Polysaccharide storage myopathy in the M. longissimus lumborum of showjumpers and dressage horses with back pain.
Authors: Quiroz-Rothe E, Novales M, Aguilera-Tejero E, Rivero J L L
Journal: Equine veterinary journal
Summary
# Editorial Summary Polysaccharide storage myopathy (PSSM) has long been recognised as a cause of exertional rhabdomyolysis in some horse populations, but its involvement in back pain remained unclear until this 2002 investigation by Quiroz-Rothe and colleagues. The researchers obtained muscle biopsies from the longissimus lumborum and gluteus muscles of nine sport horses (five showjumpers and four dressage horses) presenting with clinical back pain of unknown aetiology, comparing histopathological findings with control groups including horses with confirmed exertional rhabdomyolysis and healthy trotters. All nine horses with back pain demonstrated pathognomonic PSSM lesions—excessive glycogen accumulation and amylase-resistant polysaccharide deposits—within the epaxial musculature, findings corroborated by quantitative optical densitometry, whilst rhabdomyolysis cases showed elevated glycogen without the characteristic abnormal polysaccharides. This work has significant implications for equine practitioners: when conventional clinical examination and imaging fail to identify a cause of poor performance or back pain in sport horses, PSSM affecting the epaxial muscles should be considered as a differential diagnosis, and muscle biopsy of the longissimus lumborum offers a viable diagnostic tool to confirm or exclude this condition. Recognition of PSSM in the back musculature opens the possibility of targeted nutritional and exercise management strategies that may be overlooked if the diagnosis remains elusive.
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Practical Takeaways
- •If a performance horse presents with back pain undiagnosed by standard clinical and imaging techniques, consider PSSM as a differential diagnosis and recommend epaxial muscle biopsy
- •PSSM affects sport horses across disciplines (jumping, dressage) and may be a more common cause of back pain than previously recognized
- •Muscle biopsy from the back muscle (M. longissimus lumborum) is a practical diagnostic tool that can guide targeted dietary and management interventions for affected horses
Key Findings
- •PSSM was diagnosed in 9/9 horses (5 showjumpers, 4 dressage horses) presenting with back pain, characterized by high glycogen and amylase-resistant polysaccharide deposits in M. longissimus lumborum
- •Similar PSSM pathology was found in M. gluteus biopsies of the same horses, indicating systemic polysaccharide storage disease
- •Horses with rhabdomyolysis showed increased PAS staining but lacked amylase-resistant material, distinguishing PSSM from other myopathies
- •Epaxial muscle biopsy of M. longissimus lumborum can provide definitive diagnosis of PSSM-associated back pain when clinical examination and imaging are inconclusive