Pathogenesis and Diagnosis of Myopathies

Horses with underlying myopathies are usually recognised on the basis of their physical examination (muscle pain or stiffness, swelling, atrophy or paresis), through a history of poor performance or from clinical biochemical analysis: generally, though not always, horses have mild to massively elevated plasma activities of the muscle derived enzymes, creatine kinase (CK) and aspartate aminotransferase (AST). Recent years have seen huge strides in the veterinary profession’s understanding of the cause and pathophysiology of some of these conditions, in particular, recognising that the syndrome of ‘tying up’ has several, and perhaps multiple causes. Identifying the aetiology (whenever possible) is important for case management and owner advice, but unfortunately, establishing the precise diagnosis in individual cases remains problematic, with many cases being best described as idiopathic. This presentation covers the diagnostic approach to working up horses with a suspected underlying myopathy and covers the specific tests that can be performed to help establish the correct diagnosis.

Equine myopathies can usually be categorised (with some overlap) as exertional and nonexertional. Typically, exertional myopathies (rhabdomyolysis) are seen in athletic horses, during or shortly after exercise in animals that are otherwise regarded as normal between episodes. Whilst a variety of acquired conditions (for example hormonal and electrolyte abnormalities, infectious disease and oxidant damage) have been postulated to be associated with exertional rhabdomyolysis, many horses appear to have an underlying genetic predisposition to this trait, and it may be that the proposed acquired causes are environmental factors that can modify the phenotype in a genetically susceptible animal. Nonetheless, as in humans, over-exertion likely is a significant cause of exertional myopathy in some horses. […]