Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis

S. J. Valberg, E. C. Mckenzie, L. V. Eyrich, J. Shivers, N. E. Barnes, C. J. Finno

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Reasons for performing study: Although exertional rhabdomyolysis (ER) is common in Arabian horses, there are no dedicated studies describing histopathological characteristics of muscle from Arabian horses with ER. Objectives: To prospectively identify distinctive histopathological features of muscle from Arabian endurance horses with a history of ER (pro-ER) and to retrospectively determine their prevalence in archived samples from Arabian horses with exertional myopathies (retro-ER). Study design: Prospective and retrospective histopathological description. Methods: Middle gluteal muscle biopsies obtained from Arabian controls (n = 14), pro-ER (n = 13) as well as archived retro-ER (n = 25) muscle samples previously classified with type 2 polysaccharide storage myopathy (15/25), recurrent exertional rhabdomyolysis (7/25) and no pathology (3/25) were scored for histopathology and immunohistochemical staining of cytoskeletal proteins. Glutaraldehyde-fixed samples (2 pro-ER, one control) were processed for electron microscopy. Pro-ER and retro-ER groups were compared with controls using Mann-Whitney U and Fisher's exact tests. Results: Centrally located myonuclei in mature myofibres were found in significantly more (P<0.05) pro-ER (12/13) and retro-ER (21/25) horses than controls (4/14). Degenerating myofibres were not evident in any biopsies. Retro-ER horses had amylase-resistant polysaccharide (6/25, P<0.05) and higher scores for cytoplasmic glycogen, rimmed vacuoles and rod-like bodies. A few control horses (3/14) and significantly (P<0.05) more pro-ER (12/13) and retro-ER (18/25) horses had disrupted myofibrillar alignment and large desmin and αβ-crystallin positive cytoplasmic aggregates. Prominent Z-disc degeneration and focal myofibrillar disruption with regional accumulation of β-glycogen particles were identified on electron microscopy of the 2 pro-ER samples. Conclusions: In a subset of Arabian horses with intermittent episodes of exertional rhabdomyolysis, ectopic accumulation of cytoskeletal proteins and Z-disc degeneration bear a strong resemblance to a myofibrillar myopathy. While many of these horses were previously diagnosed with type 2 polysaccharide storage myopathy, pools of glycogen forming within disrupted myofibrils appeared to give the false appearance of a glycogen storage disorder.

Original languageEnglish (US)
JournalEquine Veterinary Journal
StateAccepted/In press - 2015


  • Desmin
  • Exertional rhabdomyolysis
  • Horse
  • Intermediate filaments
  • Myofilaments

PubMed: MeSH publication types

  • Journal Article

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    Valberg, S. J., Mckenzie, E. C., Eyrich, L. V., Shivers, J., Barnes, N. E., & Finno, C. J. (Accepted/In press). Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis. Equine Veterinary Journal.