Cooperative Extension Fact Sheet FS1277

Nettie Liburt, Support Staff, Equine Science Center

Carey Williams, Extension Specialist in Equine Management

Muscles are complex tissues that require a multitude of different proteins, electrolytes, sugars, and repeated usage in order to function normally. There are many medical conditions that can directly affect muscle function.

When abnormal alterations occur in the physiology of muscles, a horse may experience painful, uncontrollable, and continuous muscle contractions. This condition is called tying-up, or rhabdomyolysis, and is comparable to severe cramps that a human might experience. During these episodes, muscles become damaged and unable to function properly.

Tying-up can be extremely painful and horses experiencing an episode can refuse to move, or even act colicky depending on the severity of the episode and the specific muscles that it affects. Tying-up can be triggered by strenuous exercise in an unfit horse, stress, or even dietary imbalances.

Defining Types of Rhabdomyolysis

Equine tying-up episodes can be categorized depending on the cause of the muscle damage:

1. Exertional rhabdomyolysis (ER)
   1. Sporadic rhabdomyolysis
   2. Chronic rhabdomyolysis
      1. Recurrent exertional rhabdomyolysis (RER)
      2. Polysaccharide storage myopathy (PSSM)
2. Non-exercise associated rhabdomyolysis
   1. Inflammatory myopathy
   2. Nutritional myopathy
   3. Toxic myopathy
3. Traumatic rhabdomyolysis

This fact sheet will focus on exertional rhabdomyolysis (ER). Exertional rhabdomyolysis refers to an episode of tying-up induced by activity and/or exercise. It has also been referred to as azoturia and “Monday Morning Sickness.” Tying-up can occur sporadically or chronically (recurrent).

Sporadic ER is likely to occur when a horse is asked to exercise excessively beyond his normal capacity, or when dealing with a lot of stress due to exercise or environment. While any horse may experience an episode of ER at some point during their lifetime, there are instances where horses will experience repeated episodes of ER. Repeated occurrence of ER is known as chronic ER.

Chronic ER can be broken into two specific diseases: Recurrent exertional rhabdomyolysis (RER) and Polysaccharide storage myopathy (PSSM). In cases of RER, further investigation into the cause of repeated episodes is required. Some research suggests that RER can be caused by abnormal regulation of calcium concentration inside the muscle cell. (Calcium plays an important role in normal muscle contraction and relaxation). In other cases, horses lack the ability to utilize sugar appropriately within their muscles, effectively starving the muscles and creating severe damage during activity. This condition is called PSSM.

PSSM can be classified into two categories, Type I and Type II. Type I PSSM horses have a genetic mutation of the GYS1 gene that causes an enzyme to excessively convert sugar into its storage form (glycogen), rendering the sugar unavailable to meet the energy needs of the muscle. The mutation that causes Type 1 PSSM is a dominant trait that can be passed to a foal from an affected mare or stallion. Type II PSSM horses lack this genetic mutation but still experience abnormal sugar storage in the muscle, making it unavailable for use as energy. The cause, or causes, of Type II PSSM are currently unknown. Both types of PSSM horses will have similar clinical signs of tying-up.

What to Look For

When a horse ties-up, he/she experiences a variety of physical symptoms. Some are obvious, while others require careful examination skills. Some signs are so mild that only medical diagnostics can determine if a horse is or has experienced an episode.

Physical Signs

• Typically occurs shortly after the onset of exercise or after exercise completion.
• Slight, sometimes unnoticeable cramping, or in more severe cases the horse may be reluctant to move.
• Lower back, gluteal, and thigh muscles become firm and painful.
• The gait becomes short and stiff.
• Excessive sweating, quick, shallow breathing, and an increased heart rate are observed, mimicking colic signs.
• Dark brown or reddish-colored urine in severe cases, indicating the breakdown of a muscle protein called myoglobin. Myoglobin is toxic to the kidneys.

Diagnostics

• Blood work reveals increased concentrations of the enzymes creatine kinase (CK), aspartate transaminase (AST), and lactate dehydrogenase (LDH), among others. These enzymes are normally found in muscle cells, and are released when muscle cells become damaged during an episode of tying-up. As a result, abnormally high concentrations of CK, AST, and LDH will be measured in the blood.
• A urinalysis is also commonly performed to detect electrolyte imbalances, as well as the presence of myoglobin, which would also indicate tying up.
• Muscle biopsy samples are taken from the semimembranosis or semitendonosus muscle (hamstring muscles) and stained and observed under a microscope. Muscle tissue from horses with RER looks different from that of horses with normal muscle tissue, and a trained scientist or veterinarian can tell the difference.
• Genetic testing is accomplished by having your veterinarian collect a sample of whole blood or hair with roots still attached and sending it to a genetic testing laboratory. Analysis will detect the presence or absence of a mutation of the glycogen synthase-1 gene.

Treatment

Tying-up is considered a veterinary emergency, especially if the horse is exhibiting signs such as profuse sweating, reluctance to move, and dark urine. A veterinarian should always be consulted when it is suspected that a horse is experiencing an episode of tying-up. Some episodes require immediate veterinary attention while mild RER cases can be handled by an experienced horseperson. When any exercise-induced tying-up episode occurs, the activity should be immediately stopped and the horse evaluated. Treatments can vary from case to case, depending on the cause, however general parameters for treatment include:

1. Maintain hydration
2. Replenish electrolytes
3. Provide pain management
4. Provide muscle relaxation
5. Repeatedly monitor blood and urine indicators of muscle damage
6. Prevent further muscle damage while promoting blood flow and muscle movement
7. If necessary, gently rub the horse dry, then blanket if temperatures warrant

Management

Management of a single tying-up episode or of a chronic case is dependent on the cause. Training programs should be designed with care, and intensity and duration increased slowly to avoid overexertion, particularly in a horse with chronic ER.

Often, incidences of tying-up are triggered by a stressful event (trailering, separation from stablemates, overexertion) or environment, so it makes sense to keep the horse’s living space as comfortable as possible (see Rutgers NJAES fact sheet FS656 on managing stress). Some ways to do this include feeding the at-risk horse first before its barn mates, providing company when alone indoors, or even using mild sedatives if necessary.

Of course, a nutritionally-balanced diet that is low in non-structural carbohydrates (starch and sugar) is essential for assuring optimum health, but can also prevent electrolyte imbalances and vitamin deficiencies. Electrolytes, such as sodium, potassium, and calcium, are crucial for proper muscle function, and must be provided especially when a horse is sweating excessively and working hard. Adding vitamin E to the diet of horses prone to tying-up is also recommended to increase the muscle membrane integrity. Amounts up to 5,000 IU per day of vitamin E have been found to be effective at decreasing muscle enzyme leakage out of the cells.

Insulin is a hormone that allows the body to take the sugar (glucose) from dietary carbohydrates and store it in the form of glycogen for future use. Horses with PSSM have an enhanced ability for glucose storage into skeletal muscle, as well as a higher than normal sensitivity to insulin. For this reason, starch and sugar should make up no more than 10–15% of the diet for PSSM horses, and 20% for RER horses. The energy density of the diet can be increased by supplementing either fat from oil or rice bran, or low-carbohydrate, highly digestible fiber, such as beet pulp or soy hulls. Research indicates that equine diets that are low in carbohydrates and higher in fat may reduce incidences of tying-up, possibly because of the increased availability of free fatty acids as energy sources, and a lower response of insulin to a higher fat diet (thus less glycogen accumulation in muscle cells).

Take-Home Message

This fact sheet was intended to help you understand more about tying-up, as well as the causes, diagnostics, and management of the condition. This information is not a substitute for veterinary care; if you suspect any problem with your horse you should IMMEDIATELY contact your veterinarian.

Tying-up is a condition that causes painful muscle cramping in the horse. While the condition can take on various forms, proper management, training, and a balanced diet are essential components to minimize episodes. Increase the demands of exercise slowly and minimize stress in the horse’s environment. Knowing exactly what form of ER a horse has been diagnosed with makes management considerably easier. Proper diagnosis and veterinary treatment will help the horse to avoid symptoms and live more comfortably. Treatment of horses that experience tying-up often comes down to good management, but should be guided by your veterinarian and a professional nutritionist. You know your horse best, and always consult your vet if you have concerns about your horse’s health.

References and Suggested Reading

1. Piercy, RJ, and JL Rivero. 2004. Muscle disoders of equine athletes. In: Equine Sports Medicine and Surgery. KW Hinchcliff, AJ Kaneps and RJ Geor, eds. Saunders, London, England. Pg. 81–92.
2. Valberg, S.J. 2016. How to feed horses with exertional myopathies. Proceedings of the 2016 American Association of Equine Practitioners Convention. 62:520–522.
3. Valberg, S. 2016. Michigan State University College of Veterinary Medicine Equine Neuromuscular Diagnostic Laboratory website. Accessed on December 29, 2016 at: cvm.msu.edu/research/faculty-research/valberg-laboratory
4. Valberg, S. 2005a. Equine exertional rhabdomyolysis: management of sporadic and recurrent exertional rhabdomyolysis. Mid-Atlantic Nutrition Conference Proc. Pg. 197–203.
5. Valberg, S. 2005b. Equine exertional rhabdomyolysis: management of polysaccharide storage myopathy. Mid-Atlantic Nutrition Conference Proc. Pg. 204–208.
6. Williams, C. A. 2005. Are you ‘Stressing Out’ your horse? Rutgers Cooperative Research and Extension. FS656. New Brunswick, NJ. Pg. 1–4.
7. Williams, C. A., D. S. Kronfeld, T. M. Hess, K. E. Saker, and P. A. Harris. 2004. Lipoic acid and vitamin E supplementation to horses diminishes endurance exercise induced oxidative stress, muscle enzyme leakage, and apoptosis. In: The Elite Race and Endurance Horse. Ed. Arno Lindner. CESMAS, Oslo, Norway. pp. 105–119.

Photo credits: C. Williams.

August 2017