Muscular Causes of Poor Performance in the Horse: Making the Diagnosis

Investigation of the horse with poor performance is often challenging. this reflects that physical examination is often unremarkable, and where subtle abnormalities are identi- fied their significance may be difficult to ascertain. alongside respiratory, cardiovascular and orthopaedic disorders, muscular diseases should also be considered.

Clinical signs
Cases of exertional rhabdomyolysis are usually easy to identify on clinical examination. they may present with muscle pain or stiffness, reluctance to move, sweating, gait abnormalities, muscle fasciculations or recumbency following exercise, in association with marked increased in serum muscle enzyme activities, and in severe cases myoglob- inuria may be present. in patients that experience recurrent bouts of rhabdomyolysis further investigation of a possible underlying myopathy is indicated. Subclinical myopa- thies are more challenging to diagnose, having few localising clinical signs; poor per- formance may be the only complaint.

Muscle enzyme activity
Elevated resting muscle enzyme activities are usually present in cases of exertional rhabdomyolysis, whilst in subclinical myopathies the resting muscle enzyme activity may be within the normal reference range. Both creatine kinase (CK) and aspartate transferase (aSt) should be interpreted together, and other enzymes such as GGt, SDH and GLDH will be helpful in excluding hepatic causes of increased aSt activity. pro- longed recumbency or general anaesthesia may also result in mild muscle damage and it is therefore important to rule out another primary disease process leading to secondary muscle damage.

If resting muscle enzyme activities are within normal limits, a submaximal exercise test may be useful to detect subclinical disease. Blood samples are collected prior to, and 4-6 hours following 12-15 minutes of trot and light canter work. However there appears to be marked overlap between the response of normal horses to exercise and those with subclinical muscle disease. Whilst previous authors have recommended that a 3-4 fold increase of muscle enzyme activity should be considered to be significant, recent studies have shown that much smaller increases may be observed in horses with sub- clinical myopathies e.g. pSSM11,2 [...]