Muscle: myopathy - overview in Horses (Equis) | Vetlexicon
equis - Articles

Muscle: myopathy – overview

ISSN 2398-2977

Synonym(s): Muscle myopathy


Breed/Species predisposition

Cost considerations

  • Costs of loss of use during episodes of muscle disease.
  • Costs of diagnostic work-up, treatment and management of disease.
  • Loss of breeding potential in genetically transmitted diseases.

Special risks



  • Neurogenic or myogenic.
  • A field of considerable recent and ongoing research.
  • Some syndromes are multifactorial and difficult to specifically define etiology.

Neurogenic (hereditary or environmental, acquired or congenital)

  • Peripheral neuropathies.
  • Neuromuscular transmission disorders, eg Botulism Botulism, Tetanus Tetanus.
  • Anterior horn cell disorders.
  • Motor nerve root disorders.


Post-anesthetic myopathy Muscle: myopathy - postanesthetic

Exertional rhabdomyolysis Muscle: myopathy - exertional rhabdomyolysis 

  • Multifactorial: irregular exercise, dietary imbalances, possibly genetic in some individuals.

Hyperkalemic periodic paralysis Muscle: hyperkalemic periodic paralysis 

  • Genetic defect → abnormality in sodium channel transport.

Fibrotic and ossifying myopathy Muscle: myopathy - fibrotic 

  • Trauma to semitendinosis → contracture.

Atypical myoglobinuria Atypical myoglobinuria 

  • Ingestion of toxin hypoglycin A found in seeds, leaves and seedlings of trees of the genus Acer (maples). In UK, Acer pseudoplatanus, and in North America Acer negundo.
  • Associated with stressful environmental conditions and low plane of nutrition.

White muscle disease of foals Nutrition: nutritional myopathy 

  • Selenium deficiency plus as yet unknown factors → failure to protect muscle cell membranes from peroxidative damage.


Clinical signs

  • Many equine myopathies share common clinical presentations; therefore genetic/breeding, environmental and athletic history taking, and a thorough physical examination and laboratory workup are essential for determining the most likely underlying cause of any myopathy.

Post-anesthetic myopathy Muscle: myopathy - post-anesthetic 

  • General anesthesia +/- hypotension/prolonged recumbency.
  • Reluctance or inability to stand within normal recovery period.
  • Muscle tremor/fasciculation.
  • Pain, heat, swelling associated with dependent muscle group.
  • Peripheral nerve paresis Peripheral nerve: trauma.
  • Myoglobinuria.

Exertional rhabdomyolysis Muscle: myopathy - exertional rhabdomyolysis 

  • Gait abnormality such as stiffness, reluctance to move or in severe cases, recumbency during or immediately after exercise.
  • Hindlimbs, epaxial and pelvic muscles are the most affected.
  • Pain (muscular).
  • Signs of distress.
  • Signs of colic Abdomen: pain 01Abdomen: pain 02.
  • Myoglobinuria Urine: myoglobinuria.
  • Hyperthermia.
  • Poor performance or lack of collection.

Fibrotic and ossifying myopathy Muscle: myopathy - fibrotic 

  • Shortened pathognomonic gait.
  • Usually unilateral.
  • Muscle atrophy.
  • Palpable firmness.

Hyperkalemic periodic paralysis Muscle: hyperkalemic periodic paralysis 

  • Intermittent attacks often triggered by anesthesia, stress, or diets high in potassium:
    • Muscle tremors and sweating.
    • Weakness.
    • Involuntary recumbency.
    • Upper respiratory tract noise.
    • Prolapsed third eyelid.
    • Stiff gait.
    • Spontaneous recovery.

Atypical myoglobinuria Atypical myoglobinuria 

  • Acute onset weakness and stiffness and lowered head carriage.
  • Painful, depressed, trembling and sweating.
  • Firm muscles on palpation.
  • Recumbency.
  • Elevated heart and respiratory rates.
  • Myoglobinuria Urine: myoglobinuria.
  • Death.

White muscle disease Nutrition: nutritional myopathy 

  • Recumbent, inability to stand despite effort.
  • Cardiac dysrhythmia and cardiovascular collapse.
  • Respiratory difficulty.
  • Weakness and recumbency.
  • Sudden death.
  • Inability to stand, weakness.
  • Stiff uncoordinated gait.
  • Pain (muscular).
  • Dysphagia.
  • Poor sucking.
  • Failure of passive transfer.
  • Aspiration pneumonia.
  • Recumbency.

Diagnostic investigation


Urinalysis Urine: overview 


Scintigraphy Bone: scintigraphy 

Thermography Thermography 

  • Muscle strains, disuse atrophy, thoracolumbar gait abnormalities.
  • Areas >1°C/30.2°F warmer than other areas indicate potential inflammation.

Electromyography (EMG)

  • Needle EMG evaluates the function of the ventral horn cell, its axon, axon terminals, neuromuscular junctions and the muscle fibers it innervates.
  • A specialist technique with limited availability.
  • Muscle weakness (myasthenia), neurogenic disorders, tetanus, debilitating/metabolic disorders.

Genetic tests


  • Muscle biopsy for polysaccharide storage myopathy, recurrent rhabdomyolysis Muscle: myopathy - exertional rhabdomyolysis and glycogen branching enzyme myopathy.
  • Findings may include:
    • Increased numbers of internally localized muscle fiber nuclei.
    • Inflammatory cells.
    • Phagocytosis of muscle fibers.
    • Muscle fiber necrosis +/- vacuolization.
    • Other structural anomalies of muscle fibers.
  • Histochemical examinations can also be carried out on biopsy tissues can be used to identify morphological changes, enzyme abnormalities or disturbances in fat metabolism.


Exercise tests

  • Changes in creatine kinase (CK) Blood: biochemistry - creatine kinase and aspartate aminotransferase (AST) Blood: biochemistry - aspartate amino transferase (AST) activity after a controlled period of exercise Musculoskeletal: exercise response test.
  • Resting CK and AST levels should be assessed.
  • A period of exercise to give strenuous exercise without overexertion, chosen for intensity and duration according to the fitness and the usual level of exercise for a given animal, should be performed.
  • 2-4 h post-exercise CK levels should be assessed, and at 24 h.
  • Normal values:
    • CK activity should be less than doubled at 2-4 h and return to baseline at 24 h.
    • AST levels should increase by no more than 50%.
  • Abnormal CK/AST activity is regarded as an indicator of muscle damage but not its severity.


Initial symptomatic treatment

Standard treatment

Post-anesthetic myopathy Muscle: myopathy - postanesthetic 

Exertional rhabdomyolysis Muscle: myopathy - exertional rhabdomyolysis 

Fibrotic and ossifying myopathy

  • Surgical excision of affected muscle/tendon segment.
  • Surgical transection of semitendinosus insertion.

Hyperkalemic periodic paralysis Muscle: hyperkalemic periodic paralysis 

  • Supportive during attacks.
  • Electrolytes (low potassium).
  • Long-term: low potassium diet, potassium-wasting diuresis.

Atypical myoglobinuria Atypical myoglobinuria 

White muscle disease Nutrition: nutritional myopathy 

Polysaccharide storage disease Muscle: myopathy - exertional rhabdomyolysis 

  • Dietary adjustments - low carbohydrate diet.
  • Decrease weight of animal; commercial diets are available.
  • Consistent exercise regime.

Subsequent management


  • Varies with each disease.




  • Varies according to specific disease.

Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Valberg S J (2020) Genetics of equine muscle disease. Vet Clin North Am Equine Pract 36 (2), 353-378 PubMed.
  • Noll C V (2019) Standing myotomy to treat fibrotic myopathy: 22 cases (2004-2016) Vet Surg 48 (6), 997-1004 PubMed.
  • Valberg S J (2018) Muscle conditions affecting sport horses. Vet Clin North Am Equine Pract 34 (2), 253-276 PubMed.
  • Durward-Akhurst S A & Valberg S J (2018) Immune-mediated muscle diseases of the horse. Vet Pathol 55 (1), 68-75 PubMed.
  • Lewis S S et al (2017) Clinical characteristics and muscle glycogen concentrations in warmblood horses with polysaccharide storage myopathy. Am J Vet Res 78 (11), 1305-1312 PubMed.
  • Gonzalez-Medina, S et al (2017) Equine atypical myopathy in the UK: Epidemiological characteristics of cases reported from 2011 to 2015 and factors associated with survival. Equine Vet J 49 (6), 746-752 PubMed.
  • Valberg S J et al (2016) Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis. Equine Vet J 4