Incidence and Management of Exertional Rhabdomyolysis

Stephanie Valberg, DVM, PhD, professor of large animal medicine and director of the University of Minnesota’s Equine Center, tackled the broad subject of skeletal muscle disease related to exercise at the 2006 AAEP Convention.

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Stephanie Valberg, DVM, PhD, professor of large animal medicine and director of the University of Minnesota’s Equine Center, tackled the broad subject of skeletal muscle disease related to exercise at the 2006 AAEP Convention.

This has been a problem recognized for centuries with names such as azoturia, Monday morning disease, tying-up syndrome, myositis, and set fast, all of which are accompanied by muscle pain. But each syndrome is a little different. These are more descriptively related under the term exertional rhabdomyolysis or ER.

Valberg noted that as many as 3% of horses in exercise might have experienced an episode of ER within the past year.

Certain exercise, such as polo and racing, might elicit a greater incidence. Typical clinical signs include muscle stiffness, muscle cramping, acute onset of hind limb lameness, refusal to move, and other signs of distress, such as increased heart and/or respiratory rates, sweating, and colic.

Diagnosis is made based on clinical signs and muscle enzyme levels. Creatine kinase (CK) values greater than 300,000 u/L might be associated with muscle atrophy following several days of muscle edema and swelling.

Usually muscle tissue will regenerate in about three months, and prognosis is good for performance if CK never exceeds 250,000 u/L. For horses with CK greater than 500,000 u/L, there is a guarded prognosis for healing or future athletics.

For a few days following an episode of ER, horses should be confined with minimal hand-walking. When stiffness dissipates, turnout in a small paddock is advised and muscle enzyme values should be checked continually until they return to normal.

Sporadic cases of ER develop related to overexertion in a horse not fit for the task, or from heat exhaustion with dehydration and electrolyte imbalances. Vitamin E deficiency is also becoming more common as horses get less access to pasture.

One question often asked is if this syndrome is heritable. There is a form of recurrent exertional rhabdomyolysis (RER) in Thoroughbreds, Standardbreds, and Arabian horses, and PSSM is found in Quarter Horses and draft breeds. Both are associated with genetic susceptibility.

Recurrent exertional rhabdomyolysis is thought to be caused by an abnormality in regulation of calcium within muscle cells. It tends to be more prevalent in mares, with as many as 80% of 2-year-old Thoroughbred fillies affected. Nervous temperament increases prevalence by fivefold. The presence of concurrent lameness increases likelihood of developing RER by fourfold.

Recurrent exertional rhabdomyolysis seems to be a stress-related disorder with intermittent clinical signs, and it is especially prevalent in fit horses. Everything proceeds along okay for the first 15-30 minutes of exercise, then something triggers the muscles to expel excess calcium, leading to sustained muscle contractions and spasms. During this time, CK enzyme levels increase dramatically.

Muscle lactate concentrations in RER horses are 5-10 times lower than in healthy horses after racing, so there seems to be no relationship between an episode of RER and lactic acidosis (accumulation of lactic acid more rapidly than it can be metabolized), an important point when devising management strategies.

Treatment recommendations for RER include minimizing stress and continuing with light training, as rest makes this syndrome worse.

A horse should be warmed up well before being asked for more exertion. Any lameness condition should be accurately diagnosed and treated. In some cases, administration of low-dose (1-2 mL) acepromazine given 30 minutes prior to exercise is helpful. In Standardbreds, interval training using sprinting episodes seems to help diminish occurrence, and trot time should be limited to 15-20 minutes.

In some cases, treatment with daily progesterone might improve a nervous mare by controlling her heat cycles. There are reports that dantrolene (trade name Dantrium, a direct-acting skeletal muscle relaxant) given 90 minutes before exercise (and on a fasted stomach) has been shown to deter episodes.

In all cases, dietary management is important. Limiting starch not only modifies fuel energy use in the muscle, but it also decreases excitability and nervousness. Because it’s difficult to keep weight on a racehorse or a horse engaged in rigorous training and athletics, a commercially prepared high-fiber, high-fat, low-starch diet should be offered, as these diets are palatable, nutritionally balanced, and designed to maintain a horse’s weight. There are presently some commercially available feeds that specifically address ER horses.

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse’s AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads

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Written by:

Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.

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