Kirsten Voigt, DVM

Tying up is a general term for muscle pain and damage after exercise.  The medical term is Exertional Rhabdomyolysis or ER.  There are two broad categories of the disorder; sporadic and recurrent.  Sporadic cases happen infrequently, maybe only once in a horses lifetime, and are associated with environmental factors.  These cases have no underlying abnormalities of the muscle so they only occur under extreme conditions.  Examples include overexertion, heat exhaustion or dietary imbalances of electrolytes, vitamin E or selenium. 

Recurrent cases appear the same as sporadic cases but happen more frequently.  Horses with recurrent ER have a defect in the muscles ability to regulate calcium, which is necessary for contracting and relaxing the muscle.  Changes associated with the abnormality can be seen on muscle biopsy which is the best way to definitively diagnose the condition. 

Tying up can look a lot like colic.  The horse may be breathing rapidly with a high heart rate.  They often sweat, have tightened or tremoring muscles, are reluctant to walk and if made to have a very short stiff gait.  It occurs after exercise.  In sporadic cases it is usually heavy exercise that the horse is not yet conditioned for.  Recurrent cases are different in this aspect.  Episodes are more common after a day or longer of rest then return to work.  They usually don’t happen if the horse is allowed to work at a maximum exertion but if held back during the workout the likelihood increases.  Sporadic cases can happen to any horse at any age where as recurrent cases are more common in young fillies with nervous dispositions.  The breeds most commonly affected are Thoroughbreds, Standardbreds and Arabians.  The muscle defect is genetic and tends to run more frequently in certain family lines within a breed as well. 

A horse that is tying up should not be walked as this can be very painful and does not encourage the muscles to relax and normalize.  Standing quietly in a cool, relaxed environment is best.  Several medications are available to veterinarians to help with pain and relaxation of the muscles.  Blood tests are very helpful in determining the diagnosis and the severity of the problem.  During an episode there is damage done to the muscles that causes certain blood levels to rise in relation to the amount of damage done.  If the damage is severe enough, the urine may be discolored and damage to the kidneys is possible.  Blood tests could help determine if the horse may need hospitalization.  They can also be helpful to monitor recovery and know when the levels are normal enough for the horse to return to work.  A final diagnosis can be made by sending a biopsy, or sample, of the muscle to a lab that will determine the underlying cause of the problem. 

If a horse is suspected of having recurrent ER, exercise tests can be done where blood samples are taken before and 4-6 hours after fifteen minutes of controlled trotting.  In horses with ER, a significant change will be seen in the blood levels even if the horse is not visibly tying up. 

To prevent sporadic cases from occurring horses should not be exerted far beyond their current conditioning level.  Proper hydration and electrolyte supplementation is important with heavy exercise especially in hot and humid weather.

Maintenance and prevention of recurrent cases includes regular daily exercise at an appropriate level for the horse. Prolonged periods off work lead to more episodes of tying up when suddenly exerted.  Reduce stress and keep them as calm as possible which includes feeding feeds that have minimal sugars and simple carbohydrates and a higher percentage of fats and fiber for an energy source.   Diets with balanced vitamins, minerals and electrolytes are also helpful.  Regular turn out in a pasture is better than stall confinement.  

One other form of Exertional Rhabdomyolysis is worth mentioning here.  It is a disorder known as polysaccharide storage myopathy or PSSM.  It looks very similar if not identical to the recurrent form but the underlying problem with the muscle is due to inappropriate glycogen storage in the muscle.  It differs in that the most commonly affected breeds are Quarter Horses, draft breed and Warmbloods.  PSSM is a genetic disorder as well.  It is often first seen at around five years of age in affected Quarter Horses.  Exercise tests and blood tests can help with the diagnosis but the only way to differentiate it from other muscle disorders is by muscle biopsy.   

Management is similar in that regular consistent exercise and diets low in sugars and simple carbohydrates are best.  PSSM horses may however need a much more careful program for conditioning beginning with ground work only, just 4 minutes at a time and working up to a half of an hour over a three week span.  Once ridding starts workouts should only increase by two minutes at a time.  Length of time working is a bigger factor in causing and episode than the intensity of the work so conditioning must occur gradually.  Once in shape the best prevention is to keep them in shape with daily work. 

Exertional Rhabdomyolysis can be a worrisome condition but with prompt treatment, accurate diagnosing and appropriate diet and exercise management, owners can still enjoy there ridding companions for a variety of riding disciplines.  For more information on this subject go to www.cvm.umn.edu/umec/lab/home.html to view fact sheets written by Dr. Stephanie Valberg, one of the nations leading research experts on muscle disorders, from the University of Minnesota.