Exertional rhabdomyolysis (ER) is a form of exertional myopathy in horses that is represented by muscle fatigue, pain, or cramping associated with exercise. Less common exertional myopathies that cause exercise intolerance without muscle necrosis include mitochondrial myopathies and forms of polysaccharide storage myopathy in Warmblood horses.¹ While exertional rhabdomyolysis was once considered a single disease described as azoturia, tying-up, or cording up, it is now known to comprise several different myopathies which differ significantly in etiopathology despite similarities in clinical presentation.² Exertional rhabdomyolysis can be sporadic (with single or very infrequent episodes), or chronic (with repeated episodes occurring).

Any breed of horse can develop sporadic exertional rhabdomyolysis. The most common cause is exercise that exceeds the horse’s state of training. The incidence of muscle stiffness also has been found to increase during an outbreak of respiratory disease. Dietary deficiencies of sodium, vitamin E, selenium, or a calcium-phosphorus imbalance can also contribute to this condition. Some horses have recurrent episodes of rhabdomyolysis even with light exercise, which is known as chronic exertional rhabdomyolysis.¹

Diagnosis of Exertional Rhabdomyolysis in the Horse

Approximately 3% of exercising horses are reported to have had an episode of ER in the last 12 months.³ Clinical signs of exertional rhabdomyolysis are usually are seen shortly after the onset of exercise. Horses with ER usually show signs of muscle stiffness, shifting hindlimb lameness, elevated respiratory rate, sweating, firm painful hindquarter muscles and reluctance to move that can last for several hours. Subclinical episodes can occur in some horses, causing decreased performance and painful muscles without other overt signs.

A diagnosis of ER is usually based on clinical signs of muscle stiffness and pain after exercise in conjunction with elevations in serum creatine kinase (CK) and aspartate transaminase (AST) activities. The degree of elevation of these enzymes in serum is dependent on the severity of muscle damage as well as the length of time that has elapsed between the sample collection and the occurrence of muscle damage.³

Severe exertional rhabdomyolysis can lead to renal compromise due to ischemia and the combined nephrotoxic effects of myoglobinuria, dehydration, and NSAID therapy.¹ In horses with hemoconcentration or myoglobinuria, a top priority will be to reestablish fluid balance and induce diuresis. In severely affected animals, regular monitoring of BUN and/or serum creatinine is advised.^1,2

Methocarbamol for Treating ER in Horses

Contraindications include food animals, animals with renal disease (injectable forms), and animals with a hypersensitivity to methocarbamol.²

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.