Exertional Rhabdomyolysis in Horses

What is Exertional Rhabdomyolysis in Horses?

Exertional rhabdomyolysis (ER) also called “tying-up” is a syndrome in horses that causes muscle pain and cramping associated with exercise. The words exertional rhabdomyolysis mean the dissolving of muscle cells with exercise.

ER has been recognized in the equine world for over 100 years and continues to be a performance and career-limiting disorder in horses. ER can have several different causes and by determining the form of ER, you can implement management strategies to help control and minimize the episodes and symptoms.

Types of ER in Horses

There are two types of ER in horses:

Sporadic ER: If a horse has a single episode of tying up or infrequent episodes, they are categorized as having sporadic

Chronic ER: A horse that has repeated episodes of tying up along with increased muscle enzyme activity is classified as having chronic ER.

Symptoms of Exertional Rhabdomyolysis in Horses

Typically, clinical signs of tying up occur shortly after beginning exercise including:

• Firm and painful muscles

• Stiffness

• Excessive sweating

• Quick, shallow breathing

• Rapid heart rate

• Muscle tremors

In extreme cases horses may show these signs:

• Reluctance/refusal to move

• Discolored (red/brown) urine

Severe cases can progress to massive muscle necrosis and renal failure causing damage to the kidneys.

Causes of Exertional Rhabdomyolysis in Horses

Horses that have sporadic ER have no defect in their muscle, instead something in their environment has caused the muscle cells to be damaged. Sporadic ER occurs most commonly in horses that are exercised at a rate higher to their current condition. Other conditions that may lead a horse to have an episode of sporadic ER include:

• Training program that is accelerated too abruptly or after a period of no exercise for a few days, weeks, or months

• Competitions or exercise on hot, humid days when a horse’s body temperature is high; loss of fluid and electrolytes in sweat occur causing depletion of muscle energy stores

• Metabolic imbalances leading to muscle dysfunction and damage

• After or during respiratory infections where they have a fever, cough, nasal discharge, or other signs of respiratory compromise

Chronic ER can occur in horses due to the following causes:

• Recurrent Exertional Rhabdomyolysis (RER)

• Type 1 and 2 Polysaccharide Storage Myopathy (PSSM1, PSSM2)

• Malignant Hyperthermia (MH)

• Myofibrillar Myopathy (MFM)

How Veterinarians Diagnose Exertional Rhabdomyolysis in Horses

A veterinarian can diagnose your horse with an episode of ER through history, physical exam, and blood sample to determine muscle protein levels. When muscle cells are damaged during an episode of tying up, the levels of proteins will increase.  

If your horse has had several episodes of ER then your veterinarian may recommend further diagnostics to look for an underlying cause. Genetic testing (using a sample of tail hair) can be used to diagnose a horse with PSSM1. If this test returns normal then a muscle biopsy may recommend to look for another reason of tying up such as PSSM2 and RER.

Treatment of Exertional Rhabdomyolysis in Horses

Initial treatment for a horse with an episode of ER includes stall rest for 12-48 hours until they can move comfortably. During this time, a non-steroidal anti-inflammatory such as Banamine® can be administered to help with pain and inflammation. Depending on the cause and severity of the episode, oral or IV fluids may also be used to help with dehydration or flushing of myoglobin from the kidneys. A muscle relaxer such as Acepromazine may also be administered. Removing all grain and allowing access to fresh water and hay is also recommended for the initial stage of treatment.

If the horse is able to walk freely, moving the horse to a small paddock or turnout area where it can walk at will is important. Hand walking at 10-minute increments can begin at this time but should be implemented with caution, as recurrence of ER may occur. Slow progression of exercise over the next 1-2 weeks should occur according to your veterinarian’s recommendations. Your veterinarian may want to do a follow-up examine before the horse returns to their normal activity level.

Recovery and Management of Exertional Rhabdomyolysis in Horses

After a horse has an episode of ER it is important to implement management practices to help reduce the risk of recurrence. The following are tips for proper management for horses with ER.

• Nutrition: A balanced diet with appropriate caloric intake and adequate vitamins and minerals are vital to managing and preventing future episodes of ER

• Vitamin E and selenium: Ensuring your horse has adequate amounts of vitamin E and selenium can be important for normal muscle function and recovery. Your veterinarian may want to measure your horse's selenium and vitamin E levels to determine if they could be deficient. Your veterinarian can help to design a supplement plan.

• Electrolytes and minerals: Horses often exercised in hot weather may develop electrolyte imbalances. Offering free choice access to loose salt or salt blocks should be provided for these horses. Fresh water should always be available, especially if electrolytes are being supplemented. Dietary imbalances of electrolytes can cause episodes of ER, correction of these imbalances through supplementation may be critical to management of ER for these horses. Your veterinarian can help to design a nutritional plan to incorporate electrolytes.

• Exercise: Horses, just like humans, should be slowly and gradually increased in their level of work. After an episode of ER, a program should be designed to slowly increase the horse's duration and intensity of exercise to ensure adequate fitness and prevent further episodes of ER.

Exertional rhabdomyolysis may lead to muscle necrosis, renal failure, and even death in some instances.

Prevention of Exertional Rhabdomyolysis in Horses

For horses with sporadic ER, prevention can start by providing your horse with adequate nutrition, water, and daily consistent exercise as mentioned above in the management section.  

For horses diagnosed with chronic ER, prevention will likely be dependent on the underlying cause. Genetic testing as well as any other diagnostic tests your veterinarian recommends will help to diagnose and implement the best preventative measures for your unique horse.

Aspects of ER prevention will be similar to the above-mentioned management and will include:

• Adequate/balanced nutrition

• Strict diet restrictions (often a low-starch diet)

• Consistent daily exercise

• Daily turnout

• Access to fresh water

• Supplementation such as a muscle specific vitamin E, and/or selenium

• Fat supplementation may also be recommended