Exertional Rhabdomyolysis

Kim Snedden

Sarah Aucoin

Dr. Rustin M. Moore

Traditionally, exertional rhabdomyolysis, more commonly known as tying up, was termed Monday Morning disease because it was mainly seen in horses that were worked on Monday morning after having rested on Sunday. Today this disease can be seen in any breed of horse. Predisposing factors include horses fed a high grain diet then worked on irregular schedule or given a day or two off, anxious or nervous horses, and genetics. Although specific genes are not yet identified there is an increase in some family lines in Quarter horses, Standardbreds, Belgian, and Percherons, and Thoroughbreds.

Cause

The exact mechanism of extertional rhabdomyolysis is still unknown. One of the current theories is related to how horses store energy as glycogen in their muscles. A problem occurs when horses are fed a high grain diet and is rested for a day or two. The amount of energy needed is less so there is a net increase in glycogen stored in the muscles. The next time the animal is worked, the muscle fibers will use the increased amount of stored energy and convert it to a large amount of lactic acid. This increase in lactic acid overwhelms the muscle fibers and causes damage to the muscle fibers.

Clinical Signs

This disease has fairly specific signs that should raise a red flag in the owner/trainer’s mind. After mild to moderate exercise the horse will begin to sweat profusely, tremble, and have an increased heart rate. The next sign is weakness in the hind limbs that will cause the horse to have a short choppy gait and be reluctant to move. Cramping in the muscles will cause them to be hard to the touch. If the disease is severe, the horse will show signs of forelimb weakness and the urine will be a dark brownish-red color due to an accumulation of myoglobin, a pigment from muscle breakdown. If these signs are noted, immediate action should be taken or the disease may be fatal.

Diagnosis

The diagnosis of exertional rhabdomyolysis can usually be made through the animal’s history and clinical signs that are exhibited. Several laboratory tests may aid in the diagnosis and are helpful in monitoring recovery. Elevations of muscle enzymes found in the blood following acute exertional rhabdomyolysis are useful indicators of the extent of muscle damage. Muscle enzymes commonly evaluated are CK, AST and LDH. In addition, analysis of muscle biopsy tissue may be of value. The muscle tissues that are collected from affected horses will reveal a number of lesions. Evidence of ongoing inflammation and regeneration will occur within these tissues. In animals with polysaccharide storage disease, an increase in muscle glycogen content and storage inclusion will be found. Collection and evaluation of urine may also help to detect early kidney damage due to changes in the circulation and precipitation of myoglobin in the renal tubules.

Treatment

The goal of initial therapy is to limit further muscle damage, reduce pain and anxiety, and restore fluid and electrolyte balances. The treatment required will vary with the individual and clinical severity. Common recommended therapeutic agents are listed in table 1. Caution should be exercised with the administration of Flunixin meglumine and other NSAIDs in dehydrated animals or animals in renal failure. The animal should be rehydrated with IV fluids before the drugs are given. Other therapies that have been used include sodium bicarbonate, oral vitamin E and selenium, B vitamins, diethyl sulfoxide (DMSO), and seratonin antagonist. Complete rest is recommended for all cases. Forcing the horse to move or transporting the horse in a trailer can increase the severity of muscle damage.

The feed intake should be reduced in severe cases to grass, hay, and water. The animal should be kept warm and dry. If the horse becomes recumbent, provide a well-padded stall with access to food and water. Assist the horse to rise, but do not force, as soon as possible. Continue to monitor hydration status and urine output.

After the initial therapy the horse should be restricted to stall rest. Return to activity depends on clinical improvement of the horse and return of serum CK levels to near baseline. Return to exercise should be gradual, starting with a short turnout of approximately 5 minutes duration in a paddock and increase the amount of time slowly over weeks.

Table 1: Commonly Used Therapeutic Agents in Equine Rhabdomyolysis Syndrome

Classification	Drug	Comments
NSAIDs	1. Phenylbutazone	May be toxic with kidney dysfunction or dehydration.
	2. Flunixin meglumine	May be toxic with kidney dysfunction or dehydration.
	3. Meclofenamic acid
Analgesics	4. Butorphanol
Sedatives/Tranquilizers	1. Phenothiazine derivatives	May lead to low blood pressure and shock.
	2. Xylazine	Has an analgesic effect
	3. Detomidine	Has an analgesic effect
	4. Diazepam
Corticosteroids		Used during initial acute stages in moderate to severe cases.
IV Fluids		Balanced polyionic solution. Corrected for acid-base abnormality if occurs.

Control/Prevention

Exercise- Once the horse has recovered from the initial bout, instigate a regimen of regular daily exercise. Avoid any breaks from exercise, even of only 1-2 days.

Diet- The diet should be as balanced as possible, with limited energy provided from soluble carbohydrate sources. The use of cereal grains, especially oats, should be limited or avoided. Increasing fat in the diet using rice bran or corn oil may help provide adequate calories and decrease the incidence of this disease. The diet should also be balanced to work load, and any reduction in work should be accompanied by a reduction in feed intake. Oral supplementation of vitamin E and selenium have been noted to have value in treatment due to their action against free-radicals induced muscle damage. Oral electrolytes in water may also be provided in a separate bucket than normal water source, to theoretically prevent imbalances.

Medications- In addition to diet and exercise control measures there are various therapeutic agents that have been proposed to prevent the recurrence of rhabdomyolysis, most lack controlled studies in the equine. Dantrolene has been used in humans to treat a similar condition. It is believed to decrease the rate of calcium released from inside the muscle cell, preventing prolong muscle contraction. Because this drug is toxic to the liver, long-term therapy is not recommended, and liver function should be monitored regularly. Phenytoin has also been suggested as beneficial in the prevention of rhabdomyolysis. Also, an amino acid, dimethylglycine, has been used as a nutritional supplement for horses with the aim of delaying fatigue by decreasing lactate production. It has also been reported to decrease the incidence of this disease.

Prognosis

Prognosis depends on the extent of muscle damage; it is good for those animals that remain standing, and is fairly good for those animals that go down due to loss of use of their hindquarters, but stay quiet and content when down. The prognosis is poor for nervous, restless, recumbent animals that continue to struggle and are not quieted by sedatives and tranquilizers. The prognosis is also poor in those horses that are forced to continue to move after initial clinical signs.