Bit & Bridle

Equine Exertional Rhabdomyolysis (“Tying-Up”)

Other Names

  • Azoturia
  • Setfast
  • Monday-morning disease
  • Rhabdomyolysis
  • ER (Exertional Rhabdomyolysis)

What is Tying-Up in Horses?

Exertional rhabdomyolysis (ER), commonly known as “tying-up,” is a syndrome of exercise-induced muscle damage in horses. It occurs when muscle cells break down during or shortly after exercise, releasing myoglobin and muscle enzymes into the bloodstream. The term “azoturia” or “Monday-morning disease” historically referred to cases in working draught horses that had been given full rations over a rest day and then returned to heavy work — the result being a sudden, severe episode of muscle cramping.

Today, tying-up is recognised as a spectrum of conditions with different underlying causes. It is one of the most common reasons for poor performance in Thoroughbreds and other performance horses. The condition ranges from a single, isolated episode triggered by overexertion, to a chronic, recurring form rooted in underlying muscle disorders.

Two distinct chronic forms are recognised in the veterinary literature:

  • Recurrent Exertional Rhabdomyolysis (RER): Most common in Thoroughbreds and Standardbreds, thought to involve abnormal calcium regulation within muscle cells. Often linked to nervous temperament and management factors.
  • Polysaccharide Storage Myopathy (PSSM): Seen in Quarter Horses, warmbloods, and draught breeds. Caused by abnormal glycogen accumulation in muscle cells, with a genetic basis in many cases.

Causes

Tying-up can be triggered by a range of factors, and in horses with an underlying muscle disorder, even mild exercise may precipitate an episode:

  • Overexertion: Excessive exercise relative to the horse’s current fitness level
  • Electrolyte imbalances: Deficiencies in sodium, potassium, calcium, or magnesium impair normal muscle function
  • High-starch diets: Excessive non-structural carbohydrates (cereal grains) can exacerbate PSSM and RER
  • Stress and anxiety: Nervous horses, particularly fillies, are at higher risk of RER episodes
  • Inadequate warm-up: Going straight into hard work without adequate preparation
  • Dehydration: Impairs electrolyte balance and muscle cooling
  • Hormonal factors: Oestrus cycles appear to increase susceptibility in some mares
  • Genetic predisposition: PSSM Type 1 involves a mutation in the GYS1 gene affecting glycogen synthesis

Symptoms and Signs

Episodes typically occur during or within 15-30 minutes of the start of exercise. Signs include:

  • Sudden stiffness, reluctance to move, or complete refusal to walk
  • Muscle cramping and hardness, most noticeably over the hindquarters and back
  • Profuse sweating out of proportion to the exercise performed
  • Rapid heart rate and respiration
  • Anxiety or distress
  • In severe cases: inability to stand, myoglobinuria (dark reddish-brown “coffee-coloured” urine) indicating kidney stress
  • Pain on palpation of the large muscle groups (gluteals, longissimus dorsi)

Severity can range from mild stiffness that resolves with gentle walking, to a horse unable to move and requiring emergency veterinary attention.

Diagnosis

Diagnosis is based on clinical signs combined with laboratory testing:

  • Blood tests (serum chemistry): Elevated creatine kinase (CK) and aspartate aminotransferase (AST) confirm muscle cell damage. In a severe episode, CK can be ten to one hundred times the normal range.
  • Urinalysis: Dark urine may indicate myoglobinuria, which can cause kidney damage if not promptly managed
  • Muscle biopsy: For horses with recurrent episodes, biopsy of the semimembranosus or semitendinosus muscles can identify PSSM or other myopathies
  • Genetic testing: A DNA test for the GYS1 mutation can confirm PSSM Type 1
  • Exercise testing: Monitoring CK levels before and after a standardised exercise test helps identify horses with sub-clinical muscle enzyme elevations

Treatment

The immediate priority in a tying-up episode is to stop all exercise and keep the horse calm and warm:

  • Stop exercise immediately: Do not walk the horse out — movement while muscles are actively cramping worsens damage
  • Keep warm: Rug the horse, particularly over the hindquarters, to prevent muscle chilling
  • Call the vet: Any moderate to severe episode requires veterinary attendance
  • Intravenous fluids: Large volumes of IV fluids protect the kidneys from myoglobin damage and restore electrolyte balance
  • NSAIDs: To reduce pain and inflammation
  • Sedation: May be required in severely distressed horses
  • Vitamin E and selenium supplementation: In horses with deficiencies, these antioxidants support muscle health

For horses with chronic or recurrent tying-up, long-term management includes dietary modification (reducing starch and sugar, increasing fat and fibre), structured daily exercise even on rest days, and stress reduction. Horses with PSSM benefit significantly from a low-starch, high-fat diet. Those with RER often improve with consistent daily turnout and routine.

Prevention

  • Maintain consistent exercise — avoid sudden increases in intensity after days off
  • Ensure adequate warm-up before demanding work
  • Feed appropriate rations relative to workload — reduce hard feed on rest days
  • Provide daily turnout to allow free movement and reduce anxiety
  • Supplement electrolytes and ensure adequate hydration, particularly in hot weather
  • Consider genetic testing for Quarter Horses and warmbloods showing recurrent episodes
  • Work with a veterinary nutritionist to formulate a suitable diet for horses with PSSM or RER

Prognosis

A horse that experiences a single, isolated episode of tying-up triggered by overexertion has an excellent prognosis with appropriate management. Horses with chronic RER or PSSM can often be managed successfully with dietary and exercise modifications, though they may never be entirely “cured” and require careful, consistent management throughout their career. Severe episodes that result in significant myoglobinuria carry a more guarded prognosis due to the risk of acute kidney injury. The British Equine Veterinary Association (BEVA) recommends ongoing monitoring of muscle enzyme levels in horses with recurrent episodes.

When to Call the Vet

  • Your horse seizes up during or after exercise and refuses to move
  • The horse is sweating heavily with hard, painful muscles over the back and hindquarters
  • You notice dark, reddish-brown urine after an episode — this is an emergency
  • This is the second or subsequent episode — investigation of an underlying cause is warranted
  • The horse is in significant pain or distress and not improving with rest

Frequently Asked Questions

Can tying-up damage the kidneys?

Yes. In severe cases, the large quantities of myoglobin released from damaged muscle cells can overwhelm the kidneys, causing acute kidney injury. Dark, coffee-coloured urine is a warning sign and should be treated as an emergency. Prompt intravenous fluid therapy is the primary treatment to protect kidney function.

Is tying-up more common in mares?

The RER form appears to be more prevalent in fillies and mares, particularly those with nervous temperaments. Hormonal fluctuations during oestrus may increase susceptibility. The PSSM form has no sex predisposition.

What should I feed a horse prone to tying-up?

For horses with PSSM, a low-starch, high-fat diet is essential — reducing cereal grains and substituting fat (such as vegetable oil) and high-fibre forage. For RER horses, consistent feeding routine and reducing starchy feeds on rest days is important. A good-quality hay should form the basis of the diet. Always consult your vet or an equine nutritionist before making significant dietary changes.

How long does recovery from tying-up take?

Mild episodes may resolve within 24-48 hours with rest. Severe episodes can take days to weeks for muscle enzyme levels to normalise. Return to exercise should be gradual and guided by follow-up blood testing — do not resume work until CK and AST levels have returned to normal ranges.