Diagnosis and Treatment of Muscle Diseases in Horses

RECURRENT EXERTIONAL RHABDOMYOLYSIS OF THOROUGHBREDS Recurrent exertional rhabdomyolysis (RER) may occur in up to 5% of racing thoroughbreds. Pedigree analysis has shown a probable autosomal dominant mode of inheritance with variable expression. The syndrome is characterized by mild, moderate or severe rhabdomyolysis that begins shortly after the onset of aerobic exercise. Muscle cramping is usually accompanied by tachycardia, tachypnea, sweating, reluctance to move, and occasionally recumbency. Laboratory diagnosis is similar to other causes of rhabdomyolysis. Since the rhabdomyolysis occurs early after the onset of exercise, systemic dehydration and acidosis rarely occur. Thoroughbreds with RER will often have elevations in CK that are not associated with any clinical signs. This subclinical manifestation of the disease can lead to CK activities as high as 10,000 U/L. Biopsy specimens show an increased number of centrally located nuclei within the muscle fibers. In vitro contracture studies on intercostal muscle fibers show an increased sensitivity to potassium, caffeine and halothane. These horses also have quicker times to 50% contraction and 90% relaxation compared to normal horses and horses with other underlying muscle diseases. The disease is most commonly seen in young thoroughbred fillies with nervous temperaments. Stress is thought to be one of the most important predisposing factors. High carbohydrate diets may lead to higher CK activity. Horses are more likely to develop clinical signs if they are exercised at a medium gallop or a trot than if they are racing or at a full gallop. Lameness, inclement weather and time of year have also been cited as possible predisposing causes. Treatment of RER is aimed at decreasing the incidence of the episodes by changes in management including a high-fat, low-starch diet, changes in exercise and turnout regimens, and treatment of underlying lameness. Dantrolene at 4 mg/kg to fasted horses 90 minutes prior to exercise, has been shown to lower post-exercise serum CK activities. For horses with severe episodes, treatment of RER should be the same as for other causes of acute rhabdomyolysis and should include management of pain, correction of dehydration, muscle relaxants, and management of the recumbent horse, if necessary.