Equine Tying Up Syndrome

There are two simple words that describe painful, exercise-associated muscle cramping in a horse: tying up. While the traditional tying up usually occurs after a long hard ride, some horses can tie up repeatedly for no immediately obvious reason. 

Regardless of the underlying cause, the clinical signs are similar. And in most cases, affected horses require immediate veterinary care, says Dr. Fabienne Uehlinger of the Western College of Veterinary Medicine (WCVM) in Saskatoon, Saskatchewan. 

“The horse becomes very stiff — it’s usually reluctant to move, it may show signs of colic and be sweating, and it may be breathing hard,” says Uehlinger, a board-certified specialist in large animal internal medicine.

Affected horses often look like they are experiencing colic or laminitis (founder). In severe cases, a horse may pass very dark urine as substances released from damaged muscles are flushed through its kidneys. Damaged muscles release myoglobin, a muscle protein which is toxic to the kidneys and can cause permanent damage.

More often than not, tying up occurs as a one-time event due to physical exertion beyond the horse’s fitness level or due to an underlying condition, explains Uehlinger. For example, a tying-up episode can occur if a horse is being exercised after it has been on stall rest, if it has suffered a recent bout of equine influenza, or if it has been exercised too strenuously outside its regular exercise routine. 

However, there are times when a horse ties up repeatedly for no apparent reason. For these chronic cases, veterinarians usually consider two conditions: polysaccharide storage myopathy (PSSM) and recurrent exertional rhabdomyolysis (RER). In some cases, these conditions can be inherited and passed down between generations.

PSSM refers to abnormal storage of sugars in the muscle,” says Uehlinger. “That sugar is providing the fuel necessary for the muscle to work. When the horse exercises or moves around, it is not actually able to use that sugar appropriately. It doesn’t have the energy to do what it needs to do, and it ties up.”

PSSM is divided into Types 1 and 2. Type 1 most commonly occurs in Quarter Horses and related breeds such as Appaloosas and Paints. Type 1 is also common in draft horse breeds of European descent, such as Belgians and Percherons, as well as in some warmblood breeds. There is a known genetic mutation that is inherited by the offspring of affected horses. 

Type 2 PSSM tends to occur in light horse breeds such as Thoroughbreds and Arabians, but it may also occur in some Quarter Horses. So far, researchers have not identified the genetic mutation that causes this type of PSSM.