Is Muscle Disease Contributing to a Horse’s Poor Performance?

Attention to certain details during exams and careful consideration of test results can help a veterinarian arrive at a diagnosis, making way for an appropriate management.

Posted by Erica Larson, News Editor | Feb 16, 2019 | AAEP Convention 2018, Anatomy & Physiology, Conditioning, Diagnosing Lameness, Horse Care, Injuries & Lameness, Lameness, Monitoring Exercise Performance, Muscle and Joint Problems, Musculoskeletal System, Sports Medicine

Lack of energy under saddle, a poor attitude during exercise, chronic back pain, hollowing over jumps. These are all possible signs of low-grade or chronic muscle disease, which can be difficult to diagnose. But one researcher reports that attention to certain details and careful consideration of test results can help a veterinarian arrive at a diagnosis, making way for an appropriate management.

At the 2018 American Association of Equine Practitioners Convention, held Dec. 2-5 in San Francisco, California, Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, the Mary Anne McPhail Dressage Chair in Equine Sports Medicine at the Michigan State University’s College of Veterinary Medicine, in East Lansing, reviewed how she determines whether muscle disease is behind a horse’s poor performance.

“Optimal function of skeletal muscle is essential for successful athletic performance,” she said. “Even minor derangements in locomotor muscle function will impact power output, coordination, stamina, and desire to work during exercise.”

And, while many muscle disorders cause readily apparent clinical signs—such as sweating, muscle twitching and stiffness, and a reluctance to move—these low-grade and chronic signs can be much more subtle, Valberg said, and veterinarians face the challenge of determining:

  • Whether a horse is simply exhibiting uncooperative behavior or experiencing exertional muscle pain;
  • If musculoskeletal or orthopedic pain is contributing to muscle pain (i.e., if the bones or joints are involved);
  • To what degree, if any, muscle disease is causing or adding to lameness or other pain; and
  • Whether muscle weakness is altering the gait, among other points.

“The diagnostic approach to poor performance involves, by necessity, a detailed evaluation of all body systems,” Valberg said. “Frequently, poor performance in athletes is due to a combination of orthopedic and muscle pain, and a systematic evaluation of both systems provides the most satisfactory outcome for the client and the horse.”

She begins her exams by collecting a detailed history, including:

  • The horse’s training and fitness level;
  • His diet, turnout, and exercise schedule, along with any recent changes to them;
  • Any behavioral changes;
  • Past lamenesses, medications, diseases, and/or injuries;
  • Duration, severity, and frequency of the poor performance, and whether bouts are consistently or intermittently associated with exercise;
  • How rest periods impact the poor performance; and
  • Whether it developed following a particular event, such as after the horse fell or completed a day of particularly strenuous exercise.

Next, she advocates for thorough physical and lameness exams. She said she pays close attention to details such as:

  • Muscle mass and whether it is appropriate for the horse’s discipline and training level;
  • Muscle symmetry, tone, swelling, and/or wasting;
  • Heat, knots, and pain on palpation; and
  • If the horse has muscle fasciculations (involuntary twitching).

In addition, she said, she assesses the horse’s muscles in movement.

“The horse should be observed at a walk, trot, and canter for any neurologic or gait abnormalities, the degree of overreach from hind hoof to fore-hoof impressions, rounding or hollowing of the back, degree of pushing off with the hind limbs, facial expressions of pain, reluctance to exercise, and gait transitions,” Valberg said. “In complex cases repeated clinical evaluations might be required, including examination of the horse under saddle.”

This can help the veterinarian determine whether saddle fit or the rider is contributing to muscle pain, she added.

If the veterinarian suspects muscle disease, an important part of a poor performance exam is an exercise test. This allows the practitioner to assess horses’ blood creatine kinase (CK) and aspartate transaminase (AST) levels. These enzymes leak out of muscles and into the bloodstream if muscles are damaged. She advised practitioners to collect blood samples before exercise, as well as four to six hours after, when serum CK levels peak, for comparison. Horses can experience muscle pain without elevations in CK and AST, Valberg said, so this test does not rule out muscle disease. Rather, it allows the veterinarian to determine if there is evidence of rhabdomyolysis (tying-up, or muscle damage).

It doesn’t take much exercise to cause elevated CK levels in horses with chronic rhabdomyolysis, Valberg said. Veterinarians can observe unfit horses on the longe line for about 15 minutes, alternating walk and trot at two-minute intervals, and fit horses after four minutes of walk and 11 minutes of trot on the longe.

“During the test, horses should be observed carefully for exacerbation of lameness, changes in impulsion, stiffness, shortened stride, and development of a sour attitude,” she said. “In addition to quantifying the extent of rhabdomyolysis during mild exercise, the exercise test can be used to decide how rapidly to put the horse back into training.”

Four hours after the exercise test, veterinarians collect blood for analysis. Valberg said horses with abnormally elevated CK levels are likely experiencing subclinical exertional rhabdomyolysis (ER), which can be due to either external factors, such as dietary imbalances, too-strenuous exercise, viral infections, or other management issues (extrinsic) or factors within the horse, such as a genetic disorder (intrinsic).

She starts by recommending management changes to attempt to eliminate extrinsic ER. But if horses fail to respond to balancing the diet and to rest and a gradual return to exercise, they likely suffer from an intrinsic form of ER. Valberg said veterinarians can use a combination of the animal’s history; age, breed, and sex; clinical signs; and ancillary testing to narrow the diagnosis. Myopathies, or muscle diseases, that cause ER include:

  • Malignant hyperthermia (MH)—A genetic mutation causes this skeletal muscle abnormality, which is found in Quarter Horses and Paint horses. Inhalant anesthesia can trigger episodes characterized by a high fever, acidosis (a decrease in body pH), and sometimes death. Exercise can trigger signs of tying up. Veterinarians can use a genetic test to confirm MH.
  • Recurrent exertional rhabdomyolysis (RER)—RER is most likely caused by an abnormality in the regulation of muscle contraction and relaxation. Horses suffering from an acute episode of RER have very stiff, hard, painful muscles and are reluctant to move. The condition often occurs in fit animals and is usually triggered by stress and exercise. There’s no test for RER, so veterinarians diagnose it based on the horse’s history, clinical signs, and serum CK levels.
  • Type 1 polysaccharide storage myopathy (PSSM1)—Caused by a genetic mutation, PSSM1 leads to exertional rhabdomyolysis, stiffness, muscle soreness and reluctance to exercise. Stock horses and draft horses are most commonly affected, with halter lines of Quarter Horses affected far more frequently than other types. Vets can use a genetic test to confirm PSSM1.
  • Type 2 PSSM (PSSM2)—Quarter Horse-related breeds with PSSM2 can exhibit similar clinical signs as those with PSSM1, but the cause of PSSM2 is not yet known and there is not a scientifically validated genetic test for this disease.
  • Myofibrillar myopathy (MFM) in Arabians—Valberg and colleagues recently identified this muscle disorder. In Arabian endurance horses they found microscopic indications of MFM. These horses were well-conditioned, very fit, 100-mile endurance Arabians that suffered post-race stiffness, increased CK levels, and myoglobinuria (brown urine) or stiffness that developed about 5 miles into rides that followed two weeks of rest post-endurance-ride. Researchers have not yet identified a genetic mutation that is consistently present in MFM horses diagnosed by muscle biopsy, so Valberg recommends a muscle biopsy to diagnose MFM.

In some cases serum CK and AST levels are normal in horses with muscle soreness, reluctance to push with their hind limbs, reluctance to collect and engage through the back, and vague lameness that cannot be localized.  Horses with these clinical signs can suffer from other myopathies such as type 2 PSSM and MFM, which can impact Warmbloods. Warmbloods with PSSM2 and MFM typically have chronic signs related to poor performance, including undiagnosed gait abnormalities, sore muscles, mild muscle atrophy, reluctance to go forward and collect and decreased energy levels among others. They usually have normal blood CK levels. Practitioners can diagnose PSSM2 with a muscle biopsy.

Valberg said it’s important to recognize that even if CK levels remain normal after exercise, horses could still be suffering from a muscle disease; horses with PSSM2, MFM, or vitamin E deficiency (which can cause muscle and neurologic issues) can all have normal serum CK activity yet significant clinical signs of muscle disease. In these scenarios pursuing a proper diagnosis and making management changes accordingly could help alleviate signs.

Take-Home Message

Indeed, muscle diseases can cause poor performance in sport horses, said Valberg, and chronic conditions can be challenging to pinpoint. Diagnostics should include thorough physical, orthopedic, and neurologic examinations; a close look at the horse’s musculature; and an exercise test and serum CK analysis. Genetic tests and muscle biopsies might help veterinarians identify some cases, and management changes can help alleviate clinical signs in some diseases.



Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.



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