Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.
One health condition of horses that necessitates quick identification and action on the owner's part is acute rhabdomyolysis ("tying up"), an ailment that, if left unattended, can progress to the point of requiring euthanasia. At a recent veterinary conference, one researcher gave an overview of how to identify and manage episodes of "tying up."
"Severely affected horses can present a challenging medical situation, and evaluation should include attempts to identify the probable underlying cause of disease to ensure that thorough and appropriate treatment is provided," explained Erica C. McKenzie, BSc, PhD, Dipl. ACVIM, Dipl. ACVSMR, associate professor of large animal medicine at Oregon State University. She presented the lecture at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.
Before delving into identification and treatment, McKenzie discussed a few causes of rhabdomyolysis.
McKenzie said that rhabdomyolysis can be caused by either "exertional or non-exertional phenomena." Exertional causes include:
- Polysaccharide storage myopathy (PSSM, most commonly seen in Quarter Horses and related breeds often associated with a mutation of the glycogen synthase 1 gene); and
- Recurrent exertional rhabdomyolysis (RER, a calcium regulation disorder seen commonly in Thoroughbreds, Standardbreds, and potentially Arabian endurance horses).
Non-exertional causes include:
- Nutritional myodegeneration (also called white muscle disease and caused by selenium deficiency);
- Atypical myopathy (a sudden onset of acute muscle pain and damage in horses at pasture);
- Traumatic myopathy;
- Immune mediated phenomena associated with respiratory illness and Streptococcus equi infection (immune mediated or streptococcal myositis [muscle inflammation] and/or infarctive hemorrhagic purpura [localized tissue death resulting from obstruction of the blood supply to the affected site]);
- Malignant hyperthermia (a fast rise in body temperature and abnormal muscle contractions when under general anesthesia); and
- Ionophore exposure (a group of antibiotics found in many livestock feeds that horses should not consume).
The clinical signs and treatments for rhabdomyolysis are generally influenced by the severity and root of the condition, so McKenzie stressed that identifying the initiating cause is beneficial to ensure proper therapeutic options are employed.
Not surprisingly, exertional and non-exertional rhabdomyolysis may have different clinical signs to watch for, McKenzie said.
When dealing with a case of exertional rhabdomyolysis, clinical signs will develop during or shortly after exercise, she said. The most common signs to watch for include excessive sweating, increased respiratory rate, stiffness, nonspecific lameness, and a reluctance to continue working or moving. Affected horses might also produce dark colored urine, she said.
"Clinical signs associated with non-exertional rhabdomyolysis are somewhat dependent on the underlying cause," she said. For instance:
- Nutritional myodegeneration in foals will result in stiffness, weakness, abnormal head carriage, difficulty swallowing, and cardiac disease; affected adult horses often exhibit similar signs in addition to excessive salivation, muscle pain and swelling, and abnormal jaw movement;
- Horses with immune mediated phenomena related to an S. equi or other respiratory infections will often show one of more of the following signs: stiffness, lethargy, reluctance to move, gluteal and epaxial muscle atrophy, recumbency, uncontrollable pain, limb edema, and darkened mucus membranes;
- Clinical signs in horses with atypical myopathy generally appear rapidly and without warning in horses 3 years of age or less; signs include recumbency, myoglobinuria (dark, red-brown colored urine), and death within 72 hours;
- Horses with anesthetic myopathy exhibit signs including prolonged recovery from anesthesia with multiple attempts to stand, muscle fasciculations, sweating, reduced weight bearing ability, and swelling of affected muscle groups; and
- Malignant hyperthermia (a sudden increase in body temperature and the occurrence of muscle rigidity) can also occur in horses.
If the affected animal is not already under veterinary care (as they would be with anesthetic myopathy), contact a veterinarian to aid in diagnosing or confirming the problem.
"Assessment of horses with suspected acute rhabdomyolysis should commence with collection of a thorough history to elucidate any associated causes, including unaccustomed exertion, exertion after an unusual rest period, dietary factors, recent respiratory infection, trauma, anesthesia, or possible toxicosis," McKenzie said.
Once a thorough history has been collected, McKenzie said a physical examination should be carried out and should include visual inspection and palpation for muscle asymmetry, pain, tightness, swelling, or atrophy.
She also noted that using ultrasound or thermography can be useful for identifying muscular abnormalities not outwardly visible, and echocardiography should be used for horses suspected of having nutritional myodegeneration, ionophore exposure, or atypical myopathy to check for cardiac changes. Finally, abdominal ultrasound and/or a belly tap can help let veterinarians know if infarctive purpura could be present since intestine may become diseased in affected horses.
Veterinarians will also perform blood tests to check for specific indicators that could point to the root cause of the rhabdomyolysis, and in some cases, a muscle biopsy can confirm or negate a potential differential diagnosis, she said.
Once a diagnosis has been reached, treatment and subsequent management can commence.
Treatment and Management
In the last section of her lecture, McKenzie discussed common treatment and management methods of horses with acute rhabdomyolysis.
"The objectives of care in acute rhabdomyolysis include preventing further injury; maintaining mobility and circulation to muscle tissue; relieving anxiety, mania, and pain; correcting fluid, acid/base, and electrolyte derangements; and preventing renal compromise," she said.
Often times, horse owners will need to manage the first stages of acute rhabdomyolysis independently. McKenzie gave the following recommendations:
- Regardless of whether it falls under the exertional or non-exertional category, immediately cease any exertion the horse might be carrying out when rhabdomyolysis is suspected;
- If the horse needs to be moved for treatment, ensure the pace is a slow walk, and intersperse rest stops; and
- If transport to a referral clinic or hospital is necessary, ensure the trailer is well-bedded with shavings or straw in case the horse becomes recumbent during the ride.
Upon arrival at the clinic or hospital, McKenzie advocates placing an intravenous catheter to simplify diagnostics and to administer fluids. Once a diagnosis is reached (as described above) treatment can begin. Regardless of the cause, the foundation of treatment in most forms of rhabdomyolysis is based around supportive care, however specific subsets of the disorder or clinical signs sometimes necessitate specific treatment modalities including intravenous fluid therapy or vitamin E and selenium supplementation. Some additional important notes on treating rhabdomyolysis include the following:
- Horses affected by malignant hyperthermia or exhaustion-related hyperthermia require active cooling to keep their body temperature stable within acceptable ranges;
- If horses have little to no urine production and evidence of possible renal damage they should receive diuretics as long as they are concurrently appropriately hydrated;
- Horses with anesthetic myopathy, streptococcal rhabdomyolysis, and/or infarctive purpura are known to be violent and difficult to keep comfortable via standard analgesic protocols, so these animals might require multiple pain-relieving drugs. McKenzie stressed that caution must be practiced when using drugs such as flunixin meglumine and phenylbutazone, which can have adverse effects on renal function;
- Complimentary therapies including antioxidant administration, therapeutic ultrasound, massage, acupuncture, and hyperbaric oxygen therapy might be beneficial for some affected horses;
- Treatment for horses with infarctive purpura--which, if elected, includes antibiotic therapy and corticosteroids--is generally unsuccessful; and
- Monitoring blood serum creatine kinase levels (which indicate muscle damage) can help veterinarians decide their course of treatment. She noted that dantrolene sodium can be administered orally to non-fasted horses to reduce or prevent additional muscle damage.
Once an acute rhabdomyolysis episode has been effectively managed, the risk of further episodes can be minimized by protecting the muscles from inappropriate exertion or trauma. For horses with hereditary defects of muscle function, adopting a high-fat and-fiber, low-starch diet and maintaining a consistent exercise program usually provides substantial benefit.
Acute rhabdomyolysis episodes can be difficult to manage, but having a solid understanding of what to look for and what to do when the situation arises can help ensure horses receive appropriate medical care. Always seek veterinary attention if acute rhabdomyolysis is observed.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.