Tips for Diagnosing Muscle Disease in Horses

“Both the art and science of veterinary medicine are required to investigate poor performance in horses,” said Stephanie J. Valberg, D.V.M., Ph.D., Dipl. A.C.V.I.M., Dipl. A.C.V.S.M.R, during her presentation on diagnosing muscle disease at the 69th Annual Convention of the American Association of Equine Practitioners.^*

The art and science components converge when first attempting to differentiate between lameness and neurologic disease based on the patient’s history, videos proffered by owners (many muscle conditions occur intermittently), and a complete physical examination.

Once a muscle disease is identified, then veterinarians must decide which of the four categories it falls: muscle weakness (e.g., due to vitamin E deficiency), mechanical lameness (e.g., fibrosis of muscle tissue), acute muscle pain, or chronic muscle pain.

In the case of chronic muscle pain, there are several important “primary myopathies” that must be explored. These include sporadic exertional rhabdomyolysis—tying-up—due to exercise in excess of training, or recurrent exertional rhabdomyolysis (RER), polysaccharide storage myopathy (PSSM) type 1 and 2, malignant hyperthermia in Quarter Horses, or myofibrillar myopathy (MFM) in Arabians and Warmbloods.

Valberg described during her presentation the following step-by-step approach to diagnosing muscle diseases in cases of owner-reported poor performance:

1. Measure the muscle enzymes creatine kinase (CK) and aspartate aminotransferase (AST). If CK is elevated when collected from a resting horse, then a diagnosis of rhabdomyolysis is made. An elevation in AST indicates a previous bout of tying-up.
2. If the CK is normal but exertional rhabdomyolysis is still suspected, then perform an exercise test. Measure the CK in a resting sample then again four to six hours after 15 minutes of trotting exercise. If the CK value doubles in this time frame, then a diagnosis of subclinical exercise rhabdomyolysis is made.
3. If CK and AST values are elevated (indicating the presence of exertional rhabdomyolysis), then consider genetic testing for PSSM1. This is especially important in Quarter Horse-related breeds but not Standardbreds, Thoroughbreds, or Arabians as those breeds are rarely, if ever, affected by PSSM1.
4. Diet trial to assess response to therapy for horses with RER, PSSM, or MFM. “This should be done after other causes of poor performance such as gastric ulcers, lameness, and behavior are ruled out,” Valberg advised.
5. Muscle biopsies may be useful in horses with PSSM1 and 2 and MFM. Samples are taken from the semimembranosus muscle, one of the hamstrings. This is in contrast to muscle biopsies taken from the sacrocaudalis dorsalis muscle in the back for horses with muscle weakness to diagnose vitamin E responsive myopathy and equine motor neuron disease.

Once a primary myopathy is diagnosed, then specific interventions can be applied accordingly.

“Dietary changes for horses with PSSM1 and PSSM2 include limiting the amount of starch and sugar in the diet. Re-Leve is a Kentucky Equine Research feed specifically formulated with fermentable fibers and added fat rather than starch sources for most of its calories,” said Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research.

She added, “Careful management of an affected horse’s feeding program can often provide relief and allow an affected horse to train and perform at a productive level.”

For a thorough review of how to feed horses with myopathies, go to Feeding Performance Horses with Myopathies, a paper written by Joe Pagan, Ph.D., and Valberg for the 2020 American Association of Equine Practitioners Conference.

^*Valberg, S.J. 2023. How to diagnose muscle disease that impacts performance. In: Proc. American Association of Equine Practitioners Convention 69:222-227.