Technical information for veterinarians
Tying-up, muscle tension, spasm in horses
What is tying up in horses?
Tying up (muscle tension/spasm) involves chronic tension in the muscles due to overwork. When the muscles are active, they produce lactate as part of their normal metabolism. Too much lactate causes lactic acidosis, a lowering of the pH in the muscles and the body in general. This impacts the efficiency of metabolism, giving rise to fatigue. The excess lactate prevents the muscles from relaxing properly following contraction. As a result, large muscle groups tend to seize up, remaining in a state of contraction.
In mild tying up, the horse’s muscles are extremely sore and stiff. In full blown tying up, the horse is unable to move.
The areas typically affected by tying up include the horse’s neck, back, shoulders, zones of previous trauma and areas where other muscles have attempted to compensate for those already affected.
Prolonged muscle fatigue can, in time, affect the skeletal, circulatory and respiratory systems.
Symptoms of tying up in horses
The following symptoms may indicate that a horse is tying up:
- Obvious discomfort and irritability
- Abnormally short strides
- Profuse sweating
- Muscle stiffness, contraction
- Difficulty moving
- Elevated pulse and laboured breathing
- Brown coloured urine resulting from the kidneys filtering myoglobin from the blood. (Myoglobin is an indication of severe muscle damage.)
- Inability to sleep
The etiology of tying up in horses
Just like human athletes, horses (particularly racehorses) are prone to stiffness and soreness following sporting events/training. The horse’s large muscle mass undergoes intense stress during events and, therefore, is prone to cramping. Extreme cramping leads to tying up (rhabdomyolysis) or exertional myopathy.
Tying up manifests to varying degrees, from barely perceptible trembling to full-blown contraction. Catabolism may occur as a result of tying up, leading to muscle weakening and decreased performance.
There are two primary categories of causes that lead to tying up, and treatment depends on what has precipitated the condition in the specific horse in question.
The two categories are sporadic exertional rhabdomyolysis and recurrent exertional rhabdomyolysis:
Sporadic exertional rhabdomyolysis
In the sporadic form, extrinsic factors affect the muscle function. Intense exertion causes the muscles to become exhausted, leading to tetany and destruction of muscle cells. Endurance horses experience this type of tying-up when prolonged sweating alters the body’s electrolyte balance to the degree that it affects muscle cell contraction. Carbohydrate overload/excessive grain intake without sufficient exercise to use the stored energy can also lead to tying up (as well as increasing a horse’s risk of laminitis and founder).
Recurrent exertional rhabdomyolysis
The recurrent form of tying-up seems to be caused by intrinsic muscle defects (genetic variations in the structure of muscle cells). These defects include polysaccharide storage myopathy (PSSM). In the condition polysaccharide storage myopathy (PSSM), the stored muscle polysaccharide does not have the correct biochemistry and so cannot be used correctly. Another abnormality involves the abnormal movement of calcium ions in and out of the muscle cell membrane. (The number of calcium ions within the cell controls muscle contraction.)
Horses that tie up after a few minutes of trotting may have a mitochondrial enzyme defect that causes severe intolerance to exercise. This results in an excess accumulation of lactic acid in the muscles and blood.
Less common causes of tying up include neurogenic atrophy resulting from equine protozoal myelitis; myogenic atrophy associated with Cushing’s disease, influenza or muscle trauma; an immune mediated, steroid responsive myopathy in young horses possibly associated with Streptococcus equi bacteria. In Quarter Horses, the muscle abnormality known as myotonia dystrophica, can cause tying-up.
Treatments for tying up in horses
A horse that has tied up needs to relax. Letting him rest quietly in a warm, dry area is the best way to handle the situation. The horse should remain on his feet and be offered pure water to help flush the kidneys of waste.
If the stiffness has disappeared the following day, the horse should be encouraged to exercise, although not intensely. Slow walking will help to loosen up the horse’s muscles.
Dietary measures will often help solve the problem of recurrent exertional rhabomyolysis. Good quality hay should be offered and carbohydrate reduced to a minimum to maintain the right energy level. Providing a higher level of fat in the grain supplement can provide the needed energy without risking carbohydrate overload.
Anecdotal reports from horsemen seem to indicate that if their horse’s electrolytes (potassium, sodium, magnesium, calcium) are kept at optimal levels their horses may ty up less frequently.
Horses that experience recurrent exertional rhabomyolysis should be exercised daily.
Since horses that have tied up first and foremost need to relax, the tranquilizer acetylpromazine, (“Ace”), is normally indicated. This tranquilizer helps to reduce muscle contractions.Phenylbutazone, commonly known as “Bute,” is an NSAID that is used to help relieve the horse’s pain, and make it possible for him to relax. If the horse is dehydrated, intravenous fluids with appropriate electrolytes are administered. (Some veterinarians like to include DMSO in the intravenous therapy.)
Horses at risk for tying up
Tying up is more common in young Quarter Horses and Thoroughbreds that are either high strung or not fit. Fit, well-trained and well-bred horses are better able to deal with higher levels of lactate in their systems. However, if a horse develops a level of lactate significantly higher than his body has learned to cope with, he will be more likely to tie up. Five percent of Thoroughbred race horses develop the recurrent exertional rhabdomyolysis type of tying-up during the racing season. This usually occurs when they are trained at a gallop but held back from full racing speeds.
Reducing the risk of tying up in horses
Much of the treatment for tying up focuses on prevention. Horses should be warmed up and cooled down properly with at least 15 minutes of walking. Horses should never be exercised to the point where they become stressed. Turnout should be provided as frequently as possible.
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Natural options for Tying up in horses
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For more information on helping your horse stay healthy, please see Tips for a Healthier Horse