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.
