PSSM – Two Types of this Muscle Disorder Result in Tying-up

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Tying-up is a disorder that occurs when normal muscle physiology is altered. Horses that tie up have painful, uncontrollable muscle contractions that lead to damaged muscle. Muscles fail to function properly, and symptoms can vary depending on the severity of the episode—from mild signs of stiffness and reluctance to move forward to severe signs of complete refusal to move, sweating, cramping, panic and even colicky symptoms.

Tying-up has been recognized and studied as a problem in horses for centuries but it was the explosion of research in human neuromuscular disorders such as ALS, multiple sclerosis, and myasthenia gravis that became key to unraveling some of mysteries of tying-up in horses.

Dr. Stephanie Valberg of Michigan State University is one of the lead researchers in this field. Researchers like Dr. Valberg have discovered that tying-up is not one single disorder but a symptom of several different problems.

Broadly speaking, tying-up can be divided into two categories. First is sporadic, where some outside factor caused it, such as over-exertion, exhaustion, nutritional mismanagement, casting, etc. This affects horses of any age or breed. The second category is chronic, due to intrinsic defects in muscle physiology and function. This article will focus on two types of chronic tying-up: PSSM1 and PSSM2.



Polysaccharide Storage Myopathy (PSSM) was first recognized as a specific muscle disease in horses in 1992. It’s a disease resulting in abnormal energy storage (i.e., abnormal glycogen) and energy utilization in muscle. There are two subtypes of PSSM: PSSM1 and PSSM2.



Glucose is one of the “fuels” muscles use to provide the energy to contract and relax. Normal muscle cells link huge numbers of glucose molecules together, forming a polysaccharide energy storehouse called glycogen. The enzyme that enables this glycogen production is called glycogen synthase 1. When muscle cells need to burn or consume energy, the glycogen production pathway switches off and the glycogen burning pathway switches on.

Horses with PSSM1 have a genetic defect resulting in defective glycogen synthase 1. The defective gene causes the enzyme to be overactive, resulting in overproduction of abnormal glycogen. Also, the defective enzyme fails to switch off when exercise begins. Muscles in horses with this defective gene keep producing glycogen when they need to be burning it. Even though there is plenty of stored energy in these cells, it is unavailable since the “burning” pathway fails to switch on. The cell is quickly depleted of any available glucose that is not already tied up in glycogen. Continued work by energy depleted muscle results in damage to the muscle. With enough muscle damage, symptoms of tying-up emerge.

One of the functions of insulin is to move glucose into cells. PSSM horses are more sensitive to insulin, resulting in even more glucose entering muscle cells. This compounds the problem of glycogen overproduction and eventual energy depletion. Starch and sugar in the diet stimulates greater insulin release, increasing the risk of tying-up and sore muscles.

PSSM1 is found in 6% to 10% of all Quarter Horses and Paints but in 28% of the subgroup of Quarter Horses bred for halter. It is nearly nonexistent in Arabians, Standardbreds, and Thoroughbreds.

Whether PSSM horses have episodes of tying-up or not depends on several factors, such as diet and exercise. Periods of rest prior to exercise and diets high in starch and sugar are triggering factors. The type of exercise is another factor. Light exercise is heavily dependent on glycogen and therefore more likely to generate problems (strenuous exercise results in quicker transition into use of alternatives to glucose for energy). An example of light exercise is 20 minutes of walk and trot with frequent transitions.

Signs of a sudden, acute episode include tucking up in the abdomen, trembling of muscles, muscle stiffness, sweating, reluctance to move forward, gait abnormalities, discolored urine, and sometimes colic. Symptoms of subtler, lingering cases are less dramatic and include lack of energy, reluctance to move forward, a “sour” attitude, poor performance, and chronic back pain. Sometimes the horse might stop and stretch, as if to urinate. Other horses, especially drafts, tend to have subtle symptoms that might go undetected.

Tying-up can be diagnosed by blood tests, either after an episode or by a test comparing baseline muscle enzyme levels prior to 15 minutes of exercise (walk and trot only with lots of transitions), to levels four hours after the exercise session. There is also a genetic test (blood or hair sample) that can detect the defective gene that causes PSSM1.



Horses with PSSM2 have similar symptoms to PSSM1 horses but do not have the abnormal gene for glycogen synthase. In these horses, the root cause of the overproduction of glycogen and energy depletion has not been discovered yet. PSSM2 might turn out to be several different disorders. PSSM2 diagnosis is based on muscle biopsy results demonstrating abnormal glycogen and the genetic test demonstrating an absence of the defective gene of PSSM1.

Of the Quarter Horses with PSSM, only about 28% are type 2, whereas of the warmbloods with PSSM, about 80% are type 2. The Quarter Horses with this type tend to be more of the performance type, such as barrel racing, cutting, and reining.

Horses with PSSM will always have an underlying tendency for muscle soreness. Progress in our understanding of the disease has improved our ability to manage these horses to reduce the risk for tying-up and muscle soreness.


Managing PSSM

Confinement and rest tend to cause more problems with these horses. Instead, turnout with other horses to keep these individuals moving is a key element to prevention.

Exercise: Duration and frequency of exercise appears to be more important than the intensity of exercise. Make changes in exercise intensity and duration very gradual. Consistency and regularity of exercise are very important. Minimize days without exercise; even 10 or 15 minutes on otherwise “off” days can be very beneficial. Keeping them fit is the best prevention.

Diet: In general, keep non-structural carbohydrates (NSC) such as starch and sugar low so there is less glucose available and less insulin production for glycogen production.

Fat provides an alternate energy source and is beneficial for PSSM horses. For average weight PSSM horses, limit NSC to 12% fed at 1.5% of body weight and increase fat levels to 13% or higher of total digestible energy in the diet. Extra Vitamin E supplementation (1000 or more international units per day) is recommended for horses on high fat diets. Be careful with overweight horses since obesity leads to other problems. Easy keepers that are overweight need even greater starch and sugar restriction, limiting NSC to 5-10%.

Another strategy, rather than feeding more fat, to manage the overweight PSSM horse is to fast them for 6-8 hours prior to exercise. Fasting results in lower blood glucose (so less is available to produce glycogen), lower insulin levels, and higher levels of circulating fatty acids in the blood. These fatty acids provide an alternate energy source for muscle. Dietary changes alone will not work unless a strict exercise program is also implemented.

Research has expanded our understanding of tying-up in horses. Dr. Valberg’s lab and others continue to study and learn more about these and other muscle disorders in horses so we can learn how to manage these horses better now and in the future.


Published in the August 2020 Issue:

August 2020


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