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


  • 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 and Emergencies - when to call the vet 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.


This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login


This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login


This article is available in full to registered subscribers

Sign up now to obtain ten tokens to view any ten Vetlexicon articles, images, sounds or videos, or Login

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.

Related Images

Can’t find what you’re looking for?

We have an ever growing content library on Vetlexicon so if you ever find we haven't covered something that you need please fill in the form below and let us know!


To show you are not a Bot please can you enter the number showing adjacent to this field

 Security code