Frequently Asked Questions

What is tying-up?

Tying-up is a broad term used to describe a variety of muscle disorders that can affect the performance horse. Other names given to this syndrome include exertional rhabdomyolysis, Monday morning disease, and azoturia.

What are the clinical signs of tying-up?

Clinical signs vary depending on the severity of the episode. In mild cases, affected horses may be slightly stiff after exercises. At the other end of the spectrum, pain may be so intense that affected horses may be unable to stand and bear weight. Exercise may elicit a short, stiff stride. Following rest, horses may be reluctant to move and may adopt an unusual stance; males often posture as if to urinate. Horses may also sweat liberally and more than the degree of exertion would dictate. Elevated heart and respiratory rates may also be experienced by horses affected with tying-up. In severe cases, horses may pass a dark, red-brown colored urine (myoglobinuria).

What is myoglobinuria?

Discoloration of the urine is due to the presence of a muscle protein called myoglobin. As muscle fibers become damaged, large quantities of myoglobin, as well as other muscles proteins, leak into the bloodstream and are excreted through the kidneys. If dehydration accompanies myoglobinuria, kidney damage may occur because of protein accumulation.

What muscles are affected by tying-up?

The muscles of the hindquarters are the most severely affected. The muscles are typically firm and painful on palpation. Cramping of the muscle may also occur during palpation.

How can tying-up be confirmed and differentiated from muscle soreness or strain?

Diagnosis of tying-up is confirmed by detection of elevated serum activities of creatine kinase (CK) and aspartate aminotransferase (AST) in the blood. CK and AST are muscle proteins that are released into circulation when muscle cells are damaged. Elevations in CK increase quickly following an episode of tying-up; peak values usually occur four to six hours following the attack. Trends in AST are realized more slowly with levels remaining elevated for seven to 10 days following an episode.

Equine athletes often encounter muscle strain. While muscle injuries can cause lameness and require a period of rest, pain is less severe than in tying-up. Most definitively, however, is the fact that CK and AST levels will be only mildly increased, if at all, in horses suffering from soreness or strain.

What causes tying-up?

Tying-up can be subclassified as either sporadic or chronic exertional rhabdomyolysis. Sporadic exertional rhabdomyolysis applies to horses that, on rare occasions, experience an episode of generalized tying-up. A common cause of sporadic tying-up is exercise that exceeds the level of fitness. Electrolyte imbalances, especially low sodium, and deficiencies of vitamin E and/or selenium may predispose a horse to tying-up. Horses with concomitant illnesses, such as a respiratory infection, may also be more prone to tying-up than healthy counterparts.

Horses that endure repeated episodes of tying-up, called chronic tying-up, may have limited athletic careers due to the start and stop nature of training schedules. Some horses are affected each time they exercise, while others might have only intermittent episodes. Over the years, chronic tying-up has been attributed to hormonal and electrolyte imbalances, lactic acidosis in muscles as well as vitamin E and/or selenium deficiencies, but few of these theories have been proven scientifically. Of these, however, electrolyte balance has been implicated most in the development of chronic tying-up.

Can electrolyte imbalances initiate chronic tying-up episodes?

Scientific investigations in England have suggested imbalances of electrolytes (sodium, calcium, and phosphorus) contribute to tying-up. Horses affected with tying-up showed marked improvement after sodium was added to the ration or the ration was balanced for a proper calcium:phosphorus ratio. In particular, endurance horses require electrolyte supplementation before, during, and after competition. Other equine athletes may need additional salt in their diets as well. In the United States, however, electrolyte imbalance does not seem to the be the primary cause of recurrent tying-up.

What is the difference between recurrent exertional rhabdomyolysis (RER) and polysaccharide storage myopathy (PSSM)?

RER is characterized by a defect in the mechanism of muscle contraction. The muscle is supersensitive to certain stimuli and responds to this stimulation by remaining in a state of contraction. A similar abnormality called malignant hyperthermia is common in humans and pigs. RER is a common cause of tying-up in Thoroughbred, Standardbred and Arabian horses. RER is most prevalent in young, nervous fillies. Interestingly, training at a gallop, but keeping the horse from top speed, seems to trigger an episode. Other factors that may incite tying-up include prolonged periods of inactivity, high-grain diets, excitement, and pre-existing unsoundness.

PSSM, on the other hand, is distinguished by accumulation of glycogen (a form of carbohydrate) and an abnormal form of polysaccharide (a complex carbohydrate) in muscle. PSSM primarily affects Quarter Horses and related breeds (Paints and Appaloosas), warmbloods, draft horses, and a few Thoroughbreds. PSSM can develop at a young age, frequently when a horse enters training. A notable precipitant of PSSM is exercise following a period of rest. Unlike RER, horses generally have a quiet disposition.

Is there a cure for tying-up?

No, there's no cure for recurrent exertional rhabdomyolysis or polysaccharide storage myopathy. Through regular exercise, daily turnout, changes in environment, and most notable, changes in diet, it has been possible for many horses with tying-up to return and to continue in athletic endeavors.