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Tying Up: Rhabdomyolysis in horses

Hard, painful muscles in the rear are among the signs of tying up.

Hard, painful muscles in the rear are among the signs of tying up.

Tying up syndrome is a preferable name to exertional rhabdomyolysis because not all horses that tie up have exercised, writes Dr Robert N Oglesby, DVM.

Tied up horses develop pain and stiffness in the lower back, gluteals and muscles of the thigh region. The problem can vary from a transient cramping to a serious problem where muscle destruction occurs. In cases of muscle destruction the urine turns dark with myoglobin, a component of muscle cells.

There have been many proposed mechanisms for tying up but frequently electrolyte concentrations are a factor in horses that tie up during intense exercise. In the 1990s it became evident that many cases are due to a defect in carbohydrate metabolism within affected skeletal muscle fibers. This condition has been dubbed Equine Polysaccharide Storage Myopathy (EPSM), and has led to some new treatment options that have shown great success. It has been proposed that this may be an inherited disease with an autosomal recessive pattern.

Symptoms

Typical signs of tying up are:

  1. A stiff walk, that at its worse can immobilize the horse.
  2. Hard painful muscles in the rear.
  3. If the pain is severe enough sweating may occur.

Horses that tie up usually have a recurring problem, associated with a specific set of circumstances. When horses worked for a living, a common problem was “Monday morning’s disease.” This problem occurred when horses, which were working all week, did not have their grain cut back while resting over the weekend. Some horses tie up when they become excited, at a show, breeding, or traveling. Other horses develop the problem when exercised. Occasionally you run into the horse that ties up for no reason you can detect.

Moderate to severe cases are not hard to diagnose. The stiffness in the rear limbs and hard painful muscles are easily seen. But mild cases may present as decreased performance or a vague lameness originating in the back or hind end. In cases of possible tying up the diagnosis can be confirmed with blood tests for increase levels of muscle enzymes.

Possible Causes

There have been several proposed causes for tying up. The different histories, and varying treatments that horses respond to, strongly suggest that we are looking at several different diseases. Some proposed causes are:

  • Altered carbohydrate metabolism
    Recent work has identified abnormal carbohydrate metabolism as a cause in many breeds of tying up. The others on the list are at this time unproven causes of the problem.
  • Hypotension and Electrolyte imbalances
    Horses which tie up during or following exercise frequently have electrolyte imbalances. These imbalances cannot be diagnosed with simple blood testing.
  • Lactate build up (unproven)
  • Too little oxygen getting to the muscles (unproven)
  • Vitamin E / selenium deficiencies (unproven)
  • Hypothyroidism (unproven)
  • An interesting observation by the University of Calif. is that ear ticks (Otobius megnini) have been associated with muscle spasms in more than half a dozen cases. These muscle spasms were very much like tying up. In each case muscle enzymes were elevated. When the ear ticks were treated the horses got over the spasms. Other signs identified with theses horses were colic, tremors, and seizures. (JAVMA, v207, n1)

Diagnosis

Diagnosis is generally not complicated except in mild cases which may resemble founder or generalized stiffness. The muscle enzymes CPK and AST are invariably elevated during a tying up episode and shortly after. Strenuous exercise can elevate these enzymes up to 4 fold normal while most tying up episodes result in a 10 fold rise or greater.

EPSM

Diagnosis of EPSM form of tying up can be made with a muscle biopsy of the semitendinosus muscle preserved in 10% buffered formalin. The samples are stained with hematoxylin and eosin, periodic acid-Schiff (PAS) stain and PAS followed by amylase digestion. Abnormal findings in the Type 2 glycolytic fibers are:

  • subsarcolemmal and intra-cytoplasmic pale staining zones,
  • fiber hypertrophy,
  • internal nuclei,
  • excessive glycogen.

Currently this type of pathology is being carried out at Cornell University. Doctors may contact the Dept of Pathology, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853, atten: Beth A Valentine, DVM, PhD.

Fractional Excretion Test

Once a horse has fully recovered from an episode of tying up a fractional excretion test for electrolyte imbalances should be done. Electrolyte imbalances are difficult to detect simply and routinely. The estimation of electrolyte concentrations in the blood cannot be used to detect deficiency or excess with any accuracy because of the homeostatic mechanisms which maintain normal blood concentrations. Fractional excretion test is a practical means of assessing certain alterations in electrolyte status.

Recently a muscle enzyme that tracks the progress of tying up syndrome in horses has been found. This enzyme, carbonic anhydrase III (CA-III), differs from other enzyme markers in that it correlates with the severity of the disease almost instantaneously. Most other enzymes, used to monitor tying up, lag behind the disease by days or weeks. Another advantage to CA-III is that levels are markedly elevated in horses with histories of tying up but are not currently showing signs of the disease. This makes it possible to tag susceptible horses for preventive care. Currently no labs in the USA are doing this testing regularly.

Treatment

First aid will depend on severity of signs. When just a little stiffness occurs, hand walking will help a horse loosen up. Recently it has been suggested that walking may be harmful but many mild cases have improved by being walked. Bute should be given for pain. Acepromazine (1 to 2 mg. /1000 lbs.) is helpful to relax the horse and increase peripheral perfusion. If the stiffness is severe current recommendations are to not move the horse.

Many severe cases are associated with intense exercise. If dehydration or electrolyte imbalances are possible, IV fluids should be used to correct these problems. Corticosteroids make sense for early treatment to help prevent further cellular damage. Phenylbutazone at the upper end of recommended doses will help to relieve the pain. Acepromazine (given after correcting dehydration) at the above rates will help increase blood flow to the muscles. If additional sedation or pain relief is desired narcotics (butorphanol or Valium) would be preferable over high doses of acepromazine.

Muscle relaxers seem to be rational therapy but questions of efficacy and potential toxicity do not make them a clear choice. Currently two muscle relaxers are advocated: dantrolene and methocarbamol. Check with your vet for the latest on these two.

Prevention

Electrolyte Imbalances

If the results of the fractional excretion (see above) test indicate a nutritional imbalance these should be corrected first. Routine use of electrolyte additions to the feed are not neccassarily good. However electrolyte supplementation during times of maximal stress is recommended.

As a horse sweats more and his electrolyte concentrations deplete his sweat becomes less salty thereby conserving needed electrolytes. On the other hand, when the system senses an excess of an electrolyte it cranks up mechanisms to rid the body of them.

If you supplement electrolytes on a daily basis you weaken the system’s ability to respond to stress.

The best time for supplements is just before and during a time of maximal stress. Electrolytes should be added to the feed and not the water. An inexpensive but effective electrolyte mixture can be made using 1 tb. table salt and 1 tb. Salt Substitute (Lite Salt or NoSalt: reg. TMs) mixed in the feed twice daily.

EPSM

Since the discovery of EPSM and its relation to tying up, dietary management has been very effective at managing this problem. To clear out the abnormal amount of glycogen and carbohydrate from the muscle cells requires 3 to 4 months with improvement noted for up to one year after instituting the diet. So do not expect rapid results, but with patience they will come with most horses being normal within a year.

Current recommendations are to remove as much carbohydrate as possible from the diet and substitute oil and protein as an energy source. Substituted for the grain portion of the diet is 2 cups (app. 1 lb) of vegetable oil per 1000 lbs and alfalfa pellets. A basic formula for initial estimation of amounts for a 1000 lb horse would be:
Current daily grain ration (in pounds) – Oil (in cups) = Alfalfa Pellets (in pounds)

For example take a 1200 lb horse receiving 5 lbs of grain a day: 5 (lbs of grain a day) – 2.4 (2 cups a day /1000 lbs x 1200 lbs) = 2.6 lbs of alfalfa pellets a day.

Of course this is best fed divided twice daily and changes should be made slowly. A problem in this diet is that high fat diets are may not be palatable to some horses and you may have to reduce the oil portion slightly to get the horse to eat it, particularly if the pellet portion is small. Some adjustments may need to be made. The horses weight should be watched and adjusted using the pellet portion of the diet. Of course during this time grass and hay should be available as usual. If you love in a selenium deficient area of the country you should consider having vitamin E (normal = 2.5 -4.0 ug/ml) and selenium (normal = 180 – 240 ng/ml) values run on the blood.

Other Possibilities

With some of the causes of tying up being unknown, and probably multifactorial, other preventive measures are trial and error. Identifying predisposing factors may allow prevention. For instance, if nervousness in certain situations results in tying up, mild tranquillizers where practical, might help. If intensive exercise brings on the problem consider better conditioning regimens and electrolyte supplementation.

Drugs reported to help in some cases are:

  1. Vitamin E \ Selenium (1 to 5 micrograms / lb. of horse / day) as a feed supplement. Selenium is potentially toxic so review any supplement plans with your veterinarian.
  2. Bicarbonate (180 mg. / lb. of horses / day or app. 1/3 lb. per 900 lb. horse per day) as a feed supplement
  3. Electrolyte supplementation (1 tsp. of table salt and 1/2 tsp. of Lite salt in the feed daily is an inexpensive sodium and potassium supplement)
  4. Oral phenytoin has been very useful in some cases. Start at 5 mg.\ lb. twice daily for three days then reduce by half every three days. The dosage is reduced to the lowest effective dosage.

Other Resources

Myopathies in a riding horse stable.
Tierarztl Prax 1991 Apr;19(2):167-169
Zentek J
Institut fur Tierernahrung Tieraztlichen Hochschule Hannover.

In this case report on myopathies in 6 saddle-horses, a combined dietary vitamin E and selenium deficiency is presumed. Other disorders, such as exertional myopathy (“Monday morning disease”) due to excessive energy intake or ionophore intoxication could be excluded by calculating the energy supply or by a simple colour test.

The selenium requirement of horses is estimated to 0.1-0.2 mg/kg dry matter (Meyer 1986). If natural feed compounds are low in selenium, adequate amounts of this trace element can be supplied by means of supplemental feeds, sodium selenite (20 mg/500 kg BW/week) or bruised linseed (cooked, 200-500 g/d). In horses tolerance of increased selenium intake is low. After oral administration of sodium selenite lethal dose was 3.3 mg/kg bodyweight. In feedstuffs selenium concentration must be below 5 mg/kg dry matter (Traub-Dargatz et al, 1986).

The intake of vitamin E should be 0.25-1 mg/kg bodyweight. The requirements are influenced by feed compounds, especially the concentration of dietary unsaturated fatty acids, and the performance of the horses (Meyer 1986). Foals, pregnant or lactating mares, and racing horses have higher requirements. In acute disease vitamin E and selenium are given parenterally (5 mg Se/horse) or by oral application of sodium selenite (46% selenium, 20 mg/horse), in addition to symptomatical treatment.

 

This © 2005 article reprinted with permission from Horseadvice.com, an internet information resource for the equestrian and horse industry since 1994. On the WWW at www.horseadvice.com we have tens of thousands of documents on the web about horse care, diseases, and training. 

First published on Horsetalk in 2005

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