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‘Tying Up’ AKA Exertional Equine Rhabdomyolysis.

Although it sounds complicated the term ‘rhabdomyolysis’ refers to the breakdown of muscle. This can be for a variety of reasons but is most commonly related to exercise. However, there are many different classifications and causes of tying up as illustrated in Box 1:

Generally there can be a marked variation in the severity of the clinical signs of horses tying up and signs can range from a slight change in gait, or reluctance to move through and progress to recumbency and even, in rare cases, death. The severity of clinical signs is obviously dependant on the degree of muscle damage. As stated before the majority of myopathies are exercise related and therefore signs will occur during or shortly after exercise.

Clinical signs include:

Diagnosis

Diagnosis is based on the history and clinical signs but often as a first line blood biochemistry can help determine the levels of muscle enzymes found in the blood – the degree of increase in these enzymes will reflect the level of muscle damage but may not always equate to the severity of the clinical signs. In some cases it may be beneficial to perform an exercise test coupled with testing for muscle enzymes to detect whether the muscles’ response to exercise is ‘normal’ – this is particularly useful in those cases of chronic equine rhabdomyolysis when the initial changes may be more subtle or missed.

Urinalysis can be a helpful diagnostic tool because when the muscles become damaged there will be an increase in myoglobin (a breakdown product from muscles) in the urine and in severe cases it will turn the urine dark red/brown.

In cases of recurrent equine rhabdmyolysis a muscle biopsy can provide very useful information and therefore facilitate a more successful treatment. Especially in those cases with a genetic link, such as PSSM.

Treatment:

Sporadic exertional rhabdomyolysis

Chronic exertional rhabdomyolysis

Severe rhabdomyolysis

Overally sporadic exertional rhabdomyolysis is the classic ‘tying up’ and is the most common type of the disease seen – and despite our lack of knowledge about exactly how it occurs, it is generally easily treated. However, the more severe cases can be more problematic and often carry a poorer prognosis. Therefore, a full investigation into any underlying metabolic abnormalities will help identify the best course of treatment.