1. What is exertional rhabdomyolysis (ER)?What is exertional rhabdomyolysis (ER)?
  2. What are the symptoms of ER?What are the symptoms of ER?
  3. How do I know if my horse is having an episode of tying-up?
  4. How do I know what caused ER?
  5. What if the episode occurred weeks ago and I want to know if my horse is susceptible to tying-up?
  6. Can my horse's muscles heal after an episode of tying-up?
  7. What causes ER?
  8. How do I manage a horse with sporadic ER?
  9. What causes chronic ER?

1. What is exertional rhabdomyolysis (ER)?

Exertional rhabdomyolysis (ER) or “tying-up” has been recognized in horses for more than 100 years as a syndrome of muscle pain and cramping associated with exercise. ER continues to be a performance-limiting or career-ending disorder for many equine athletes. The term ER literally means the dissolution of muscle cells with exercise. Through Dr. Valberg’s research it is clear that ER is a syndrome, meaning that there are several different causes for the clinical signs of muscle pain that typify ER. By determining the underlying form of ER that a horse is suffering from, specific management strategies can be employed to control symptoms.

2. What are the symptoms of ER?

Clinical signs of ER usually occur shortly after the beginning of exercise. The most common sign is firm and painful muscles over the lumbar (loin) and sacral (croup) regions of the topline, including the large gluteal muscles. Excessive sweating, quick, shallow breathing, rapid heart rate, and muscle tremors are also noticed. In extreme cases, horses may be reluctant or refuse to move and may produce discolored urine due to the release of myoglobin from damaged muscle tissue. Episodes of ER vary from subclinical to severe in which massive muscle necrosis and renal failure from myoglobinuria occurs.

If a horse shows signs of ER a veterinarian will determine how severe the problem is, relieve the muscle pain and cramping, provide oral or intravenous fluids to correct dehydration and flush myoglobin from the kidneys and put a management plan in place.

A horse show classic signs of ER; pain, stiffness, sweating and reluctance to move.

3. How do I know if my horse is having an episode of tying-up?

A blood sample can be used to determine what is happening inside your horse’s muscles. Proteins measured in a blood sample can determine the extent of the muscle damage. This is a separate test from genetic testing for PSSM1 and is used to see if muscle damage has occurred.

Two muscle proteins often used as a measure of muscle damage include creatine kinase (CK) and aspartate transaminase (AST). When muscle cells are damaged, they release these proteins into the bloodstream within hours. A blood sample taken to measure these proteins can determine how much muscle damage has occurred. This is not a specific test for PSSM1 as the presence of CK and AST in the blood occurs with any muscle damage. With many forms of tying-up, blood CK activity returns to normal within days if horses are rested. It is very common for horses with PSSM1 to have high CK activities even if they are rested for weeks after an episode of tying-up.

4. How do I know what caused ER?

To determine the specific cause of ER a complete veterinary evaluation is needed. Depending on the horse’s breed, age, the degree of recurrence of ER and serum CK and AST activity, veterinarians will select a specific diagnostic plan that may include and exercise challenge, genetic testing or muscle biopsy. Diagnostic work up of myopathies

5. What if the episode occurred weeks ago and I want to know if my horse is susceptible to tying-up?

One way to see if tying-up occurs is to do an exercise test. This is useful if a horse shows no signs of muscle stiffness when a veterinarian performs a physical exam but still suspects the horse suffers from a form of tying-up. The horse should be worked in a round pen at a walk and trot for 15 minutes. If the horse shows any signs of persistent muscle stiffness, the test should stop. The idea here is not to produce signs of stiffness, but to see if this mild form of exercise cases slight muscle damage. In cooperation with your veterinarian, have a blood sample drawn 4-6 hours after the exercise test. A significant result indicating sub clinical muscle damage would be a serum CK activity greater than 800 U/L 4 hours after 15 minutes of exercise at a trot. The blood sample is not informative if taken right after exercise, as the CK protein has not had enough time to leak out of the muscle cells.

For more details on the exercise challenge test, see the recommended diagnostic work-up.

6. Can my horse's muscles heal after an episode of tying-up?

Muscle has a remarkable ability to repair itself. After an episode of tying-up, the muscle cells usually heal completely within three to four weeks without any scarring. If the damage is severe, you may notice that the horse's muscle mass decreases in size as the body removes the damaged proteins. The muscle mass usually returns within two to four months.

7. What causes ER?

ER initially can be subdivided into sporadic and chronic forms. Horses that experience a single episode or infrequent episodes of ER are categorized as having sporadic ER, whereas horses that have repeated episodes of ER accompanied by increased muscle enzyme activity, even with mild exertion, are classified as having chronic ER.

Sporadic Exertional Rhabdomyolysis
Horses with sporadic ER have no intrinsic defect in their muscle. Rather something extrinsic in the environment has caused muscle cells to be damaged. Sporadic ER occurs most commonly in horses that are exercised in excess of their level of conditioning. This happens frequently when a training program is accelerated too abruptly, particularly after an idle period of a few days, weeks or months. Endurance competitions held on hot, humid days may elicit sporadic ER in susceptible horses because of high body temperatures, loss of fluid and electrolytes in sweat, and depletion of muscle energy stores. These metabolic imbalances can lead to muscle dysfunction and damage. In some instances, horses seem more prone to ER following respiratory infections. Therefore, horses should not be exercised if they have a fever, cough, nasal discharge, or other signs of respiratory compromise.

8. How do I manage a horse with sporadic ER?

With the first episodes of ER horses should be rested for the first 12-48 h in a stall until they can move about comfortably without pronounced stiffness. Small paddock turn out where the horse is quiet can be provided. Hand walking should be done with caution as even 10 min of forced walking can trigger another episode of ER at this stage. Re-examination by a veterinarian one to two weeks after the episode of ER will determine when to put the horse back into a gradual exercise program.

Nutritional Management of Sporadic ER
A well-designed exercise program and a nutritionally balanced diet with appropriate caloric intake and adequate vitamins and minerals are the core elements of preventing future episodes of ER.

Vitamin E and selenium
Adequate amounts of vitamin E and selenium prevent the detrimental interaction of peroxides with lipid membranes of the muscle cell. In adult horses, vitamin E and selenium deficiency don’t likely cause ER but deficiencies are not good for normal muscle function and recovery. Many feeds, particularly those designed for horses with ER provide adequate selenium supplementation and caution should be taken not to provide excessive selenium in the diet. Vitamin E is provided in most diets by green grasses, well-cured hay, and rice bran. The best approach is to measure whole blood selenium and serum vitamin E to determine if a horse is deficient and then to design supplementation based on those levels. More information about selecting the correct Vitamin E supplement can be found here.

Electrolytes and minerals
Horses performing in hot weather often develop electrolyte imbalances, particularly if exercise continues for several hours. Free-choice access to loose salt or a salt block should be provided to these horses, or alternatively, one to four ounces of salt can be added to the feed daily. Extreme climatic conditions may necessitate the use of commercial electrolyte mixtures containing a 2:1:4 ratio of sodium:potassium:chloride. Fresh water should be available to horses at all times, especially if they are being supplemented with electrolytes.

Dietary imbalances of electrolytes, particularly deficiencies of sodium, potassium, and calcium, have been implicated in ER (Harris 1991). Correction of imbalances may be crucial in the management of some ER cases.

Exercise programs
Similar to humans horses need to be conditioned in order to comfortably perform a given level of exercise. Exercise regimes should gradually be increased after an episode of ER and a program designed to incrementally increase the duration and intensity of exercise to ensure adequate fitness and prevent further ER.

9. What causes chronic ER?

There can be several causes for chronic ER in horses. These include recurrent exertional rhabdomyolysis (RER), type 1 polysaccharide storage myopathy (PSSM1), type 2 polysaccharide storage myopathy (PSSM2), malignant hyperthermia (MH) and myofibrillar myopathy (MFM).