Bit & Bridle

Equine Exertional Myopathies

Other Names

  • Tying-up
  • Monday Morning Disease
  • Azoturia
  • Myositis
  • Cording-up
  • Exertional rhabdomyolysis (ER)
  • Recurrent exertional rhabdomyolysis (RER)
  • Polysaccharide storage myopathy (PSSM)

What are Equine Exertional Myopathies?

Exertional myopathies are a group of muscle disorders in horses that cause muscle pain, stiffness, and cramping in association with exercise. The common underlying mechanism is damage to skeletal muscle cells (myocytes) during or after exertion — a process known as exertional rhabdomyolysis. This results in the release of muscle enzymes (particularly creatine kinase, CK, and aspartate aminotransferase, AST) into the bloodstream, and in severe cases, myoglobin (the red pigment in muscle cells) can spill into the urine, causing the characteristic red-brown discolouration known as myoglobinuria.

The condition encompasses a spectrum of presentations ranging from mild, subclinical muscle soreness detectable only through blood tests, to severe, incapacitating episodes in which the horse is unable to move, sweating profusely, in obvious pain, and at risk of life-threatening kidney damage from myoglobin-induced nephrotoxicity.

Exertional myopathies can be sporadic (a one-off event in a previously normal horse) or recurrent (a persistent problem linked to an underlying genetic or metabolic predisposition). Understanding which type of myopathy is present is essential for effective long-term management.

Causes

Several distinct causes and syndromes exist within the exertional myopathy umbrella:

  • Sporadic exertional rhabdomyolysis: A single episode triggered by exercise that exceeds the horse’s current level of conditioning. Common after a period of rest without reducing feed (the classic “Monday Morning Disease” — draught horses given a rest day without reducing grain rations, then worked hard on Monday). Also seen in horses returning to work after illness, in horses worked hard in hot weather without adequate fitness, and in horses with electrolyte deficiencies.
  • Recurrent exertional rhabdomyolysis (RER): An inherited condition predominantly affecting Thoroughbreds, Standardbreds, and Arabians. Affected horses experience repeated tying-up episodes, often triggered by stress, excitement, or moderate exercise. It involves an abnormality in intracellular calcium regulation within muscle cells, making them hyperexcitable. Nervous, excitable mares on high-grain diets are particularly prone.
  • Type 1 Polysaccharide Storage Myopathy (PSSM1): A genetic condition caused by a mutation in the GYS1 gene (glycogen synthase), leading to abnormal accumulation of glycogen and an abnormal polysaccharide in muscle cells. Most common in Quarter Horses, draught breeds, and warmbloods. Affected horses typically show signs of muscle stiffness and cramping with mild exercise and may also display reluctance to move and back pain.
  • Type 2 Polysaccharide Storage Myopathy (PSSM2): A more recently characterised condition in which muscle biopsies show similar polysaccharide accumulation without the GYS1 mutation. The exact genetic basis is still being investigated. More commonly seen in warmbloods and some draught breeds.
  • Mitochondrial myopathies: Rare conditions affecting mitochondrial function within muscle cells, causing poor exercise tolerance and early fatigue.
  • Malignant hyperthermia (MH): An extremely rare, life-threatening condition in Quarter Horses caused by a mutation in the RYR1 gene. Triggered by certain anaesthetic agents or extreme stress, it causes a dramatic, uncontrolled skeletal muscle hypermetabolic crisis with rapidly rising body temperature.

Symptoms and Signs

  • Stiffness and reluctance to move: The horse becomes stiff and unwilling to walk forward, often appearing as though it cannot move its hindquarters properly.
  • Hard, painful muscles: The large muscles of the hindquarters (gluteal and lumbar muscles) become firm, swollen, and painful to the touch — this is sometimes described as feeling like “wood”.
  • Sweating: Profuse sweating disproportionate to the level of exercise.
  • Elevated heart and respiratory rate: As a consequence of pain and metabolic stress.
  • Muscle tremors: Visible trembling of the affected muscles.
  • Reluctance to bear weight: In severe cases the horse may be reluctant to move at all, sometimes confused for colic.
  • Dark urine: Red-brown or port wine-coloured urine (myoglobinuria) in severe cases, indicating significant muscle breakdown. This is a veterinary emergency as it can cause acute kidney injury.
  • Anxiety and distress: Particularly in nervous Thoroughbreds with RER, anxiety may precede or accompany an episode.

Diagnosis

  • Clinical examination: History and physical findings — particularly the timing relative to exercise, breed, and management history — provide important diagnostic clues.
  • Blood tests: Measurement of serum CK and AST levels. Elevated CK (often several thousand to tens of thousands of units per litre in acute episodes) confirms muscle damage. Serial measurements help assess the severity and monitor recovery.
  • Urinalysis: Detection of myoglobin in urine confirms severe rhabdomyolysis. A dipstick positive for blood in the absence of actual red blood cells on microscopy is characteristic.
  • Genetic testing: DNA tests are available to detect the GYS1 mutation (PSSM1) and MH mutation. These tests require a blood sample or hair with follicles and are available through UC Davis and other veterinary genetics laboratories. PSSM1 and MH are included in the AQHA 5-panel test.
  • Muscle biopsy: Biopsy of the semimembranosus or gluteal muscle allows histopathological assessment for polysaccharide accumulation, mitochondrial abnormalities, and other structural changes. This is the gold standard for diagnosing PSSM2 and other myopathies not detectable by genetic tests.
  • Exercise testing: Measuring CK and lactate at rest and following a standardised exercise test helps identify subclinical muscle damage in horses suspected of having chronic myopathy.

Treatment

Acute episode management:

  • Stop exercise immediately: Do not continue to work a horse that is tying-up. Continued exercise dramatically worsens muscle damage. If the horse is some distance from the stable, walk it slowly or ideally transport it rather than riding home.
  • Keep warm: Apply rugs to prevent chilling, which worsens muscle cramping.
  • NSAIDs: Flunixin meglumine or phenylbutazone for pain relief and anti-inflammatory effect.
  • Intravenous fluids: Essential in moderate to severe cases to correct dehydration, support kidney function, and facilitate clearance of myoglobin. Aggressive fluid therapy is particularly important if myoglobinuria is present.
  • Sedation: In very painful or anxious horses, mild sedation (acepromazine, detomidine) may be needed.
  • Restricted movement: Box rest initially, with gradual introduction of gentle hand-walking as the horse improves.

Long-term management (varies by underlying cause):

  • RER: Reducing dietary starch and sugar, increasing fat as a calorie source, consistent daily exercise, and stress reduction. Dantrolene (a muscle relaxant) given before exercise has shown benefit in some cases.
  • PSSM1 and PSSM2: A strict low-starch, low-sugar, high-fat diet is the cornerstone of management. Affected horses should receive minimal grain or starchy feeds, with fat supplementation (e.g. soya oil, rice bran oil) as an energy source. Daily exercise is also important — even short daily turnout or light work helps maintain muscle function.
  • Vitamin E supplementation: Has shown benefit in some horses with exertional myopathies, particularly those with concurrent vitamin E deficiency.

Prevention

  • Match exercise levels to the horse’s current level of fitness — avoid sudden large increases in workload.
  • On rest days, reduce concentrate feed to avoid the classic Monday Morning Disease scenario.
  • Ensure adequate electrolyte replacement, particularly in horses working hard in hot weather or sweating heavily.
  • For horses with known PSSM or RER, follow a consistent management programme as advised by your veterinarian — dietary modification and daily exercise are the most important preventive measures.
  • Genetic test horses of susceptible breeds before committing to a performance career, particularly for PSSM1 in Quarter Horses and draught breeds.
  • Warm up and cool down properly before and after strenuous exercise.

Prognosis

Prognosis depends on the underlying cause, severity of individual episodes, and quality of ongoing management. Horses with sporadic rhabdomyolysis secondary to overexertion generally recover fully. Horses with RER or PSSM can often return to full work with appropriate dietary and management modifications, though some remain prone to occasional episodes. PSSM2 can be more challenging to manage. Horses that experience repeated severe episodes with myoglobinuria are at risk of cumulative kidney damage. Early diagnosis and targeted management significantly improve the long-term outlook.

When to Call the Vet

Any horse that becomes stiff, reluctant to move, or in obvious pain during or immediately after exercise should be assessed by a veterinarian as soon as possible. Dark or discoloured urine is an emergency — contact your vet immediately. For related conditions, see our entry on HERDA for another inherited muscle-related genetic condition, and our guide to equine musculoskeletal health. The British Equine Veterinary Association can help locate a specialist in equine sports medicine and internal medicine.

Frequently Asked Questions

How do I know if my horse has PSSM or just regular tying-up?

Clinical presentation can be similar, but PSSM horses often tie-up with relatively low-intensity exercise — even gentle walking — whereas sporadic rhabdomyolysis typically follows unaccustomed strenuous work. PSSM horses may also show stiffness, back pain, and reluctance to engage the hindquarters outside of acute episodes. Definitive differentiation requires genetic testing for the GYS1 mutation (PSSM1) and/or muscle biopsy. Your veterinarian can advise on the most appropriate diagnostic pathway based on breed, history, and clinical signs.

Can tying-up be fatal?

Isolated episodes rarely cause death directly, but severe cases with extensive muscle breakdown and myoglobinuria can lead to acute kidney failure if fluid therapy is not provided promptly. Horses with the malignant hyperthermia mutation can experience a fatal anaesthetic crisis if appropriate precautions are not taken before general anaesthesia. Always inform your anaesthetist if your horse has been diagnosed with MH or is of a breed at risk.

Should I continue riding a horse that ties-up occasionally?

Ongoing riding without identifying and addressing the underlying cause is not appropriate. Some horses with well-managed PSSM or RER can continue in ridden work with proper dietary and exercise management, but this requires a confirmed diagnosis and a tailored management plan developed with your veterinarian. Continuing to work a horse through repeated tying-up episodes causes cumulative muscle damage.

Is tying-up more common in certain breeds?

Yes. Thoroughbred and Standardbred racehorses and Arabians are most commonly affected by RER. Quarter Horses, draught breeds (Warmbloods, Percherons, Belgians), and cob types are most commonly affected by PSSM. However, exertional myopathies can occur in any breed. Breed should inform the initial diagnostic approach, but all horses that tie-up repeatedly deserve a thorough investigation regardless of breeding.