Similar Problems, Several Syndromes
Tying-up and exertional rhabdomyolysis (ER) are terms used to describe the sudden and severe muscle cramping that sometimes occurs in performance horses. Studies over the last decade have allowed researchers to identify several types of chronic exertional rhabdomyolysis. Although the syndromes are caused by different cellular and metabolic abnormalities, careful management of the feeding program can often provide relief and allow an affected horse to train and perform at a productive level. Polysaccharide storage myopathy (PSSM) is a type of chronic exertional rhabdomyolysis that is most common in stock-type horses (especially those of Quarter Horse, Paint, and Appaloosa breeding), drafts, and warmbloods. The problem is caused by an abnormal accumulation of glycogen in muscle cells, possibly linked to enhanced insulin sensitivity and rapid clearance of glucose from the blood. After only a few minutes of mild exercise, horses with this condition may exhibit stiffness and reluctance to move. Signs in less severely affected animals include muscle twitching, pawing, and stretching out as if to urinate. While PSSM has been diagnosed in horses less than a year of age, the problem often becomes evident when a young horse is first put into training. Episodes may also be triggered when a fit older horse has a change in management such as being kept in a stall or resuming work after a layoff.
A different glycogen storage disorder, equine polysaccharide storage myopathy (EPSM), is found in drafts, draft crosses, and warmbloods. Horses with EPSM show weakness, muscle wasting, twitching, and difficulty with limb control when backing or holding up a leg. Some important contrasts have been found between EPSM and PSSM, but similar dietary management steps have proven helpful to horses with either condition.
Thoroughbreds, Arabians, Standardbreds, and some warmbloods exhibit yet another muscle problem known as recurrent exertional rhabdomyolysis (RER). The cause of this syndrome is a genetic defect in the regulation of intracellular calcium. The muscle stiffness and cramping of RER are commonly manifested when young horses begin race training and may be seen more frequently as fitness increases. The signs are reportedly more common in horses of nervous temperament and are frequently triggered by some type of stress- or fear-producing incident.
Diagnosis of Specific Syndrome
Although nutritional and exercise history are important clues, an examination of blood and muscle cells must be carried out to determine which type of rhabdomyolysis is the cause of muscle problems in a particular horse. While there is no cure as such for ER, a carefully designed program of nutrition and exercise may provide positive results for many horses. A distinction must be made, and horses diagnosed with PSSM must not be fed typical grain diets.
An athletic horse needs a forage-based diet with some mixture of fermentable fiber, fat, and grain-based concentrate properly fortified to provide vitamins, minerals, and sufficient energy for the demands of performance. The need for additional energy is determined by an individual horse’s metabolism and level of exercise. The ration of a horse with any form of rhabdomyolysis must be designed to meet basic requirements but avoid loading the horse’s system with an excess of any nutrient that may contribute to muscle problems. These points need to be taken into consideration:
- Recommended levels of selenium and vitamin E are usually met by grazing, hay consumption, and minimum levels of RE•LEVE®. Supplementation of selenium and vitamin E over this level is not generally helpful, with the exception of draft horses that may benefit from elevated levels of vitamin E.
- Electrolytes (sodium, chloride, potassium, calcium, magnesium) are quickly depleted as a horse exercises, particularly in hot and humid conditions. Use of an electrolyte can help to prevent some episodes of tying-up.
- Chromium supplementation at the rate of 5 mg per day has proven helpful, especially to nervous horses, possibly by assisting in glucose and glycogen metabolism. Conversely, because of its influence on insulin activity, the use of chromium may be counterproductive in horses with PSSM.
- RE•LEVE®, a concentrate with decreased starch and high levels of fat and fermentable fiber, is helpful to Thoroughbreds with chronic RER as well as Quarter Horses and warmbloods with PSSM.