8.2.1 Musculo-skeletal disease
Musculo-skeletal disease is probably an under-diagnosed cause of poor performance. Although lameness is usually recognised, some animals will win races or be placed when, under close examination, they are slightly lame. This is not to say that they would not do better if they were sound.
Horses with a unilateral lameness are unlikely to be presented as a poor performance case because this is usually apparent to the trainer or jockey. However, in some cases it will have been overlooked, or will have been dismissed as unimportant. Some animals may only be lame at high speeds and this may be difficult to detect without examination on a treadmill. Bilateral lameness is a common cause of poor athletic performance and it is frequently overlooked. This may be particularly true in young racehorses which have bilateral injury due to poor adaptation to stresses placed on bone. Careful clinical examination may detect changes suggestive of stress fracture of the dorsal metacarpus, but is less likely to detect remodelling changes in the proximal sesamoid bones or third carpal bone. Scintigraphy is particularly useful in demonstrating areas of high bone turnover in these cases. However, there is no clear demarcating margin between extremes of normal bone remodelling and a skeletal abnormality which is a cause of pain. The more widespread use of scintigraphy will no doubt increase the rate at which musculo-skeletal causes of poor performance are detected.
Myopathies are recognised to be a cause of lameness, stiffness, and poor perHowever, they are perhaps under-diagnosed in the latter category. Measurement of plasma CK before exercise and 6 hours later is usually sufficient to make a diagnosis. AST and LDH assays may help to identify the duration of the injury, or detect previous injury, because their levels decline more slowly than CK, which peaks after six hours. AST levels peak around ~4 hours and may remain high for 7 days; LDH peaks within 12 hours and remains high for around 7 days. LDH is much less specific for muscle damage than CK or AST.
Aorto-iliac thrombosis is an uncommon cause of poor athletic performance, but should be considered in those animals which show hindlimb gait abnormalities after exercise (see section 6.10.5).