Tying up, or Exertional Rhabdomyolysis, in horses is characterized by muscle pain, stiffness, excessive sweating, and a reluctance to move associated with exercise. Once known as “Monday Morning Sickness”, it is not the simple muscle soreness a person might experience the day after running or lifting weights, especially if they hadn’t done so in a while.
Tying up in horses can be a serious, even life-threatening condition with multiple causes. This article will describe the two main types of tying up (sporadic and chronic), the underlying causes, the breeds of horse affected, and the signs and symptoms. The diagnosis and treatment, as well as specific diet and management practices in the hopes of preventing tying up, will also be covered.
Tying up has been recognized as a serious condition in horses for over a century. The name “Monday Morning Sickness” comes from the time when draft horses were worked six days a week and given Sunday off to rest in their stalls while fed their normal ration of grain. These horses often displayed signs of tying up when asked to work again on Monday morning. The trigger for an episode seemed to be the combination of returning to work or exercise after a period of enforced idleness while on a high-grain diet.
For many years, tying up was thought to be the result of the build-up of lactic acid in muscles. However, due to advances in exercise physiology and muscle diagnostic testing, it has now been shown that lactic acid build-up in horses does not cause tying up. In fact, veterinarians and scientists now know that tying up is a syndrome with two main types – sporadic and chronic -- and multiple causes within each type.
The term “Sporadic Tying Up” is used when a horse has a single episode not because of an internal defect in their muscles, but because something external -- such as exercise, the environment, or the diet -- triggered muscle damage. Sporadic tying up is a temporary or occasional problem in muscle cells that may be caused by things like by fatigue, heat exhaustion, or electrolyte imbalance. It may occur in any age, breed, gender, or discipline and is not an inherited condition. Examples include a polo pony used in a match before he was fit enough to compete, a three-day eventer on an especially hot and humid day, or an endurance horse depleted of electrolytes after a 50-mile race.
On the other hand, “Chronic Tying Up” is an internal problem with the muscle tissue itself. It may still be triggered by exercise and affected by diet, management, and other factors. However, with this form of tying up there is some underlying defect in muscle structure or function causing horses to have repeated episodes.
To further complicate matters, there are different forms of chronic tying up, including Recurrent Exertional Rhabdomyolysis (RER), Polysaccharide Storage Myopathy (PSSM) Types 1 and 2, Myofibrillar Myopathy, and Malignant Hyperthermia.
Recurrent Exertional Rhabdomyolysis (RER) is the term used to describe repeated episodes of tying up specifically due to an inherited abnormality with how calcium is regulated in muscle. This form of chronic tying up has been studied largely in Thoroughbreds, but has also been found in Standardbreds and Arabians. Other breeds that may be affected include Quarter Horses and warmbloods.
The type of horse most associated with this disease is the young (two-year-old), nervous, high-strung thoroughbred filly at the track. Excitement, especially when combined with exercise, seems to be a common trigger for an RER episode in genetically susceptible individuals. Other triggers include stress, lameness, high-grain diets, and specific types of exercise that may create anxiety, such as interval training, being held back (restrained) from a full gallop, and galloping with one or more other horses.
With Polysaccharide Storage Myopathy, or PSSM, the abnormality lies in the way muscle cells handle and metabolize energy. With this form of chronic tying up, glucose is either packaged into glycogen incorrectly and then stored in excessive amounts in muscle (PSSM Type 1), or simply stored incorrectly as “clumps” of glycogen (PSSM Type 2).
Certain breeds will display signs and symptoms differently. For example, Quarter Horses tend to show classic signs of tying up (muscle stiffness, cramping, and pain) with either type of PSSM. Signs in other breeds, such as warmbloods, are more along the lines of poor performance, vague or undiagnosed gait abnormalities, a reluctance to collect and engage, loss of muscle mass, progressive weakness, and recumbency (lying down).
PSSM is included in this article because it causes signs of classic tying up in some breeds. For more information about this specific condition; the different types; and its diagnosis, treatment, and management; see the separate article on this topic.
The classic signs of a horse experiencing an episode of tying up usually occur shortly after the beginning of exercise and include:
A horse exhibiting signs of tying up should be handled as an emergency. If the horse is currently under saddle or being exercised, any activity must immediately be stopped. Next, call the horse’s veterinarian and describe the situation. While waiting for the vet to arrive, make the horse more comfortable by removing tack, blanketing if it’s cold or providing shade if it’s hot, and holding up water for the horse to drink (he may not be able to lower his head to the ground). If possible, try not to move the horse. Instead, make his surroundings safe and quiet by removing other horses and activities. Follow any specific instructions from the vet such as administering medications, taking vital signs, etc. Do not offer any hay or grain at this time.
When the veterinarian arrives, they will assess the horse, take steps to relieve pain and anxiety, and possibly administer fluids if dehydration is an issue. With vet guidance, the horse may be put into a stall or other confined area since the horse should move as little as possible for the next 24 - 48 hours. Blood samples may be drawn immediately as well as later in the episode to confirm the diagnosis. Looking at levels of two indicators of muscle damage, creatine kinase (CK), which peaks 6-12 hours after an episode, and aspartate transaminase (AST), which peaks 24 - 48 hours after an episode may give a better understanding of the episode.
If this is the first time this horse has experienced a bout of tying up, the veterinarian may ask questions to try and figure out what triggered the episode. The diagnosis may be confirmed as “sporadic tying up” and therefore recommendations for the horse may include:
In addition, the vet may recommend a gradual return to exercise, beginning with something as simple as small paddock turnout or hand-walking. The vet may perform rechecks of CK and AST blood levels during this time to know when the horse is ready for more work.
If the horse has had several episodes of tying up in the past, then the veterinarian may lean toward “chronic tying up” as a diagnosis. In this case, they may recommend additional diagnostic testing based on the horse’s history, age, breed, gender and other factors. Some of these extra tests include genetic assays (test) of blood or hair, muscle biopsy, exercise challenge, and more. It is important to get to the root cause of chronic tying up as the treatment and management for one type -- for example, RER -- is different from the treatment and management for other types, such as PSSM1 or PSSM2.
The focus of this section is on horses with the RER form of chronic tying up for two reasons. One, horses with sporadic tying up may simply need a nutritionally balanced diet with optimal levels of vitamins and minerals along with a thoughtful turnout schedule, conditioning program, and competition schedule -- taking into account extreme changes in weather or terrain -- in order to prevent future episodes of tying up. Two, horses with either type of PSSM have very specific diet and management recommendations covered in detail in a separate article.
Horses prone to chronic tying up due to RER seem to do better when the sugars and starches in the diet are limited. This may be done by making high-quality forage the foundation of the diet; rounding out the protein, vitamins, and minerals with a low-NSC grain or ration balancer (not sweet feed) and supplementing with fat if the horse needs more calories. Sources of fat include stabilized rice bran, commercial feeds specifically made for these horses that are low in sugar and high in fat, and powdered fat or oil supplements. When adding fat to the diet of a horse with RER, here are three things to consider:
Finally, make sure the horse is getting at least 10 grams of sodium daily by topdressing about 2 tablespoons of table salt with regular meals. When the horse is sweating heavily from intense exercise or hot temperatures, supplementing with additional sodium and/or a commercial electrolyte supplying potassium, calcium, and magnesium in addition to sodium and chloride may also be appropriate.
For many RER horses, the combination of stress and exercise may lead to an episode of tying up. The following suggestions from the Michigan State University College of Veterinary Medicine specifically target the Thoroughbred racehorse lifestyle but may be used to help avoid triggering an episode of tying up in any horse prone to recurrent exertional rhabdomyolysis:
In general, horses that experience the RER form of chronic tying up should receive daily exercise in some form, such as turnout, lunging, and/or riding. Consistent, calm exercise is an important part of preventing an episode of tying up in these horses.
Article First Published 2012