Take a look at the variety of muscle disorders that fall into the category described by this broad and generic term.
It’s Wednesday night and you’d planned to ride right after work. It is a gorgeous hot and humid summer day, but you were held up at the office so you didn’t make it out to the barn on time. With your limited time frame shortened even more, you decide to forgo a warm-up and get right down to business. But 15 minutes into the ride, your horse starts to lose impulsion and develops a stiff, stilted gait. Thinking he is just a little sore from having skipped the warm-up, you ask him to continue. He starts to sweat heavily, his respiration increases and when you stop exercising he seems locked in place and refuses to move forward at all. What is happening? He is tying up.
What Exactly is Tying Up?
This is a generic term used to describe a variety of muscle disorders in the horse that cause cramping and muscle cell destruction during aerobic exercise. Tying up has had many names over the years, including set fast, azoturia, and Monday Morning Sickness. The medical term used to describe this disorder is exertional rhabdomyolysis – exertional meaning it occurs during exercise, and rhabdomyolysis meaning rapid breakdown (lysis) of muscle cells. Some horses may have one or two tying up episodes in their lifetimes (sporadic form), while for others it can become a recurrent, frustrating problem (chronic form).
The most common cause of sporadic tying up is overexertion (exercising beyond the horse’s current level of conditioning). It has been reported in a wide variety of breeds and occurs in around 3% of all exercising horses. However, it does seem to occur more often in nervous, high-strung fi llies. Other risk factors include:
• advancing training too quickly
• continuing to feed full rations to horses that are being ridden sporadically
• high grain diets
• exercising in hot, humid conditions/electrolyte imbalances
• pre-existing lameness • stall rest more than 12 hours per day
• a recent history of a viral respiratory infection (EHV-1 or influenza)
Contrary to popular belief, tying up is not caused by lactic acid buildup within the muscles, but more likely arises from muscle fatigue and/or electrolyte imbalances.
Most horses with the chronic form of tying up have characteristic features in their muscle cells. The human muscle biopsy technique that was adapted to horses in the 1960s allowed scientists to begin studying the microscopic structure of muscle in horses. This research has revealed that there are many different muscle disorders in the horse that can lead to the same painful muscle contractures of the back and hindquarters we collectively call “tying up”. Today we recognize four distinct causes, but there are probably more that have yet to be discovered:
Polysaccharide Storage Myopathy (PSSM)
PSSM is the most common cause of tying up in Quarter Horses and draft breeds. It is characterized by the excessive accumulation of unusable carbohydrate (glycogen and polysaccharide) in skeletal muscle. There are at least two forms of PSSM in horses based on current genetic research.
Type 1 PSSM is due to an inherited mutation in the glycogen synthase gene (GYS1). This mutation causes the enzyme that builds up glycogen stores in the muscle cell to be constantly in the “on” position. All the muscle cell’s energy is placed into storage form, resulting in a deficiency of energy during muscle cell contraction when it is needed, leading to cramping. A test now available through the University of Minnesota Veterinary Diagnostic Laboratory can tell you if your horse carries the mutated gene.
Horses with type 2 PSSM have the same problem with carbohydrate buildup in the muscle, but do not carry the GYS1 mutation. The specific cause for the type 2 form is not yet known, although it accounts for 80% of warmbloods and 25% of Quarter Horses with PSSM.
Recurrent Exertional Rhadomyolysis (RER)
Recent research investigating RER suggests the problem lies within the process of muscle cell contraction and relaxation. It may be a problem with the way calcium is regulated inside the muscle cell, but the specific defect that causes it has yet to be identified. A muscle biopsy that shows centrally located nuclei in muscle cells without abnormal polysaccharide buildup is currently the only way to confirm a diagnosis of RER. This type is most commonly found in Thoroughbreds.
Malignant hyperthermia Malignant hyperthermia is also due to genetic mutation in skeletal muscle. It is present in 1% or fewer of Quarter Horses, but can occur simultaneously with the PSSM type 1 mutation. Therefore, it is recommended that Quarter Horses with episodes of tying up that are hard to control through diet and exercise changes be tested for this mutation as well.
What Should You Do During a Tying Up Episode?
Stop exercising immediately and put the horse in a box stall to limit movement. Small quantities of water may be offered on a frequent basis while waiting for the veterinarian to arrive. He or she will take a blood sample to confirm the diagnosis of exertional rhabdomyolysis by measuring the levels of muscle specific enzymes that may have leaked into the bloodstream through damaged muscle cell membranes. Monitoring these enzymes through the recovery process will tell your veterinarian when your horse is able to return to work.
Non-steroidal anti-inflammatories and muscle relaxants will be administered to help with the pain. A sedative may be required to calm your horse if anxious. Fluid therapy is required in severe cases to correct dehydration and electrolyte imbalances and to protect the kidneys as they filter myoglobin out of the bloodstream. Myoglobin, an important component of the muscle cell, stores oxygen for use by the muscle during heavy workloads. It is released into the bloodstream during the destruction of muscle cells and is what causes the urine to turn the color of port wine. In large amounts it can be damaging to the kidneys, so adding fluids to the bloodstream helps dilute the myoglobin into manageable amounts that the kidneys can process, while correcting any electrolyte imbalances that may be present.
As an alternative to the traditional medications used for pain and muscle relaxation, acupuncture, massage, therapeutic ultrasound and/or low level laser therapy have proven beneficial for some horses. All these modalities help relax the muscles and relieve pain and can also be used in conjunction with traditional pharmaceutical therapy.
How Can You Prevent an Episode of Tying Up?
Make sure you always warm up your horse before asking for more strenuous work. A ten to 15 minute open walk is sufficient most of the time. And do not ask your horse to suddenly work beyond his current level of conditioning – in other words, don’t expect your pasture ornament to carry you on a ten-mile trek in the mountains this weekend. Increase training gradually, not abruptly.
For horses that chronically tie up, daily exercise and turnout access are essential to successful management, as is a change in nutrition to a high fat, low carbohydrate diet. Nonstructural carbohydrates (sugars and starches) should make up less than 15% of the total daily calories, while 20% to 25% of total daily calories should be from fat. Sporadic episodes may be prevented by following the same diet and exercise recommendations.
Recent research suggests that proper amounts of vitamin E and lipoic acid, both found in plant matter, may decrease the incidence of tying up. Daily supplementation with 1,000 IU of vitamin E may benefit horses that are prone to this problem. Selenium supplementation, especially in selenium deficient areas, has also been found helpful in preventing episodes in some horses.
With proper diet management and daily exercise programs, many horses with this frustrating and painful disorder can return to work and live a normal, full life.
Dr. Kelli Taylor is a 2008 Summa Cum Laude graduate of Washington State University’s College Of Veterinary Medicine. She was born with a love of horses and has striven to be near them her entire life, even when it was impossible for her to have her own. Just after graduation, she completed an internship in equine medicine and surgery at Pilchuck Veterinary Hospital in Snohomish, Washington and obtained certification in animal chiropractic through the IVCA. She has also completed her certification in veterinary acupuncture. Dr. Kelli is very excited to be announcing the opening of her own mobile veterinary chiropractic and acupuncture practice – mindful healing veterinary care. When not working, you can find Dr. Taylor trail riding or hiking with her husband in the great outdoors of the pacific northwest.