HOW TO DIAGNOSE POOR PERFORMANCE IN THE EQUINE ATHLETE


perfectly square for symmetry of muscle mass and evidence of fasciculations. Knowledge of normal muscle mass for that breed at that fitness level and the individual’s previous muscle mass prior to poor performance are essential. The entire muscle mass should be palpated for tone, heat, pain, fascicula- tions, and swelling or atrophy by comparing with contralateral muscle groups. Flexibility of head, neck, back, and limbs should be investigated through range of motion evaluation. Running a blunt instrument over the lumbar and gluteal mus- cles should illicit extension (swayback), followed by flexion (hogback) in healthy animals. Lateral mo- tion through the spine can be assessed by push and pull of tuber coxae and withers simultaneously. Guarding against movement may reflect abnormali- ties in the pelvic or thoracolumbar muscles, or pain associated with the thoracolumbar spine or sacroiliac joints. Evidence of pain or resentment should be as- sessed several times to ensure it is repeatable. Blood samples should be drawn prior to any exercise for a complete blood count and serum biochemistry profile. The horse should be observed at a walk, trot, and


canter for any neurologic or gait abnormalities, the degree of over-reach from hindhoof to forehoof im- pressions, rounding or hollowing of the back, degree of pushing off with the hindlimbs, facial expressions of pain, reluctance to exercise, and gait transitions. In complex cases, repeated clinical evaluations may be required, including examination of the horse un- der saddle. Viewing the horse under saddle with its usual rider can help to determine if saddle fit or the rider itself are contributing to muscle pain. A full lameness exam with flexion tests is part of


a muscle workup, as most poor performance issues in sport horses are not restricted to one body system. Use of bone scans to localize inflammation, local blocks to confirm the source of pain, and radiogra- phy and ultrasound examination to characterize le- sions are often required. In addition, if horses have exercise intolerance, a standardized incremental ex- ercise test that incorporates monitoring heart rate and blood lactate may be of value to assess horses for cardiovascular and metabolic derangements.


Exercise Challenge


An exercise challenge can be helpful in detecting subclinical exertional rhabdomyolysis and in observ- ing the progression of a gait abnormality with exer- cise. Blood samples to evaluate peak changes in creatine kinase (CK) activity should be taken before exercise and about 4–6 hrs after but not immedi- ately after exercise.1 The exercise test in unfit horses involves 2-min intervals of walk and trot for up to 15 min. Clinical judgment should be used and horses should not be pushed if they seem reluc- tant to continue to exercise. Rather, allowing horses to stand for a minute and then asking the horse to recommence a trot often differentiates a lazy horse from one that will continue to show a stiff and stilted gait from muscle pain. For fit horses, 4


min of walk and 11 min of continuous trot can be used. During the test, horses should be observed carefully for exacerbation of lameness, changes in impulsion, stiffness, shortened stride, and develop- ment of a sour attitude. In addition to quantifying the extent of rhabdomyolysis during mild exercise, the exercise test can be used to decide how rapidly to put a horse back into training. Horses with marked elevations in CK 4 hrs following light exer- cise should be very gradually reintroduced to exer- cise once a therapeutic regime is instituted. A 2- to 3-fold increase in CK 4 h after exercise indicates chronic subclinical exertional rhabdomyol- ysis. A normal serum CK response to exercise is still compatible with myopathies such as type 2 poly- saccharide storage myopathy (PSSM2),2 myofibril- lar myopathy,3 and vitamin E-deficient myopathy4 that do not consistently disrupt the muscle cell membrane. In addition, horses with type 1 PSSM (PSSM1) and recurrent exertional rhabdomyolysis can have normal CK responses to an exercise test when on a well-controlled diet and exercise regime.


Ancillary Diagnostic Tests


Depending on the breed, hair roots or ethylenedi- aminetetraacetic acid (EDTA) blood for genetic test- ing may be reasonable next steps in investigating a myopathy. Malignant hyperthermia5 can cause in- termittent exertional rhabdomyolysis and is found in Quarter Horse-related breeds. Genetic testing for PSSM16 is a good choice for Quarter Horse-re- lated breeds, Belgian and Percheron draft-related breeds, and some Warmbloods (see Table 1 for breed prevalence). PSSM1 is not found in light breeds such as Arabian, Thoroughbred, and Standardbred horses. Serum vitamin E concentration is indi- cated in horses with reduced muscle mass or weak- ness or horses with little access to fresh green pastures. Whole blood selenium concentrations are indicated for horses not receiving supplementation if they are in a selenium-deficient region. Muscle biopsy for horses with suspicion of a chronic myopathy where genetic testing proves neg- ative, chronic muscle pain is impeding performance, or focal or generalized muscle atrophy is present. It is important to thoroughly investigate lameness before using a muscle biopsy as a diagnostic tool. This is because, for some disorders such as PSSM2, assessment of glycogen stains has relatively low sen- sitivity and specificity, and, therefore, muscle biopsy should only be done in the face of a high degree of suspicion of clinical disease. For horses with atrophy, biopsies of the atrophied


muscle (focal atrophy) or the sacrocaudalis dorsalis muscle (generalized atrophy) should be performed. The sacrocaudalis is often the only muscle that will show lesions of vitamin E-deficient myopathy or equine motor neuron disease.4,7 For horses with exercise intolerance or exertional rhabdomyolysis, gluteal or semimembranosus biopsies are prefera- ble. A portion of muscle that is shipped fresh on ice


AAEP PROCEEDINGS  Vol. 64  2018 215


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