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VETERINARY VIEWS
A cause of sporadic cases of exertional rhabdomyolysisisVitaminEdeficiency,whichis possible in horses with restricted pasture access.
disease, tying-up syndrome, myositis, set fast. While each syndrome is a bit different, all are accompanied by muscle pain. These are more descriptively related under the term exertional rhabdomyolysis or ER.
As many as 3% of equine athletes may experience an episode of ER while exercising. The highest incidence occurs with certain activities, such as racing and polo.
Diagnosis is based on clinical signs and muscle enzyme levels, especially creatinine kinase (CK). In the initial days following an ER episode, a horse should be confined to a stall with minimal hand walking. Once stiffness dissipates, the horse may be turned out into
a small paddock with monitoring of muscle enzymes until their values return to normal.
Sporadic cases of ER may develop due to overexertion in a horse not fit for the task, or from heat exhaustion related to dehydration and electrolyte imbalances, as occurs in horses involved in prolonged endurance athletics. Vitamin E deficiency, possible in horses that have restricted pasture access, can also affect muscle function.
RECURRENT EXERTIONAL RHABDOMYOLYSIS (RER)
A form of recurrent exertional rhabdomyolysis (RER) has a genetic susceptibility and is present in 5-10%
of Thoroughbreds, as well as some Standardbreds and Arabian horses. Equine polysaccharide storage myopathy (PSSM) in Quarter Horses and Draft breeds is also associated with genetic susceptibility.
RER is caused by abnormal calcium regulation within muscle cells. It is more prevalent in mares and affects up to 80% of 2-year-old Thoroughbred fillies. Nervous temperament increases its prevalence by five-fold. The presence of concurrent
lameness increases likelihood of developing RER by 4-fold.
RER is also associated with stress and develops with intermittent clinical signs, especially in fit horses. During the initial 15-30 minutes of exercise, the horse proceeds just fine. Then a trigger in the muscles expels excess calcium, leading to sustained muscle contractions and spasms – CK enzyme increases dramatically. Muscle lactate concentrations in RER horses are 5–10 times lower than in healthy horses after racing, which suggests that there is
no relationship between an episode of RER and lactic acidosis, an important point when devising management strategies and nutritional approaches. Muscle biopsy is a diagnostic tool to rule out polysaccharide storage myopathy (PSSM, discussed below) as a reason for tying-up.
Treatment recommendations for RER include minimizing stress and continuing with light training since rest worsens this syndrome. A horse should be warmed up well before being asked for more demanding exertions. Administration of low-dose (10–20 mg) acepromazine (tranquilizer) given 30 minutes
prior to exercise may be helpful. Interval training with sprint bouts helps to diminish occurrence; trot time should be limited to 15– 20 minutes. Treatment with daily progesterone may control heat cycles in a nervous mare. The muscle relaxant dantrolene, given 90 minutes before exercise (and on a fasted stomach), may deter episodes. The decision to use medication must be in compliance with drug testing regulations for your particular sport.
DIETARY MANAGEMENT OF ER
Besides control of environmental and behavioral stressors, dietary management
is important in all cases of exertional rhabdomyolysis. Limiting dietary starch not only modifies use of fuel energy in the muscles, but also decreases excitability and nervousness. Because it is difficult to keep weight on a horse engaged in rigorous training and athletics,
a commercially-prepared palatable diet of
high fiber, high fat (> 10%), low-starch (<
10% non-structural carbohydrates or NSC) can be offered that is nutritionally balanced
to maintain a horse’s weight. Vitamin E
and selenium are other important dietary components to manage muscle problems.
Besides control of environmental and behavioral stressors, dietary management is important in all cases of exertional rhabdomyolysis.
Recurrentexertionalrhabdomyolysisismost prevalentinmaresandaffectsupto80%of 2-year-oldThoroughbredfillies
Nervoustemperamentincreasestheoccurenceof recurrentexertionalrhabdomyolysisbyfive-fold.
SPEEDHORSE June 2022 111
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© Getty Images
© Getty Images