
Authors: Firshman Anna M., Valberg Stephanie J.
Journal: Veterinary Clinics of North America: Equine Practice
Summary
# Editorial Summary: Polysaccharide Storage Myopathy Polysaccharide storage myopathy (PSSM) represents a significant metabolic disorder affecting equine muscle metabolism, with PSSM1 being an autosomal dominant glycogen storage condition present across more than 20 breeds that manifests clinically as exertional rhabdomyolysis and other performance-limiting signs. Diagnosis differs between types: PSSM1 can be confirmed through genetic testing or muscle biopsy showing abnormal amylase-resistant polysaccharide accumulation, whilst PSSM2-ER, recently identified in Quarter Horses and distinguished as a separate glycogen storage entity, requires muscle biopsy for diagnosis since its genetic basis remains undetermined. Both PSSM1 and PSSM2-ER respond favourably to identical management strategies—specifically, a low nonstructural carbohydrate, high fat dietary protocol combined with consistent exercise programmes—offering farriers, veterinarians and equine nutritionists a clear evidence-based approach to managing affected horses. These findings are particularly relevant for practitioners working with breeds showing higher prevalence, as appropriate dietary modification and conditioning can substantially improve clinical outcomes and athletic function. Understanding the distinction between PSSM types and their shared response to management interventions allows for targeted client education and more effective treatment protocols in practice.
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Practical Takeaways
- •Horses presenting with exertional rhabdomyolysis should be screened for PSSM1 via genetic testing or muscle biopsy, particularly if from affected breeds
- •PSSM management is straightforward: implement low-NSC/high-fat feeding protocols and ensure consistent exercise — dietary and exercise modifications are effective for both PSSM1 and PSSM2-ER
- •Quarter Horses with ER warrant muscle biopsy investigation for PSSM2-ER even without positive genetic testing, as the genetic basis remains unidentified
Key Findings
- •PSSM1 is an autosomal dominant glycogen storage disorder affecting more than 20 horse breeds, diagnosed by genetic testing or muscle biopsy showing abnormal amylase-resistant polysaccharide
- •PSSM2-ER is a glycogen storage disorder identified in Quarter Horses with unknown genetic basis, diagnosed by muscle biopsy
- •Both PSSM1 and PSSM2-ER respond well to low nonstructural carbohydrate, high fat diet combined with regular exercise