Tying-Up Syndrome

Fit horses are also at risk for tying up. A regular workout, or at least daily turnout for exercise, can help enormously.

The medical term for tying up is rhabdomyolysis-which means skeletal muscle (rhabdo) breakdown/damage (lysis). Tying up is characterized by muscle stiffness and pain, sweating, blowing, trembling, reluctance to move, and often discolored urine (brown) that's triggered by exercise. The horse may be agitated and even paw. Blood tests will show elevated levels of muscle enzymes.

Tying up is best described as an energy crisis in the muscle cell. It takes energy for the muscle to contract. However, it also takes energy for the muscle cell to be able to release the contraction. Tying up results when the energy supply to perform these functions is insufficient.

To understand and correctly treat or prevent tying up, you need to know the causes. Tying up can occur as an isolated event or be a recurrent problem. The causes of each are very different.

Causes of Isolated Episodes
Poor Conditioning/Overwork. During the process of properly conditioning/training a horse, his body goes through many changes. More blood vessels appear in the muscle, bringing the oxygen needed to efficiently generate energy. The muscle stores more energy in the form of fat around the muscle cells and glycogen directly inside the cells. Another very important change is that the levels of antioxidant enzymes, such as superoxide dismutase (SOD), glutathione, and glutathione peroxidase, also increase. When a horse burns glucose or fat as a fuel, an inevitable end product is "free radicals," forms of oxygen that are very unstable. They can attack and destroy the cellular machinery and membranes if not captured and neutralized by antioxidant defenses.

A horse who is taken out and worked beyond his level of conditioning can run into problems on two fronts. One is an energy shortage. When oxygen supply isn't optimal, glucose can still be used, but the pathways that don't require oxygen are less efficient and critical energy shortages develop more quickly. This is complicated by the fact that the unfit muscle has lower levels of glycogen in the first place. Another problem is the free radicals generated when fat or glucose is burned with oxygen. If the antioxidant protection systems aren't up to speed, cellular damage occurs.

Nutritional Antioxidant Deficiency. Vitamin E and selenium are the two antioxidant nutrients most often associated with muscle. Vitamin E can trap free radicals that are attacking cell membranes. Selenium is necessary for the functioning of the glutathione antioxidant system inside the cells. Good protection doesn't stop there. Vitamin C is important in helping to regenerate Vitamin E and other antioxidants, by taking the free radicals they've captured and restoring them to fighting form. The trace minerals copper and zinc are needed for the SOD enzyme systems to function. Manganese is the active metal inside a special form of SOD that traps free radicals inside the mitochondria, the "factories" inside the cell that burn fat and glucose.

More isn't better with antioxidant nutrients, because excesses won't be used. On the other hand, less isn't better either! Most hays and grasses contain more than enough manganese, but deficiencies of Vitamin E, selenium, zinc, and copper are common. Although a horse can manufacture his own Vitamin C, the level may be suboptimal for an exercising horse when he's on hay rather than fresh grass.