Am I dealing with a myopathy?

Horses with underlying myopathies are usually recognised on the basis of their history (stiffness post exercise), physical examination (muscle pain or stiffness, swelling, atrophy or paresis) and the diagnostic suspicion is strengthened by demonstration of elevated serum or plasma activities of muscle-derived enzymes such as creatine kinase (CK) and aspartate-amino transferase (AST). However, in investigating horses with poor performance, it is not uncommon to identify (apparently) sub-clinical elevations in CK and AST: determining the significance of such elevations is difficult. Furthermore, a number of myopathic disorders exist in which CK and AST activities are normal, but which could feasibly present as poor performance or paresis. This presentation will cover approaches to these scenarios.
Muscle tissue has a variety of enzymes that perform normal functions, some (such as CK being more specific for muscle damage than others, such as AST). Creatine kinase will typically peak 6–12 h following an insult, and then decline with a half-life of approximately 12 h. In contrast, AST activity peaks about 24 h following a muscle insult and can remain elevated for several days to weeks. Consequently, observation that CK remains elevated for several days (or is consistently mildly elevated in successive biochemistries taken on different occasions) likely reflects ongoing or repeated muscle-specific insults.  […]