Equis ISSN 2398-2977

Muscle: myopathy - exertional rhabdomyolysis

Synonym(s): Tying up, set fast, azoturia, Monday morning disease, chronic intermittent rhabdomyolysis, recurrent exertional rhabdomyolysis, paralytic myoglobinuria, polysaccharide storage myopathy, PSSM

Contributor(s): Prof Susan Rhind, Vetstream Ltd, Chris Whitton


  • Acute and/or mild recurrent syndrome in which striated muscle fibers 'dissolve' following exercise.
  • Cause: intense or prolonged exercise, exercise following a period of rest; in rare cases a polysaccharide storage myopathy.
  • Signs: stiffness   →   recumbency, increased respiratory rate, myoglobinuria, gluteal muscle cramps; other signs of exhaustion.
  • Diagnosis: scintigraphy Bone: scintigraphy, biochemistry.
  • Treatment: pain relief, rehydration Fluid therapy: overview - possibly with chilled fluids, cold hosing; later - mild exercise, diet.
Print off the Owner factsheet on Azoturia - Tying-up Azoturia - Tying-up to give to your clients.



  • Strenuous exercise beyond the horse's level of training.
  • Other unidentified factors.

Predisposing factors



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


  • High speed exercise → lactic acidosis and/or prolonged slow exercise → depleted intracellular glycogen → deficiency in high energy phosphates in muscle cells.
  • 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.


  • Predisposed horses have higher muscle glycogen concentrations.
  • Recognized trigger factors:
    • Carbohydrates - high carbohydrate diet plus day of rest → increased incidence of rhabdomyolysis.
    • Vitamin E and selenium deficiency.
    • Electrolyte imbalance - difficult to assess intramuscular stores (erythrocyte [potassium], fractional excretion of electrolytes); potassium deficiency → decreases local capillary vasodilation; low sodium - responds to dietary supplementation with sodium bicarbonate; low phosphorus - try calcium supplement.
    • Hormones - secondary hypothyroidism → altered muscle metabolizm; sex hormones - some fillies respond to progesterone therapy.
    • Metabolic - in humans many cases due to deficiency in glycolysis or fatty acid metabolism (see storage disease below).
    • Ion channels - abnormal membrane function - some drugs such as phenytoin and dantrolene affect membrane channels and seem to prevent rhabdomyolysis.

Storage disease

  • Probably an hereditary metabolic condition.
  • High glucose-6-phosphate concentration → glycogen synthase stimulated → abnormal polysaccharide with reduced branching → accumulation of filamentous polysaccharide in skeletal muscle.
  • Unlike the metabolic condition in humans, glycolytic pathways seem to be normal.


  • Acute.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Naylor R (2014) Managing muscle disease is horses. In Pract 36 (8), 418-423 BMJ.
  • Keen J (2011) Diagnosis and management of equine rhabomyolysis. Vet Rec 33 (2), 68-77 BMJ.
  • Johlig G et al (2011) Epidemiological and genetic study of exertional rhabdomyolysis in a Warmblood horse family in Switzerland. Equine Vet J 43 (2), 240-245 PubMed.
  • 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.
  • Stanley R L et al (2009) A glycogen synthase 1 mutation associated with equine polysaccharide storage myopathy and exertional rhabdomyolysis occurs in a variety of UK breeds. Equine Vet J 41 (6), 597-601 PubMed.
  • Valentine B A (2008) Understanding exertional rhabdomyolysis. Equine Vet Educ 20 (10), 539-541 WileyBlackwell.
  • Stanley R & Piercy R J (2007) Breed susceptibility in equine polysaccharide storage myopathy. Equine Vet Educ 19 (3), 143-145 VetMedResource.
  • Estill C T & Valentine B A (2007) Severe rhabdomyolysis due to polysaccharide storage myopathy in an Arabian mare. Equine Vet Educ 19 (3), 139-142 VetMedResource.
  • Firshman A M, Baird J D & Valberg S J (2005) Prevalence and clinical signs of polysaccharide storage myopathy and shivers in Belgian Draft Horses. JAVMA 227 (12), 1958-1964 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.
  • McGowan C M, Menzies-Gow N J, McDiarmid A M & Patterson-Kane J C (2003)Four cases of equine polysaccharide storage myopathy in the United Kingdom. Vet Rec 152 (4), 109-112 PubMed.
  • Valentine B A (2003) Equine polysaccharide storage myopathy. Equine Vet Educ 15 (5), 254-262 VetMedResource.
  • Beech J (2000) Equine muscle disorders 1 - chronic intermittent rhabdomyolysis. Equine Vet Educ 12 (3), 163-167 VetMedResource.
  • Valberg S J, MacLeay J M, Billstrom J A et al (1999) Skeletal muscle metabolic response to exercise in horses with 'tying-up' due to polysaccharide storage myopathy in horses. Equine Vet J 31 (1), 43-47 PubMed.
  • Perkins G, Valbert S J, Madigan J M et al (1998) Electrolyte disturbances in foals with severe rhabdomyolysis. J Vet Intern Med 12 (3), 173-177 PubMed.

Other sources of information

  • Hodgson D R & Rose R J (1994) Eds. The Athletic Horse. W B Saunders, USA.