Exertional rhabdomyolysis in Quarter Horses and Thoroughbreds: one syndrome, multiple aetiologies

Abstract

Summary The purpose of this study was to determine if chronic exertional rhabdomyolysis (ER) in Quarter Horses and Thoroughbreds represents one or several distinct myopathies. Eighteen Quarter Horses and 18 Thoroughbreds with ER were selected from cases presented to the Veterinary Hospital on the basis of a history of ER, assessment of muscle histopathology, and serum CK activity before and 4 h post exercise. In addition, 2 of 3 of the following parameters were evaluated: muscle glycogen concentrations, thyroid hormones (T3, T4), fractional excretion (FE) of sodium, potassium and chloride. The CK response to training, the metabolic response to a near maximal standardised exercise test (SET), blood glucose concentrations after an i.v. glucose challenge and a skeletal muscle in vitro caffeine contracture test were performed on 5 of the Quarter Horses, selected because of polysaccharide storage myopathy (PSSM), and 5 of the Thoroughbreds. Serum T3 and T4 were all within normal limits. Low FE of sodium and potassium were seen in <20% of Quarter Horses and Thoroughbreds. Four hours post exercise, CK was increased in 77% of Quarter Horses and 72% of Thoroughbreds with ER. Muscle glycogen concentrations in Quarter Horses with ER were significantly higher than in normal Quarter Horses and Thoroughbreds with ER. No Thoroughbreds, but 15/18 Quarter Horses with ER had abnormal polysaccharide accumulation in muscle biopsies consistent with a diagnosis of PSSM. PSSM Quarter Horses had higher CK activity during training than Thoroughbreds and higher glycogen utilisation with the SET. PSSM Quarter Horses also had significantly enhanced glucose clearance compared to normal Quarter Horses and Thoroughbreds with ER. Thoroughbreds with ER had significantly lower thresholds for caffeine‐induced contracture than normal horses and PSSM Quarter Horses. It was concluded that there are multiple causes for exertional rhabdomyolysis. In Quarter Horses, rhabdomyolysis is commonly due to a glycogen storage disorder, PSSM, and is readily expressed in untrained horses. In Thoroughbreds, ER is commonly due to an underlying abnormality of muscle contraction. Rhabdomyolysis in Thoroughbreds, however, is only expressed intermittently when key stressors are present.

Pathologic Findings in Equine Muscle (Excluding Polysaccharide Storage): A Necropsy Study

“…17,19,25,31,32,35 Causes of muscular disorders in adult horses include nutritional myopathy, 5,17,31 infectious disease, 10,17,32,35,38 immune-mediated disorders, 17,25,26 toxic myopathy, 19,36 inherited myopathies, 5,29 endocrinopathy, 2 pasture-associated rhabdomyolysis, 15,44,45 malignant hyperthermia, 1 and exertional rhabdomyolysis. 4,6,23,39 Disorders affecting the peripheral nerves resulting in denervation atrophy of muscle also occur. 8,13,14,34,41 Most survey studies of equine neuromuscular disease are of clinical cases.…”

Heritability of Recurrent Exertional Rhabdomyolysis in Standardbred and Thoroughbred Racehorses Derived From SNP Genotyping Data

“…Diagnosis of RER is primarily based on clinical signs and supportive evidence such as detection of elevated serum creatine kinase (CK) and aspartate aminotransferase (AST) activities after exercise. Muscle biopsy is predominantly utilized to rule-out other potential causes of ER, but may show nonspecific myopathic features such as centrally located nuclei and variable degrees of necrosis and regeneration (Valberg et al 1993;Valberg et al 1999). Estimated prevalence of RER is 6% in Standardbred racehorses and ranges from 5% to 10% in Thoroughbreds (MacLeay et al 1999b;McGowan et al 2002;Isgren et al 2010).…”

Musculoskeletal Pathology

“…The 2 most-studied causes are recurrent exertional rhabdomyolysis (RER), which affects 5% to 10% of thoroughbreds in training, and polysaccharide storage myopathy (PSSM) in quarter horses. 38 Pathophysiology Bone Exercise-induced bone injury occurs when biomechanical loading overwhelms the normal adaptive processes that function to replace pockets of weakened bone and strengthen areas subject to high strain, in accordance with Wolff law. 39 Repetitive cyclic overloading of bone results in focal microdamage, such as trabecular microfractures and focal breaks in the calcified cartilage layer identified in the metacarpal condyle of racehorses.…”