There are two main types of exertional rhabdomyolysis, Sporadic and chronic.
History and clinical signs:
There is a lot of variation in the severity of clinical signs and these do not always mirror the amount of muscle damage. Signs vary from a mild alteration of gait, reluctance to move through to recumbency and even death. Most cases occur after exercise and the hind limbs in particular the gluteal, femoral and lumbar muscles are affected. The horse is usually stiff and often reluctant to move and may sweat. Heart rate and respiratory rate may also be raised. If badly affected the horses urine may appear dark brown which indicates myoglobin (a breakdown product of muscle) in the urine. The muscles tend to be painful when palpated. If particularly mild with recurrent exertion rhabdomyolysis the only presenting sign maybe that of poor performance.
The clinical signs are usually pretty diagnostic in themselves although can sometimes be mistaken for colic. Blood samples to determine the muscle enzyme levels would confirm the diagnosis and the level of muscle damage. A repeat sample taken in 5-7 days time is useful to make sure that the levels have come down enough to recommence exercise. If the condition recurs then it would be sensible to take a muscle biopsy from the gluteal muscle because it is possible that the horse could suffer from a genetic condition called polysaccharide storage myopathy. This causes the muscle to store too much glycogen leading to a build up of lactic acid and muscle damage.
Once diagnosed by blood sample the horse should be rested for a few days as further activity is likely to exacerbate the problem.
If very mild then walking out for short periods in hand may be beneficial. It is wise to monitor the muscle enzyme levels by repeat blood sampling and only start bringing the horse back into work when the levels return to within normal range.
Anti-inflammatory drugs like phenylbutazone (Bute) are useful for pain relief. Care should be taken if the signs are particularly severe as the kidneys may have become compromised by the myoglobin produced as a result of muscle breakdown. The horse would need to be hospitalised in this instance for intaveinous fluid therapy. Sedatives can also be given in a particularly distressed horse. Corticosteroids maybe used in the short term to help stabilise damaged muscles and as a potent anti-inflammatory.
Chronic cases of rhabdomyolysis are either classed as polysaccharide storage myopathy or recurrent exertional rhabdomyolysis.
The management and dietary advice is similar for both types.
Reduce the amount of carbohydrate fed to below 10% starch. This is particularly important if the horse is in light work when forage is only required in most cases. If more strenuous work is to be undertaken then the calories can be supplied by feeding oil in the diet. It is also important to not feed a horse in anticipation of an increased workload. In all cases we recommend consultation with an equine nutritionalist to tailor a diet specific to your horse’s needs.
All horses which have repeat bouts of rhadomyolysis should be given daily exercise with adequate warm up periods. Turnout is beneficial and ideally no breaks to their routine. It is important to avoid stress particularly in nervous animals as this can trigger the disease.
ACP (Relaquin) can be used in horses to reduce stress when bringing back into training although this is unlikely to have any effect on horses with polysaccharide storage myopathy. Dantrolene has also been shown to help prevent muscle damage in horses coming back into training when given 60-90 minutes before exercise.
In horses that sweat a lot particularly when in intense training tend to lose sodium and chloride. Adding common salt to the diet can help to supplement this.
Vitamin E and selenium has also been shown anecdotally to be of some benefit.
Article by Reuben Whittaker BVSc MRCV