Exertional rhabdomyolysis (ER) is a clinical syndrome in horses that is indicated by muscular pathology stemming from a variety of possible causes. It was first described in the early part of the 20th century, and was often called “tying up.” Owners of draft teams often noticed muscular distress in their horses following a day of rest, leading the syndrome to be called “Monday Morning Disease” by some. Although any horse can develop exertional rhabdomyolysis and some breeds seem to be more prone than others, often horses of the same breed and performing identical work will give rise to only a few developing ER. Studies have thereby concluded that ER is probably associated with both heredity and husbandry, around which research on the syndrome focuses to this day.

Of the many suspected causes for ER, certain causes appear to be particular to certain breeds. The more heavily muscled, calmer breeds (e.g. Quarter Horses, warmbloods and draft breeds) are thought to be affected by a defect in muscular metabolism called Polysaccharide Storage Myopathy (PSSM or EPSM).2,3 Thoroughbreds and other hot-blooded breeds, however, are thought to suffer a defect in intracellular calcium regulation.1  Other proposed etiologies have included Vitamin E deficiency, electrolyte imbalance, viral infection, and (due to an increased incidence in mares) hormonal influences.3 Genetic studies of affected horses have revealed different patterns of inheritance for both the PSSM and calcium related forms of ER.2

ER Clinical Signs and Diagnosis

Horses with exertional rhabdomyolysis typically display a wide range of physical signs, from mild stiffness and myalgia to recumbency and death.  Diagnoses are based on clinical signs, evaluating increases in serum creatinine kinase (CK), aspartate amino-transferase (AST), myoglobinuria and lesions upon muscle biopsy. Most diagnostic biopsies are taken from the middle gluteal and semimembranous muscles. Histological signs of ER include vacuolization and fragmentation of myocytes, and macrophage infiltration.1 Consideration must be given to ruling out other diagnoses, such as colic, fractures, pleuritis or laminitis, which would also cause a horse to act painful and reluctant to move.2

Prevention and Treatment of ER

Horses acutely affected with rhabdomyoysis should be moved as little as possible.1 Mild cases may be briefly walked, but movement tends to worsen the pathology in many cases.  Treatment should be directed towards reducing pain and anxiety in these horses to keep them as quiet as possible. Correcting hydration status is crucial, especially to prevent kidney damage in cases with myoglobinuria.2

Limited studies have shown a decreased incidence of ER following a switch to high-fat, low-carbohydrate diets, based on the theory that this decreases muscle glycogen stores.4 Daily exercise and turn-out are also suggested.

Methocarbamol for ER in Horses

It is commonly used in performance horses for the treatment of skeletal muscle disorders. Current regulatory recommendations for show horses and racehorses are based on a single oral dose of 5 g, although doses in excess of this are often administered.4

Methocarbamol’s exact mechanism of action is not known, but it is used to treat muscle spasticity in a variety of clinical conditions. An increase in tonic stretch reflexes originates from the central nervous system with involvement of descending pathways and results in hyperexcitability of motor neurons in the spinal cord.3 Methocarbamol alleviates the muscle spasms by blocking interneuronal pathways in the spinal cord and in the midbrain reticular activating system. It also has a sedative effect that may be beneficial to anxious horses that are in pain. 

Methocarbamol is commonly used as an adjunct therapy of acute inflammatory and traumatic conditions of skeletal muscle to reduce muscle spasms in cases involving trauma, myositis, muscular and ligamentous sprains and strains, intervertebral disk disease, neurologic disorders, and breakdown of muscle cells.

Contraindications include food animals, animals with renal disease (injectable forms), and animals with a hypersensitivity to methocarbamol.1

NOTE: The Association of Racing Commissioners International Uniform Classification Guidelines for Foreign Substances has designated methocarbamol a CLASS 4 DRUG.

Where to buy Methocarbamol

Methocarbamol is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. 

Methocarbamol carries several potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1Merck Veterinary Manual.

2msu.edu.

3MacLeay JM, Sorum SA, Valberg SJ, Marsh WE, Sorum MD. Epidemiologic analysis of factors influencing exertional rhabdomyolysis in Thoroughbreds. American Journal of Veterinary Research. 1999 Dec;60(12):1562-1566. PMID: 10622169.

4Knych H.K., et. al. Pharmacokinetics of methocarbamol and phenylbutazone in exercised Thoroughbred horses. J Vet Pharmacol Ther. 2016 Oct;39(5):469-77.