ISSN 2398-2977      

Muscle: myopathy - sporadic exertional rhabdomyolysis

pequis

Synonym(s): Tying up, set fast, azoturia, Monday morning disease, sporadic exertional rhabdomyolysis, , paralytic myoglobinuria


Introduction

  • Exertional myopathies are defined as those myopathies that cause muscle pain and impair performance during or immediately after exercise.
  • Exertional rhabdomyolysis represents a subgroup of exertional myopathies that are characterized by the elevation of serum levels of CK Blood: biochemistry - creatine kinase and AST Blood: biochemistry - aspartate amino transferase (AST). Some exertional myopathies do not, or rarely, cause serum muscle enzyme elevation (PSSM2 and Myofibrillar myositis):
    • Exertional rhabdomyolysis can occur sporadically, so-called sporadic exertional rhabdomyolysis (SER), due to extrinsic events or factors, occurring as be a one-off episode or infrequent single episodes.
    • Exertional rhabdomyolysis also occurs as a chronic and recurrent disease due to intrinsic abnormalities in muscle function, Chronic Recurrent Exertional Rhabdomyolysis (CRER), manifesting as repeated episodes of exertional myopathy, even with mild exertion.
  • Cause: intense or prolonged exercise, often in excess of training, exercise following a period of rest, nutritional imbalances, exercise during a viral illness, electrolyte abnormalities.
  • Signs: the acute clinical signs for exertional rhabdomyolysis, whether sporadic or recurrent, are similar and consist of muscle stiffness, shortened hind limb gait, reluctance to move, firm painful muscles particularly in the hindquarters and lower back, anxiety, pain, sweating and increased respiratory rate. Myoglobinuria and recumbency are rare.
  • Diagnosis: history, clinical signs and serum biochemistry, and muscle biopsy Muscle: biopsy.
  • Treatment: pain relief, rehydration Fluid therapy: overview, later - mild exercise, diet.
  • Prognosis: generally, horses recover from SER in a few days. By definition, these episodes are one-off occurrences and unlikely to be repeated. Severe episodes can cause collapse and even death but this is rare.
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Pathogenesis

Etiology

  • In sporadic exertional rhabdomyolysis (SER) there is no intrinsic defect in the muscle, but something extrinsically has caused the muscle cells to be damaged.
  • Strenuous exercise beyond the horse's level of training. Excessive exercise at the beginning of a season or abrupt acceleration of the training program often after an idle period, may lead to one-off episodes of SER which do not return as the horse becomes fitter.
  • Dietary imbalances, including deficiencies in electrolytes, high levels of non-structural carbohydrates and low levels of forage in the diet. Inadequate levels of selenium Selenium and vitamin E Vitamin E in the diet may increase the likelihood of SER.
  • Exhaustion syndrome in endurance and sport horses, often when exercising in hot, humid climatic conditions, may develop signs of SER alongside hyperthermia, dehydration, electrolyte deficiencies, weakness, rapid respiratory rate, and ataxia and collapse. Myoglobinuria and raised serum CK Blood: biochemistry - creatine kinase levels are present in this syndrome but often the muscles are not firm on palpation.
  • Some horses are more prone to SER following viral respiratory tract infections. It is important not to exercise horses if they have clinical signs of these infections.
  • Other unidentified factors.

Predisposing factors

General

Specific

  • Competitive endurance ride.
  • Return to work after period of rest.
  • Breed.

Pathophysiology

Acute

  • Endurance riding → hyperthermia and electrolyte imbalances (low muscle pH, deficiency in ATP) → impaired function of membrane pumps such as sodium/potassium, calcium/magnesium and calcium/ATPase pumps → sarcoplasmic calcium concentration rises →:
    • Mitochondrial respiration inhibited.
    • Activation of phospholipase → cellular membranes damaged.
    • Activation of neutral proteases → disrupts myofibrils.
    • Disturbed cytoskeleton.
  • Concurrent viral infection, eg EHV1, influenza → endogenous pyrogens and viral replication in muscle cells → exacerbates rhabdomyolysis.

Timecourse

  • Acute.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Valberg S J (2018) Muscle conditions affecting Sport Horses. Vet Clin North Am Equine Pract 34 (2), 253-276 PubMed.
  • Naylor R (2014) Managing muscle disease is horses. In Pract 36 (8), 418-423 BMJ.
  • Waller A P & Lindinger M I (2010) Nutritional aspects of post exercise skeletal muscle glycogen synthesis in horses: A comparative review. Equine Vet J 42 (3), 274-281 PubMed.
  • Sponseller B T et al (2005) Severe acute rhabdomyolysis associated withStreptococcus equiinfection in four horses. JAVMA 227 (11), 1800-1807 PubMed.
  • Upjohn M M, Archer R M, Christley R M & McGowan C M (2005) Incidence and risk factors associated with exertional rhabdomyolysis syndrome in National Hunt racehorses in Great Britain. Vet Rec 156 (24), 763-766 PubMed.
  • Cole F L, Mellor D K, Hodgson D R & Reid S W J (2004) Prevalence and demographic characteristics of exertional rhabdomyolysis in horses in Australia. Vet Rec 155 (20), 625-630 PubMed.

Other sources of information

  • Valberg S J (2011) Skeletal Muscle and Lameness. In: Lameness in the horse. Eds: Ross M W D & Dyson S J. Elsevier, USA. pp 818–38.
  • Michigan State University (CVM) Exertional Rhabdomyolysis (ER). Website: www.cvm.msu.edu.

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