As many as 40% of all cases of equine back pain are the result of soft tissue injury. The primary causes include chronic and recurrent exertional rhabdomyolysis (CER and RER, respectively), and an inherited enzyme deficiency called polysaccharide storage myopathy (PSSM). Quarter Horses, Warmbloods, and draft horses are particularly susceptible to PSSM, which prevents normal metabolism of glycogen, a stored form of carbohydrate found in muscle. When glycogen builds up, the result is disruption of muscle fibers and pain.

The epaxial muscles along both sides of the back are frequent sites of glycogen deposition in PSSM horses. The longissimus dorsi, another large sacral muscle, is often sore in jumpers and dressage horses, yet hadn't been identified as a PSSM site. However, a study at the University of Cordoba in Spain evaluated biopsies of this muscle in normal and back-sore horses and showed evidence that it is involved in PSSM.

Nine of 39 horses with back pain (five show jumpers and four dressage horses) had PSSM in biopsies of the longissimus dorsi. All nine had a history of back pain and poor performance. Six of the nine also had PSSM in biopsies of their gluteal muscles. As a control measure, six Standardbreds with RER and six healthy trotters had gluteal muscle biopsies evaluated for comparison. Three of the six RER horses had some evidence of PSSM, although none was found in the sound trotters.

These results suggest that PSSM should be considered as a possible cause of back pain in any performance horse, and biopsy of affected muscles should be part of the diagnostic plan.

Quiroz-Rothe, E.; Novales, M.; Aguilera-Tejero, E.; et al. Equine Veterinary Journal, 34(2), 171-176, 2002.

About the Author

Susan Piscopo, DVM, PhD

Susan Piscopo, DVM, PhD, is a free-lance writer in the biomedical sciences. She practiced veterinary medicine in North Carolina before accepting a fellowship to pursue a PhD in physiology at North Carolina State University. She lives in northern New Jersey with her husband and two sons.

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