Tying Up and PSSM

Tying Up and PSSM


Tying Up vs. PSSM

Tying up, also known as External Rhabdomyolysis simply means muscle breakdown associated with exercise. The muscle stiffness and pain correlated with tying-up can be mild to moderate and signs can range from shortened strides to reluctance to move. Tying-up is usually present in a horse that has been off for several days and is then exercised aggressively. Tying up can occur sporadically or chronically. Sporadic tying up can have many causes, however recurrent or continual episodes of tying up are most frequently cause by an underlying genetic condition.


Polysaccharide Storage Myopathy (PSSM) is a gene mutation that causes unregulated glycogen formation in the muscles of horses with PSSM. Because this enzyme is constantly active, affected horses accumulate excess glycogen in the cells. This glycogen storage disorder affects numerous breeds including Quarter Horses and can look like a typical case tying-up or cause the horse to show other signs such as difficulty backing, picking up the hind legs, etc. The muscles of horses with PSSM are more sensitive to insulin – the hormone released to help with sugar metabolism. Two forms of PSSM have now been identified:

Type 1 PSSM horses are predominately inherited as a dominant gene.  A genetic test is available for this. Testing (hair sample) is available through AQHA as part of a 5-panel test for approximately $85.

Type 2 PSSM is where there is excessive accumulation of glycogen without the mutated gene present. Also, believed to be inherited but a specific gene has not been identified. The only way to determine Type 2 is to do a muscle biopsy.


What might cause a Tying-up event in PSSM horses?

Most research recognizes an over-load of glycogen (blood sugar) in the muscles can lead to muscle fiber contraction/stiffness and soreness. Excess glycogen accumulated in the muscle while a horse is resting/not worked (and still kept on full grain ration) may cause the glycogen to break down into lactic acid when they begin to work. If severe, this results in muscle fiber breakdown/damage and release of myoglobin into the blood and on to the kidney for filtering out (reddish/brown urine). In the event of the episode being prolonged, the kidneys may eventually fail (overload) and death may potentially result.

Insulin Resistant (IR) horses and/or those that have a lesser ability to utilize excess blood sugar are often potential candidates for Tying-Up and in many cases a more heavily muscled body type horses.


Tying Up Help

Whether a horse has Type 1 or Type 2 PSSM, the management involves the same two key elements:

Diet: Reduce and/or eliminate carbohydrates, and add fat or fiber as a source of energy. Researchers suggest that owners or managers feed PSSM horses more hay and significantly less grain (carbohydrates) as it appears to exacerbate type 1 and type 2 PSSM. Dietary recommendations include increasing dietary fat to contribute 15-20% of the horse’s daily energy requirement and reducing dietary NSC levels to ≤ 20%.

Exercise:  Daily exercise is recommended as it suppresses uptake of sugars and enhances sugar metabolism. If only the diet is changed, it was found that approximately 50% of horses improve. If both diet and exercise are altered, then 90% of horses have had no or few episodes of tying-up.


Supplements that may lend support:

Vitamin E* is an essential component to body-wide antioxidant defenses, with one of its most important duties being cell membrane maintenance. Horses with an inadequate reserve of vitamin E may experience muscle soreness or stiffness during an exercise bout and prolonged recovery following strenuous work.

Selenium works in conjunction with vitamin E to defend the body’s cells from damaging oxidative byproducts known as free radicals. 

Magnesium is by far the most important mineral, activating over 300 different biochemical reactions all necessary for the body to function properly. Magnesium is an essential macro-mineral for horses that aids in maintaining normal nerve and muscle function.

DMG (N, N-DimethylglycineHCL) serves as an antioxidant and helps support a healthy anti-inflammatory response.  It is also recognized in some research as an aid in minimizing lactic acid production.



  1. Genetic testing for PSSM Type 1 would be in order to verify condition, especially if it is a mare that might be kept for breeding later. A horse could test negative for PSSM Type 1, but have a positive test for PSSM Type 2 or (vice /versa) with tying up symptoms. Dietary and exercise management for the condition is basically the same for either condition.

  2. Lower starch (reduced grain/carbohydrates) and higher fiber (hay, beet pulp, rice bran) diet and daily exercise!

  3. Metabolic Function/Nutritional Support: In-Sync is formulated for supporting muscle tone, immune and metabolic function by combining key ingredients such as Magnesium, Selenium, DMG & *Natural Vitamin E.  The Natural Vitamin E in In-Sync is 2-2 ½ time more bioavailable than synthetic Vitamin E. It also has a high level of the amino acid (lysine) to help build healthy muscle and B vitamins to support nerves and red blood cells. In-Sync 4 costs approximately $1.91 per day to feed. Disclaimer: In-Sync is recognized solely as nutritional support adjunct with proper low carb diet, not as a treatment for PSSM metabolic dysfunction.




To learn more, visit the Michigan State University College of Veterinary Medicine Equine Neuromuscular Diagnostic Laboratory website.


Contacting your veterinarian is the first step in identifying specific health issues with your horse. This information is for educational purposes only and is not intended to diagnose or treat.