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

Reasons for performing studyAlthough exertional rhabdomyolysis (ER) is common in Arabian horses, there are no dedicated studies describing histopathological characteristics of muscle from Arabian horses with ER.ObjectivesTo 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 designProspective and retrospective histopathological description.MethodsMiddle 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.ResultsCentrally 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.ConclusionsIn 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