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What is Tying Up?

Chronic exertional rhabdomyolysis has four types, which include type 1 polysaccharide storage myopathy (PSSM), type 2 PSSM, malignant hyperthermia, and recurrent exertional rhabdomyolysis. These are usually considered hereditary and are more common in certain breeds such as Standardbreds, Western pleasure horses, Warmbloods, Morgans, Arabians. Draft horses, Quarter horses, or Thoroughbreds. Although many of the types of exertional rhabdomyolysis are genetic, acute ER can happen to any horse that is worked harder than usual, overheated, or has a diet or electrolyte imbalance.

Tying up, or external rhabdomyolysis (ER), is a muscle disorder that causes severe muscle cramps, pain, and stiffness, sometimes progressing to muscle damage, renal failure, or heart problems due to electrolyte imbalances. Acute exertional rhabdomyolysis is referred to as sporadic exertional rhabdomyolysis, which can happen after a day of strenuous exercise or the loss of certain vitamins or water (such as with heat exhaustion).

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Symptoms of Tying Up in Horses

Each type of ER has its own symptoms, which are:

Type 1 Polysaccharide Storage Myopathy

  • Abdomen tucked in
  • Muscle twitching
  • Excessive perspiration
  • Abnormal gait
  • Stiff hind legs
  • Not wanting to move
  • Rolling or pawing

Type 2 Polysaccharide Storage Myopathy

  • Stiff muscles
  • Poor performance
  • Abnormal gait
  • Exercise intolerance
  • Dark or red tinted urine
  • Muscle cramping
  • Loss of muscle mass

Malignant Hyperthermia

  • Fast heart rate
  • Abnormal heartbeat
  • Increased body temperature
  • Rigid muscles
  • Coma
  • Sudden death
  • Recurrent exertional rhabdomyolysis
  • Muscle contractions
  • Poor performance
  • Abnormal gait

Sporadic Exertional Rhabdomyolysis

  • Muscle spasms
  • Stiff muscles
  • Sweating
  • Rapid and irregular heart rate
  • Reluctance to movement
  • Renal damage in severe cases (lack of urination, increased sleepiness)


  • Type 1 polysaccharide storage myopathy is seen most often in Morgans, Western pleasure horses, Draft horses, and Quarter horses, but can affect any breed
  • Type 2 polysaccharide storage myopathy is found mostly in Quarter Horses, Warmbloods, Thoroughbreds, Morgans, and Arabians, but can affect any breed
  • Malignant hyperthermia is responsible for both anesthesia-related and non-anesthesia-related causes of rhabdomyolysis in Quarter horses
  • Recurrent exertional rhabdomyolysis affects mainly Arabians, Standardbreds, and Thoroughbreds
  • Sporadic Exertional Rhabdomyolysis can affect any breed, sex, or age of horse

Causes of Tying Up in Horses

  • Type 1 polysaccharide storage myopathy is a hereditary mutation
  • Type 2 polysaccharide storage myopathy is a negative GYS1 genetic disease
  • Malignant hyperthermia is caused by an autosomal mutation in the muscle ryanodine receptor gene
  • Recurrent exertional rhabdomyolysis is due to abnormal regulation of calcium in your horse’s muscles
  • Sporadic exertional rhabdomyolysis is usually caused by exercising more than usual, but may also be from dietary deficiencies, heat exhaustion, or electrolyte imbalances

Diagnosis of Tying Up in Horses

If your horse is one of the commonly affected breeds such as Standardbreds, Western pleasure horses, Warmbloods, Morgans, Arabians. Draft horses, Quarter horses, or Thoroughbreds, the veterinarian will automatically want to check for exertional rhabdomyolysis. After describing your horse’s past medical history and immunizations, the veterinarian will do a comprehensive physical examination which will likely include a lameness evaluation and exercise assessment. Your horse’s vital signs will be checked and recorded and radiographs (x-rays) or a body scan may be done. Although the majority of laboratory tests will likely be normal, there are some tests that will help in assessing the problem.

For example, if your horse has exertional rhabdomyolysis, the lactate dehydrogenase (LDH), aspartate aminotransferase (AST), and creatine kinase (CK) will all be increased. In addition, the urinalysis will show high amounts of myoglobin in horses with moderate to severe ER. In some cases, genetic testing, muscle biopsy and exercise tolerance testing may be beneficial in determining the type of ER your horse has. Your horse’s CK will be analyzed before and after exercise to verify. This test is also helpful in planning a training program for your horse.

Treatment of Tying Up in Horses

Treating ER depends on the type and cause of the condition, but will most often include hydration and electrolytes, medication, special diet, and a training plan.

Fluid Therapy

The veterinarian will likely give your horse intravenous fluids and electrolytes to reduce dehydration and replenish lost minerals and nutrients.


Treating anxiety and pain may be done with tranquilizers or sedatives such as butorphanol, detomidine, phenytoin, dantrolene, xylazine, or acepromazine. If the case is a mild one, nonsteroidal anti-inflammatory drugs (NSAIDS) may be the only medication needed. Vitamin E supplements can also be given, but you have to follow the veterinarian’s instructions because too much can be worse than not having enough.

Special Diet

The best diet for your horse depends on which type of ER your horse has. For polysaccharide storage myopathy, a diet high in fat and low in starch (carbohydrates) is good. However, if your horse has recurrent exertional rhabdomyolysis, the best choice of diet is similar, but with more calories, nonstructural carbohydrates, and an easily digested energy source.  

Training Plan

Your veterinarian will help with a training plan that is specific to your horse. However, the most common training plan includes several weeks to months of rehabilitation. This may include short walks several times per day and working up to more strenuous exercise.

Recovery of Tying Up in Horses

Prognosis depends on what type of exertional rhabdomyolysis your horse has, but with mild to moderate cases, your horse has an excellent chance of complete recovery with no complications. If your horse has an extreme case of ER with complications, your veterinarian will work with you to get your horse back in shape as much as possible.