Stephanie Valberg, DVM, PhD, professor of large animal medicine and director of the University of Minnesota's Equine Center, started an in-depth seminar on muscle disorders by discussing diagnosis of muscle disorders, beginning with a careful physical exam and concise history of the clinical problem. She said in most cases, a muscle disorder arises from one of the following situations: muscle strain, exertional rhabdomyolysis (tying-up), weakness and exercise intolerance, abnormal muscle contraction and nerve conduction, or muscle atrophy. Identifying the exact nature of a horse's muscle problem is dependent on characterizing the clinical signs to narrow down the problem.

Careful inspection of a horse's muscle mass and symmetry and hands-on palpation educates the examiner about the horse's muscle tone and comfort level. The examiner also evaluates the horse in motion and conducts a full lameness exam. A serum chemistry panel is helpful for biochemical analysis of muscle enzymes that provide telling information about muscle health and the duration of muscle injury.

There are some enzymes that can yield information about muscle, although not all of these enzymes are found exclusively in muscle. Serum creatine kinase (CK) is released within just a few hours of muscle damage, peaking within six hours following injury. Elevations in CK are usually consistent with training, transport, or taxing endurance or eventing competition. Higher levels from these causes generally are within an expected range and should return to normal quickly, whereas a horse experiencing rhabdomyolysis will have extreme increases in CK.

Serum aspartate transaminase (AST) is an enzyme that rises more slowly after muscle insult, peaking in 12-24 hours. Its clearance rate to return to baseline might take as long as two to three weeks after a bout of rhabdomyolysis. Serial chemistries (several taken in a row) can provide a veterinarian with knowledge about healing response.

Lactate dehydrogenase (LDH), an enzyme important to energy function in cells, is useful when evaluated in conjunction with CK.

Myoglobin (the oxygen-transporting pigment of muscle) increase in plasma or serum can alert the veterinarian to acute muscle damage even before it is visible in the urine. Urinalysis for myoglobin and fractional excretion of electrolytes (a technique used to measure electrolytes or other components of urine in comparison, or fraction, with serum values of those same components) are invaluable tools to ascertain muscle damage and electrolyte abnormalities leading up to rhabdomyolysis. Exercise testing at a trot for 15 minutes, coupled with serial muscle enzyme (CK) evaluation, help quantify the degree of exertional rhabdomyolysis in chronic cases with mild signs.

Valberg discussed using thermography to detect heat from inflammation, reminding veterinarians to compare both sides of the horse. Nuclear scintigraphy is another useful tool to zero in on a complaint of subtle poor performance possibly related to rhabdomyolysis. Release of large amounts of calcium from damaged muscle seems to be an attractant for the radioactive material injected into the horse for the nuclear scan. Therefore, areas of damage are visible when using this imaging modality.

Diagnostic ultrasound is another useful tool to look for traumatic fiber disruption and/or scar tissue. Valberg stressed that artifact (something artificial or a distortion that does not represent normal anatomy or pathology) is easily confused with reality in these scans, so it is important that a horse stands squarely with his full weight equally dispersed among his limbs, and that the veterinarian compares the same muscles on the left and right sides.

Another diagnostic technique uses electromyography (a test that measures electrical signals in the muscles) when there are signs of muscle atrophy, tremors, or dimpling. This evaluates conduction within the electrical system of muscle and nerve units. Abnormalities are seen on electromyography with hyperkalemic periodic paralysis (HYPP, a genetic disease of Quarter Horses and derived breeds, characterized by sporadic episodes of muscle tremors and stiffness, along with elevated serum potassium levels), myotonia (increased muscular irritability and contraction with decreased power of relaxation), or atrophy created by nerve injury.

One extremely helpful diagnostic tool is a muscle biopsy, particularly in the diagnosis of polysaccharide storage myopathy (PSSM) or immune-mediated myopathies.

Finally, Valberg reported on the value of genetic testing using DNA markers in hair root samples to identify genetic mutations that are characteristic of muscle diseases. Genetic tests can be used to detect HYPP, glycogen-branching enzyme deficiency (GBED), and other muscle diseases.



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.

About the Author

Nancy S. Loving, DVM

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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