Comparison of histopathologic criteria and skeletal muscle fixation techniques for the diagnosis of polysaccharide storage myopathy in horses.
Authors: Firshman A M, Valberg S J, Bender J B, Annandale E J, Hayden D W
Journal: Veterinary pathology
Summary
# Editorial Summary: Diagnosing PSSM Through Muscle Biopsy—Getting the Technique Right Polysaccharide storage myopathy (PSSM) requires accurate histopathological diagnosis, yet uncertainty persists about which fixation methods and microscopic features most reliably identify the condition. Firshman and colleagues examined muscle biopsy specimens from nine horses with confirmed PSSM and 15 controls, comparing three tissue preservation techniques (formalin fixation, immediate freezing, and delayed freezing) and scoring sections under three staining protocols (haematoxylin and eosin, periodic acid-Schiff, and amylase-PAS) using blinded assessment by multiple investigators. The presence of amylase-resistant, abnormal polysaccharide proved the most sensitive and specific diagnostic marker for PSSM across all fixation methods and observers, whilst other features commonly associated with the condition—including muscle fibre atrophy and cytoplasmic vacuolisation—showed inconsistent results depending on preservation technique and investigator interpretation. Frozen tissue sections demonstrated superior preservation of muscle architecture compared with formalin fixation, which obscured several important histological features. For practitioners and diagnosticians, this work underscores the importance of requesting frozen sections when submitting muscle biopsies and focusing diagnostic interpretation on amylase-resistant polysaccharide accumulation rather than relying on secondary morphological changes, thereby improving diagnostic accuracy and clinical decision-making for horses with suspected PSSM.
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Practical Takeaways
- •When submitting muscle biopsies for PSSM diagnosis, request frozen sections rather than formalin fixation to ensure accurate histopathologic assessment and reliable diagnosis
- •Diagnostic confirmation of PSSM should specifically look for amylase-resistant polysaccharide accumulation, not normal glycogen; this finding is consistent regardless of technical variables
- •Muscle atrophy and vacuoles alone are not reliable PSSM indicators—insist on amylase and PAS staining protocols to differentiate PSSM from other myopathies
Key Findings
- •Amylase-resistant abnormal polysaccharide is the most sensitive and specific diagnostic indicator for PSSM, detected reliably across all fixation techniques
- •Frozen muscle sections provide superior histopathologic detail compared to formalin-fixed sections for myopathy diagnosis
- •Subsarcolemmal and cytoplasmic vacuoles and muscle atrophy are inconsistent diagnostic features with variable scores depending on fixation method and observer
- •Amylase-sensitive glycogen levels were similar between PSSM and control horses, indicating abnormal polysaccharide accumulation is the key diagnostic finding