Exertional rhabdomyolysis is technically defined as degeneration of muscle. Although often referred to as "myositis", this term refers to inflammation of muscle tissue. There actually is no evidence of acute inflammation present when biopsies of muscle tissue are examined. The muscle cells gradually degenerate and disappear due to a lack of energy. Many other names for this problem exist, ie. "tying up" - because of the characteristic muscle cramping that occurs; "azoturia" because of the dark colored urine that is sometimes seen; and "Monday Morning Syndrome" from the problem observed in draft horses that were traditionally worked every day except Sunday and then tied up on Monday. All these terms refer to a syndrome with clinical signs that may vary from slight stiffness and/or mild gait abnormalities to severe muscle cramping, pain, sweating, a reluctance or inability to move, or even recumbency. Signs may develop following episodes of strenuous exercise, just before exercise, or may have no association with the exercise routine at all.
What causes the changes in muscle tissue? It is now believed that the underlying problem in most cases is an abnormality at different steps within the metabolic process within muscle cells that provides energy for muscle contraction. This can be a temporary problem brought on by particular circumstances; it may be an inherited tendency; and/or it may be an actual defect in the biochemical process itself. The end result is the same, however, the muscle cells are starved for energy and cannot function normally. Muscle fibers contract and cannot relax, resulting in the cramping and pain seen clinically.
High carbohydrate diets, irregular exercise programs, exercise in an unfit or inadequately trained horse, selenium and vitamin E deficiency, electrolyte deficiency, hormonal influences, excessive exercise in otherwise fit horses, hot and/or humid conditions during exercise, and metabolic defects in muscle are some of the many causes or factors associated with the syndrome.
At this time, "tying up" seems to occur most often during the following scenarios:
- sporadic episodes in horses on an inadequate or inconsistent diet and/or exercise program,
- occasional episodes during or following strenuous exercise under hot and/or humid conditions or at anytime in an unfit horse,
- recurrent episodes before or after exercise or unrelated to the exercise routine.
Horses in scenario #1 may be on a diet high in protein, inadequate in electrolytes such as sodium or potassium, or out of balance for calcium and phosphorus. They may be fed more carbohydrate than is necessary for their exercise requirement. They may be stalled most of the time and exercised infrequently or irregularly. These cases are often resolved by balancing the diet, providing regular exercise and pasture turnout, and are not likely to recur.
Horses in scenario # 2 have been exercised beyond the physiologic capability of the muscle tissue. Years ago, this syndrome was not uncommonly seen in horses used for endurance. These equine athletes lose tremendous amounts of fluid and electrolytes in the sweat produced during exercise. They become dehydrated and deficient in potassium, calcium, and chloride. The muscle cannot function under these conditions, and cramping results. This syndrome is sometimes called "exhausted horse syndrome" as both the muscle and the horse are exactly that - exhausted. If recognized early, it is easily treated. If not, the condition can become life-threatening due to fluid and electrolyte losses. Other clinical signs may develop related to fluid and electrolyte loss, including colic and cardiac arrhythmias. With the increased knowledge of this condition, it is now unusual to see this syndrome in endurance horses. However, even well trained and talented endurance horses may occasionally suffer problems under hot and humid conditions.
Horses in scenario #3 are the most difficult and challenging to treat. Initially, these cases are treated similarly to #1. The diet and training program should be carefully regimented. Daily exercise is necessary, and turnout for free exercise is very important. Horses with recurrent problems have recently been identified as having a defect in the metabolic process within muscle cells. The muscle cells cannot utilize carbohydrate for energy. Western horses, draft breeds, warm bloods, and others have been diagnosed with this syndrome, called Polysaccharide Storage Myopathy (PSSM). These horses may have the classic signs of muscle cramping and pain. Others may just appear stiff or uncoordinated and/or may have exercise intolerance. Although the defect has not yet been specifically identified, it is known that these horses absorb glucose into cells much faster than normal horses. However, the glucose is not utilized normally. A side effect of this problem is a build-up of glycogen and an abnormal polysaccharide in the muscle cell. These accumulations can be identified on a muscle biopsy processed with special stains. The presence of these carbohydrates is not the problem but is rather a sign that a metabolic defect exists. Fortunately, the addition of fat to the diet as an alternate energy source will often alleviate the clinical signs and enable the horse to exercise effectively and live a reasonably normal life.
Thoroughbreds and Standardbreds are thought to have a defect in the coupling mechanism between proteins inside individual muscle fibers. These proteins are responsible for producing muscle contraction. Once coupled, changes in the cells occur that do not allow relaxation and clinical signs of myopathy appear. Some of these cases have responded to management changes, symptomatic therapy, and occasionally specific medications. This type of myopathy may be inherited as specific families of thoroughbreds have been identified with a greater incidence of cases than expected. Although this defect is different than that seen in horses with PSSM, some horses also respond to dietary changes with the addition of fat and elimination or reduction of grain. Interestingly, the families identified have been successful athletes. Therefore, this defect may accompany other favorable characteristics of muscle function that has enabled some individuals to excel.
Prevention and Treatment:
- Maintain a regular exercise program. Fitness is very important - be sure your horse has attained a basic level of fitness appropriate to the type of exercise you wish your horse to perform before introducing new routines or more difficult types of work.
- Minimize confinement; allow pasture turnout as much as possible. Some cases may need 24 hour turnout when not being ridden.
- Feed a balanced diet at appropriate levels - eliminate/minimize concentrate. Add fat to the diet - gradually add up to 2 cups of vegetable oil per day or feed a commercial rice bran supplement according to directions on the bag. Feed good quality hay. Some horses may not tolerate alfalfa as it can contain higher carbohydrate levels than grass hay.
- Always warm up prior to exercise for a period of time appropriate for the horses fitness level and the type of exercise.
- Consider electrolyte supplementation especially in the warm months of the year and especially for endurance exercise.
- Maintain adequate levels of selenium and vitamin E.