Exertional myopathies in horses are highlighted by instances of muscle fatigue, pain, or cramping associated with exercise.1 Most exercise-associated myopathies result in the degeneration of skeletal (striated) muscle, which is called exertional rhabdomyolysis (ER). This is also known as “tying-up”, a condition that can be a performance-limiting or career-ending problem for racing horses.2 The term “exertional rhabdomyolysis” literally means the deterioration of muscle cells with exercise. This condition has been reported since humans first engaged horses for work, recreation and entertainment, with approximately 3% of exercising horses reported as having experienced an episode of ER in the last 12 months.2 

Clinical Signs ER in the Horse

Excessive sweating, increased breathing rate, rapid heartbeat, reluctance or refusal to move, and firm, painful hindquarters are common characteristics of ER.2 These signs are usually seen shortly after a horse begins to exercise. Their severity can vary from horse to horse, and even between episodes. Severe episodes can involve muscle damage with kidney failure and recumbency. ER is often reported as being sporadic or chronic. While horses of any breed may develop ER, it is common in Thoroughbreds and can be affected by temperament, sex, age, diet, exercise routines, and lameness.3 

In severe cases, horses may be reluctant or refuse to move, and may produce discolored urine due to the release of myoglobin from damaged muscle tissue. Episodes of ER vary from subclinical to severe in which massive muscle necrosis and renal failure from myoglobinuria occurs.1

Diagnosing and Treating ER

A diagnosis of ER is confirmed using blood screenings that test for two muscle proteins, creatine kinase (CK) and aspartate transaminase (AST). The proteins are used as a measure of muscle damage due to the fact that when muscle cells are damaged, they release these proteins into the bloodstream.2 

A complete veterinary evaluation will be required in order to determine the specific cause of ER. Depending on the horse’s breed, age, frequency of ER and serum CK and AST activity, the veterinarian will usually select a specific diagnostic plan that may include an exercise challenge, genetic testing or muscle biopsy. By determining the underlying form of ER that a horse is suffering from, specific management strategies can be used to control symptoms.2,3

Treatment of ER is often directed at relieving anxiety and muscle pain as well as replacing fluid and electrolytes. Tranquilizers such as acepromazine, xylazine and detomidine combined with butorphanol have been used to provide sedation and analgesia, as well as constant-rate infusions for extreme cases.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 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.