Exertional Rhabdomyolysis – Untying “tying up”!
There are few conditions more distressing to see than an episode of “tying up”. Watching a horse sweat and tremble with severe muscle cramps and feeling helpless is very difficult and knowing you may have been able to prevent the condition makes it even more frustrating.
“Tying up” has been recognised by horsemen and clinicians for many moons and has become an umbrella term for several muscle disorders, all characterised by similar symptoms (Table 1) but having different causes. Previously, terms like Monday morning disease and azoturia were used for “tying up”, and more recently the clinical term Exertional Rhabdomyolysis (ER) has been used. If we break the terminology down we can understand how this term was developed (Figure 1).
ER can be subdivided into sporadic ER (i.e. no underlying muscle defect) or chronic ER (an underlying genetic defect in muscle function) (Table 2). Chronic ER includes Polysaccharide Storage Myopathy (PSSM) and Recurrent Exertional Rhabdomyolysis (RER), both of which have a genetic base, although in the case of RER the problem gene and heritability factors have not yet been conclusively identified.
Occurrence of ER and RER
Few studies have investigated population occurrence rates of ER, one study reported an incidence rate of 5-7% in UK racing Thoroughbreds. The re-occurrence rate in this study was up to 17%, resulting in those racehorses not racing again that season. This has significant welfare and economic implications. Therefore, developing preventative methods to manage these conditions is now an everyday part of running a training yard.
The symptoms of ER are very similar irrespective of cause, but they will vary in severity, sometimes being so mild only a trained eye would pick up on them and other episodes being so extreme that and both can become life threatening.
Categories and subcategories of ER
|Sporadic ER||Chronic ER|
|Overexertion||Malignant hyperthermia (MH)|
|Exhaustion||Polysaccharide storage myopathy (PSSM) type 1|
|Dietary and electrolyte imbalances||PSSM type 2|
Diagnosis of ER will largely be based on the history and clinical signs. However, diagnosis should be confirmed with a blood sample where vets will test for the levels of Creatinine Kinase (CK) and Aspartate Aminotransferase (AST).
CK is an enzyme required for normal muscle function. Found inside the muscle cell, the CK can leak out into the blood when a muscle cell becomes damaged due to inflammation. CK serum level is therefore a measure of muscle breakdown and levels in serum peak 6-12hrs after injury, returning to normal 3-4 days later.
AST is another enzyme that is a useful indicator of liver and muscle damage. Unlike CK it rises more slowly after an episode of muscle damage, peaking 24-48hrs following injury and taking up to 21 days to return to normal.
The combination of CK and AST is a gold standard when diagnosing a case of ER, these will provide information on the severity of the episode allowing you to tailor your recovery programme and return to work to the individual horse. Further investigation such as a muscle biopsy may also be required. If PSSM (mostly seen in Sport horses) or MH (only seen in Quarter horses) is suspected a genetic test may be carried out, unfortunately there is currently no genetic test available for RER.
This article will focus on sporadic ER and the chronic form RER, these conditions most commonly affect Thoroughbreds and Standardbreds when in a training regime.
Although the terminology used is similar sporadic episodes of ER should not to be confused with RER, which has a genetic base. A horse experiencing a single episode of sporadic ER, due to a specific environment or management reason, may never suffer another episode and is therefore not chronic or recurring.
Sporadic cases of ER can be caused by environmental and management issues including;
Muscle trauma caused by direct or generalized injury to the muscle e.g. a horse getting cast or falling, can result in clinical signs of ER (Table 1), due to the resulting inflammation causing a breakdown of muscle cells.
Overexertion is one of the most common trigger factors for sporadic ER. It means the muscle has been pushed beyond its capabilities, resulting in cell breakdown. Overexertion can occur when increasing the exercise intensity in a horse that is not ready or conditioned for such effort.
Exhaustion is seen mostly in National Hunt horses running over longer distances. Horses suffering exhaustion can experience symptoms of ER, coupled with weakness and a lack of coordination (ataxia), with severe cases ending in collapse.
Dietary issues usually involve electrolyte imbalances/deficiencies and/or diets high in non-structural carbohydrates (sugars and starch). Electrolytes are needed for normal muscle function and recovery, while excess starch may exacerbate excitability; issues with either could trigger an episode.
When any of these issues remain uncorrected the chance of another episode occurring will be high, if they are corrected and episodes subside then a trainer can be confident their horse does not have a chronic form of ER.
Chronic ER: Recurrent Exertional Rhabdomyolysis
When repeated episodes of ER occur in Thoroughbred’s and Standardbreds, despite efforts to reduce the risk factors e.g. diet and overexertion, then this horse should be investigated for RER. Current research indicates that RER is caused by a genetic fault in the control of muscle contraction and relaxation due to dysregulation of calcium.
Tension/Excitement are major trigger factors for RER horses, commonly the episodes will occur following low intensity work, or when the rider has held the horse back and not allowed him to gallop freely. Research has shown that the occurrence of RER is more common in fillies and in horses that receive high starch rations, particularly when work levels are reduced, both factors could be linked to excitability.
Treatment and preventative management
Immediate treatment for an episode of ER is focused on reducing the severity of symptoms; non-steroidal anti-inflammatories (NSAIDS) are usually administered and in severe cases fluids may be given intravenously.
Psychologically horses can be affected by the association of work creating pain, the memory of a recent episode could result in the horse working in a tense profile, almost waiting for that onset of pain. If this theory has some truth to it, then rapid alleviation of symptoms and most importantly preventing them is crucial.
Carrying out a repeat blood test for CK and AST levels a few days after an episode, will provide information regarding recovery and allow you to plan how best to move forward. Rest, controlled exercise and dietary alterations may follow immediate treatment protocols, and these will be determined by the circumstances and severity of the episode.
Preventative management involves exercise and dietary alterations to 1) prevent an episode occurring in the first place; 2) maximise recovery if it does and 3) reduce the chances of repeat episodes.
Dietary alterations may include
|Providing a low starch concentrate ration such as Connolly’s Red Mills Horse Care 10 and 14 Cubes and Mix, to limit excitability.|
|Provide adequate forage. Increasing forage availability can reduce stress levels. Forage is also important for maintaining an optimal Dietary Cation Anion Balance (DCAB), which relates to the balance of the minerals within the diet that have either an alkalising or acidifying effect. Although not scientifically proven, low DCAB levels may increase the risk of “tying up”.|
|Optimise electrolyte intake to avoid imbalances or deficiencies, which could affect muscle function and recovery. Foran Equine Equi-Lyte G will meet the electrolyte demands of a horse in training.|
|Supplementing with antioxidants, specifically Vitamin E and selenium. Foran Equine Muscle Max provides vitamin E, selenium and essential amino acid lysine, together these support normal muscle function and recovery in the racehorse. More recently Coenzyme Q10/Ubiquinol has become available in Foran Equine’s For-Recovery, this is used to support optimal recovery in the racehorse through its potent antioxidant properties.|
Managing exercise regimes, stress levels and excitability in affected horses will reduce the likely hood of an episode being triggered. Optimise their exercise regime by avoiding over exertion and/or working in a tight profile/outline and avoid situations that will increase excitability.