By Stephanie Valberg, DVM, PhD
I just bought a Thoroughbred filly, which was in race training, but retired without running because of recurrent bouts of “tying-up.” She had been turned out for six months before I got her, and never showed any problems while on the farm. What can you tell me about this disease?
Tying-up is a syndrome or description of a horse with muscle damage that has many different causes. It probably is one of the most misunderstood and controversial syndromes in the athletic horse. Since there are several causes, some of which appear to be inherited, there is no single cure. Typical signs of tying-up include a horse which becomes stiff, sweats, and is reluctant to move. Researchers have learned a great deal about tying-up—or exertional rhabdomyolysis—in recent years. Unfortunately, the information has shown that some of the most common beliefs about tying-up have been proven wrong by scientific study. Thus, what was considered by some early researchers to be a problem that had one basic cause, e.g. lactic acid, is actually a broad-scale syndrome that will require continued research on a variety of fronts before every aspect is understood.
In other words, tying-up is not one disease, but several different diseases that have similar signs and different causes. Therefore, the management of a Thoroughbred that suffers from tying-up would differ from the management of a Quarter Horse that is tying-up, would differ from the management of a backyard pleasure horse that has the same symptoms.
Some horses are healthy athletes that tie-up sporadically likely due to exercise in excess of their training level, electrolyte depletion, or dietary imbalances. They respond well to rest, a gradual return to a graduated training regime, and balancing the diet. Other horses will suffer from chronic episodes of tying-up that can be debilitating. Our research suggests that there might be several inherited reasons for chronic tying-up.
Certain lines of Thoroughbreds seem more susceptible to one form of tying-up that has a basis in abnormal regulation of muscle contraction. Muscle contractions are initiated by propagation of electrical impulses along the outer cell membrane and along membranous connections extending into the cell, which then stimulate the release of calcium from intracellular membranous storage sites. Muscle relaxation requires energy-dependent pumping of calcium back into storage sites.
Our research indicates that a common cause of tying-up in Thoroughbreds is an inherited abnormality in the way calcium is regulated by membrane systems in the skeletal muscle. The narrow genetic origin of Thoroughbreds and the common lineage of the pedigrees of horses with tying-up would support the possibility of an inherited trait. The disease might lie dormant unless specific factors trigger the calcium regulatory system to malfunction. Triggering events include stress, excitement, lameness, high grain diets, and exercise at submaximal speeds.
Young fillies are most commonly affected and usually are the most nervous and high strung. Prevention of further episodes of tying-up in susceptible horses should include standardized daily routines and an environment that minimizes stress and excitement. The diet should be adjusted to include a balanced vitamin and mineral supplement, high-quality hay, and a minimum of carbohydrates (such as grain and sweet feed). Feeding less than five pounds of sweet feed and adding additional calories in the form of fat decrease muscle damage with exercise. Daily exercise is essential, either in the form of turnout, longing, or riding. Sometimes medications such as dantrolene given to fasted horses 90 min before exercise can be helpful in preventing anticipated episodes of tying-up.
Another form of tying-up is polysaccharide storage myopathy (PSSM). It is characterized by the accumulation of glycogen (storage form of sugar) and an abnormal sugar (polysaccharide) in skeletal muscle. The recent identification of a genetic mutation for PSSM by Drs. McCue, Mickelson and Valberg the University of Minnesota revealed that there are actually at least two forms of this disease. A form of PSSM called type 1 PSSM is caused by a mutation in the glycogen synthase gene. Type 1 PSSM occurs in many Quarter Horses and related breeds, draft breeds, some warmblood breeds and several other breeds as well. It has not been identified in Thoroughbreds to date. This mutation causes PSSM horse’s muscle to continually make glycogen. Consequently when beginning exercise they have trouble switching over to burn glycogen for energy rather than storing glycogen in their muscles. Affected horses develop stiffness, muscle cramping and soreness with light exercise probably due to a deficit of energy generation in their muscles. Some horses with this disorder also have an increased sensitivity to insulin further increasing storage of sugar (glycogen) in the muscle. PSSM type 1 horses are often calm, sedate horses that tie-up after a lay-up, especially when fed grain. They tend to be found in disciplines such as halter and pleasure horse performance and do not usually perform well at speed. Type 2 PSSM has a slightly different microscopic appearance and is due to a yet unknown genetic cause. It occurs in Quarter Horse-related breeds, Warmbloods and likely other light horse breeds possibly including Thoroughbreds. While type 1 PSSM can now be diagnosed by a DNA-based blood or hair root test, type 2 PSSM still requires examination of a muscle biopsy. Horses with severe signs of PSSM should also be tested for a second genetic mutation called MH. When MH and type 1PSSM occur together horses may develop severe episodes of tying up which can be fatal.
Treatment of both forms of polysaccharide storage myopathy involves supplying horses with an alternative source of energy such as fat rather than sugar. Eliminating grain and sweet feed completely and feeding fats such as rice bran or vegetable oils stabilizes blood sugar and provides fat for energy metabolism. It is essential that horses with PSSM be turned out as much as possible and exercised often, even if only for 10 minutes every day. Horses with mild to moderate clinical signs might be able to return to full athletic performance with careful dietary and management changes, which include regular daily exercise without extended periods of inactivity.
Breeding horses with polysaccharide storage myopathy has at least a 50 percent chance of passing the trait on to offspring. Pre-screening horses for type 1 PSSM is now possible at the University of Minnesota Diagnostic Laboratory. Details on testing and forms can be found at http://www.cvm.umn.edu/umec/lab/Advances_in_PSSM.html.
If your horse ties up, here are suggestions of what to do:
1) Stop exercising the horse and move it to a box stall. Do not force the horse to walk.
2) Call your veterinarian.
3) Blanket the horse if the weather is cool.
4) Determine if the horse is dehydrated due to excessive sweating.
5) Provide fluids with small, frequent sips of water to hot horses, and provide free access to water once the horse has cooled out.
6) Relieve anxiety and pain. Drugs such as acepromazine might be prescribed by your veterinarian.
7) Remove grain and feed; provide only hay until signs subside.
8) Small paddock turnout is good once the horse walks freely, usually in 12 to 24 hours. If the problem recurs, have the horse evaluated for a specific cause of exertional rhabdomyolysis.
About the Author: Stephanie Valberg, DVM, PhD, of the University of Minnesota is a leading researcher on the subject of tying-up. Dr. Valberg, together with Drs. McCue and Mickelson, are holders of the patent for the PSSM genetic test. A portion of the profits from ReLeve are provided to Dr. Valberg and her research.