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.
- Tying up is best described as an energy crisis in the muscles.
- Incidences of tying up can be isolated or chronic.
- Don't overwork your horse as you try to get him in shape.
- Supplement your horse's diet with vitamins and minerals, as needed.
- A horse on good hay with a salt source should be able to maintain mineral and electrolyte balance.
- Try to give fit horses exercise time, even on their days off.
- If your horse ties up repeatedly, have him evaluated by your veterinarian for RER or EPSM.
- Carefully monitor exercise and diet for horses with recurrent tying-up issues.
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.
Electrolyte and Mineral Imbalances and Deficiencies. The minerals sodium, potassium, chloride, calcium, and magnesium all play important roles in the contraction and relaxation of muscles. Low sodium, potassium, or magnesium predisposes the muscle to cramping, as does high calcium relative to magnesium. Chloride comes into the picture, because when it's low, bicarbonate is high. High bicarbonate can tie up available magnesium-and calcium too, but it's usually magnesium that's in more marginal supply.
As long as a horse is getting generous amounts of hay and has access to salt, magnesium is the mineral most likely to contribute to muscular cramping at any level of exercise. When exercise duration or intensity increases and he's sweating, loss of the key minerals in sweat becomes an issue as well.
Symptoms of Inadequate Magnesium Intake
Horses vary quite a lot in how sensitive they are to low body levels of magnesium; that is, how likely they are to show symptoms. Some of this is just individual variation, some related to differences in other mineral levels in their diets. Either way, the point is that even though this is a nutritional issue, it's entirely possible to have only one horse in a group showing symptoms.
Signs that may be related to inadequate magnesium include jumpiness/spookiness, sensitivity to touch or sound, fine muscle twitching even at rest (commonly seen in the triceps muscle above the elbow, the chest, or in the quadriceps muscle above the stifle), and exercise-related muscle cramping. If you suspect this, try supplementing your horse with 5 to 10 grams of magnesium for a few days (ground-up human pills work fine for a trial). If it helps, talk to your vet or a nutritional consultant about determining an appropriate dose for long-term use.
Fit-Horse Tie Ups. Overworking an unfit horse may cause tying up, and so can underworking a fit one. Horses that are fit can be more prone to tying up if they spend one or two days confined to a stall, especially on a full grain feeding. This phenomenon has never been formally studied, but any trainer can tell you it's true. Some of these horses may have a mild form of genetic tying up (see below), or there may be changes in the amounts of rapidly broken-down glycogen types that build up in their muscles.
Minimizing Risks of Tying Up
Any horse could tie up for the reasons just listed. Fortunately, you can eliminate or minimize tying up related to those factors by following a few simple rules:
• Supplement horses in work with 1,000 IU of Vitamin E per 500 pounds of body weight. If you use powdered Vitamin E, mix it with a little oil for improved absorption.
• If your area is selenium deficient, supplement with 1 mg of selenium per 500 pounds of body weight, or as your vet recommends. Monitor blood selenium levels.
• Copper and zinc deficiencies are common. Supplement with 50 to 100 mg of copper and 150 to 300 mg of zinc per day. If using a multi-ingredient supplement, choose one that contains low manganese (e.g., much less than zinc).
• Never let your horse run out of salt. Consider adding 1 oz of salt (2 tablespoons) per day directly to meals year round to make sure your horse's appetite for salt stays normal. Monitor how long it takes him to eat free-choice salt supplies. In summer salt requirements may be double or triple the minimum 1 oz per day, even more if he sweats heavily.
• Consider adding magnesium if your horse has deficiency symptoms.
• Try to give very fit horses plenty of turnout time rather than stall confinement, especially on days they don't work. If you must confine them to stalls, drastically cut or eliminate grain on days they're not working, and at least hand walk them.
Repeated Tying-Up Problems
Horses with recurrent tying-up episodes are more likely to have a genetic problem with their muscle metabolism. One form is called Recurrent Exertional Rhabdomyolysis (RER). This is a problem in Thoroughbreds, Standardbreds, and possibly other breeds as well. Muscle contraction is triggered when calcium is released from storage areas in the muscle cell. RER seems to be related to an abnormally large and rapid release of calcium, or possibly a failure to take it back up. The exact details haven't yet been figured out.
Equine Polysaccharide Storage Myopathy (EPSM, also called Polysaccharide Storage Myopathy or PSSM) is another cause of recurrent tying-up episodes. In this disorder, the muscle cell stores abnormally high levels of glycogen and another form of stored carbohydrate called polyglucosan bodies.
Contrary to other types of glycogen storage disease in other species, horses with PSSM don't have any trouble utilizing glycogen (a storage form of glucose) for energy. In fact, they've been found to break down larger amounts of it than normal horses. These horses also have very "glucose hungry" muscles and take up glucose from the blood more easily than normal horses.
Until recently muscle biopsy was the only reliable way to differentiate between these conditions. Horses with RER have normal levels of glycogen in their muscles and damaged muscle cells on biopsy. Horses with polyglucosan bodies seen on biopsy have PSSM. There's currently controversy among researchers as to whether horses that have only increased glycogen but no polyglucosan bodies should also be diagnosed as having PSSM. Some say yes, and call this "type 2"; others say no.
Treatment and Prevention of RER and EPSM
Horses with severe RER can be treated with the drug Dantrium, generic name dantrolene sodium. I first used this drug in RER horses back in the 1970s, and it's experiencing a bit of a resurgence. Dantrolene works by slowing the release of calcium into the muscle cells. As is true of any drug, there's always a potential for side effects, and long-term use hasn't been studied in horses. Until we know more, its use should probably be reserved to treatment of acute tying-up episodes. If long-term use is elected, perform blood chemistry testing regularly.
Exercise is a very important part of managing horses with recurrent tying up. If at all possible, these horses should live outside 24/7, where they are free to move around. Formal exercise daily helps tremendously. In fact it's at least as important as diet, if not more so.
Limiting the amount of foods high in simple sugars and starch is also helpful in controlling symptoms. This means no (or very limited) grain and no molasses. The bulk of the diet should be hay or pasture. Fat is often used to replace grain calories. The amount of fat is a bit controversial.
Quarter Horses maintained in a program of regular work do well on much less fat than is commonly needed by other breeds. The usual recommendation is that 20% of the horse's daily calories come from fat. For a 1,000-pound horse, this is about two cups of oil (1 pound), or 5 pounds of a 20% fat feed, or 3.3 pounds of a 30% fat rice bran.
Why fat? By replacing grain with fat, glycogen won't accumulate as easily. Even more importantly, feeding fat trains the muscle to rely more heavily on fat than glucose as an energy source.
Interestingly enough, a feeding trial of horses with RER also showed lower muscle enzyme release when on a similar high-fat diet. This doesn't make much sense on the surface, since horses with RER don't have a higher reliance on glucose and glycogen like horses with PSSM do. However, studies in other species have found that high-fat feeding directly interferes with the muscle's ability to take in glucose. This could slow down energy production and therefore slow down contracture.
Finally, all the things we mentioned that can cause intermittent tying up would only make a horse with one of these conditions even worse, so pay attention to those, too.