PSSM in Horses - Polysaccharide Storage Myopathy 1 & 2
Updated January 19, 2023
Special thanks to equine neuromuscular disorder expert Dr. Stephanie Valberg, formerly director of the Neuromuscular Diagnostic Laboratory at Michigan State University College of Veterinary Medicine, for her thoughtful and thorough review of this article.
Polysaccharide Storage Myopathy (PSSM) is a chronic muscular condition in horses that comes under the broader heading of “tying up,” or exertional rhabdomyolysis. This article will describe the signs and symptoms of PSSM and the two different types, explain options available to veterinarians to diagnose PSSM, and outline the use of diet in combination with exercise to manage affected horses.
What is PSSM in Horses?
Polysaccharide Storage Myopathy was identified in the early nineties as an equine muscle disease (myopathy) having to do with the excessive storage or abnormal accumulation of glycogen, a polysaccharide (“poly” = many, and “saccharide” = complex sugars) in the muscle cells.
Signs and Symptoms of PSSM
- reluctance to move forward on the lunge or under saddle
- topline atrophy (loss of muscle over the back)
- difficulty striking off into canter, keeping canter leads, or performing flying changes
- an undiagnosed subtle or mild, shifting lameness
- an abnormal gait or gait asymmetry
- resentment towards saddling
- sensitivity to grooming
- sore, painful, firm back and hindquarter muscles
- reluctance to collect and engage
- reduced enthusiasm for exercise
- drop in energy level
- unwillingness to perform after 5-10 minutes of exercise
Signs of PSSM in Different Breeds of Horses
Seen in a number of breeds, the most common signs of PSSM in Quarter Horses and Arabians are the classic symptoms of tying-up: muscle pain, stiffness, excessive sweating, and a reluctance to move, all associated with the onset of exercise. These signs are associated with muscle damage measured in blood tests as an increase in the proteins creatine kinase (CK) and aspartate transaminase (AST).
However, other breeds, such as warmbloods, more often display vague signs of poor performance or a change in behavior or attitude toward exercise. Many of these affected horses are deemed lazy due to the effort it takes on the part of the rider or handler to encourage them to go and to keep them going. Horses with this form of PSSM do not have increased CK and AST in the bloodstream.
Signs of PSSM can appear as a training or behavioral issue, or even as another medical condition such as colic, laminitis, gastric ulcers, EPM, and others. Therefore, it is important to record observations that seem unexpected for a particular horse’s performance level or personality and to share these findings with a veterinarian to find the underlying cause.
Diagnosing PSSM in Horses - Types 1 and 2
When PSSM was first recognized as an emerging muscular condition in horses, veterinarians diagnosed it starting with a thorough history from the owner and trainer, a complete physical examination including a full lameness workup, and bloodwork to test for the muscle enzyme CK (with or without an exercise challenge depending on the situation).
If all signs still pointed to PSSM, the vet then sent a muscle biopsy (a surgically obtained sample of muscle tissue) to a lab. There, a special stain was applied which allowed identification of both abnormal levels and abnormal appearance of glycogen under the microscope. When combined with the veterinary workup, this would confirm the diagnosis of Polysaccharide Storage Myopathy.
Note that a horse actively tying up is an emergency that requires immediate veterinary attention. Exercise should be stopped immediately, and the horse kept quiet until he can be examined and treated.
Discovery of a Gene Mutation
The step-by-step process that a vet uses to diagnose PSSM changed in 2008 when a team of researchers identified a mutation in the gene that carries instructions for making the enzyme glycogen synthase (GYS1). This is the enzyme responsible for “packaging” glucose into its storage form glycogen in muscle cells for use later as fuel for exercise.
When the GYS1 gene has a defect in it, the enzyme is more active than normal – taking up more glucose after a meal and then packaging more glycogen in muscle cells. This enhanced activity is what leads to the abnormally high levels of glycogen seen in biopsied muscle cells under a microscope with special stains.
By going back and comparing abnormal muscle biopsies with genetic (blood or hair) samples from the same horses, researchers were able to figure out that some horses with abnormal biopsies carried the mutated GYS1 gene, but others did not. That was when they first realized they might be dealing with two different types of PSSM:
- Type 1 PSSM - abnormal muscle biopsy and mutated GYS1 gene
- Type 2 PSSM - abnormal muscle biopsy but no genetic mutation
PSSM 1 is now considered a “disease” because a specific cause has been identified. However, PSSM 2 remains a condition or syndrome because it appears to have different causes depending on the breed - these causes have not yet been identified. In fact, because muscle biopsies from horses believed to have Type 2 PSSM demonstrate abnormally appearing glycogen but not always abnormally high levels, researchers suggest PSSM 2 is likely to have one or more different mechanisms leading to clinical signs. That is, although we know what does NOT cause Type 2 PSSM (an overly active glycogen synthase enzyme caused by a mutation in a gene), we do not know what DOES cause PSSM 2.
DNA Testing Laboratories
Type 1 PSSM can now be diagnosed relatively inexpensively and painlessly via genetic (DNA) testing of blood or hair roots. In fact, muscle disease expert Dr. Stephanie Valberg, formerly of Michigan State University College of Veterinary Medicine says: “In horses with chronic tying-up that are among breeds known to carry the PSSM 1 mutation, genetic testing is a good place to start.”
Two laboratories in the US currently offer genetic testing for Type 1 PSSM:
- UC Davis Veterinary Genetics Laboratory (and can also be ordered through the AQHA)
- Animal Genetics Laboratory
At this time there is no scientifically validated genetic test for Type 2 PSSM and a muscle biopsy is used to establish a diagnosis.
Genetic Testing for PSSM 1 in Different Breeds of Horses
The advice to start with genetic testing applies to Quarter Horses, Paints, Appaloosas, and their crosses, which predominantly have Type 1 PSSM. Since draft horses, Morgans, Haflingers, warmbloods, and gaited breeds such as Rocky Mountain Horses and Tennessee Walking Horses have been diagnosed with both Type 1 and Type 2 PSSM, a complete diagnostic workup followed by the genetic test for PSSM 1 is a sound strategy.
If the GYS1 test comes back negative but PSSM still seems likely, then the next step may be a muscle biopsy to identify abnormally appearing glycogen, which may lead to a diagnosis of Type 2 PSSM.
Since PSSM 1 is rare to nonexistent in Thoroughbreds, Standardbreds, and Arabians, genetic testing for horses that tie up in these breeds is not recommended. In warmblood horses, PSSM 2 may be an early form of a recently described muscle disease (myofibrillar myopathy) which can be identified by a specific stain for an abnormal appearing structural protein in muscle cells called desmin.
Management of Horses with PSSM
Unfortunately, there is no cure for Polysaccharide Storage Myopathy. The best we can do at this time is carefully manage diet and exercise to reduce the frequency and severity of tying up (in horses that demonstrate this clinical sign) and improve poor performance and attitude (in horses that demonstrate these behaviors). Note that diet alone will not “fix” a PSSM horse – it is the combination of the appropriate feed along with physical movement that leads to a change for the better.
The clinical trials in horses that led to the nutritional and activity advice currently recommended were all done in horses with Type 1 PSSM. Quarter Horses and similar stock breeds diagnosed with Type 2 PSSM respond quite well to the same strategies. While warmbloods with PSSM 2 do improve, it is not to the same extent, most likely because the underlying cause does not appear to be a defect in the glycogen synthesis pathway.
Warmblood horses with myofibrillar myopathy are managed in a different fashion than horses with classic tying up, high CK in blood tests and either a positive GYS1 test or abnormal polysaccharide in muscle biopsy samples.
Diet for Horses with PSSM
The nutritional needs of the PSSM horse can easily become quite complicated, so owners may want to consider seeking the advice of a certified equine nutritionist.
Successfully managing a PSSM horse begins with a complete and balanced diet low in amounts of sugars and starches to avoid the build-up of glycogen in muscle tissue. One approach is to feed 1.5 to 2% of the horse’s body weight each day in forage that is less than 12% NSC (non-structural carbohydrates) paired with a ration balancer that contains vitamins, minerals, and protein. Ideally, hay would be analyzed for sugar and starch content and soaked in water, if needed, to further lower the NSC.
Traditional grains such as corn and oats as well as commercial concentrates such as sweet feed are replaced with a ration balancer pellet so that necessary nutrients are supplied without unnecessary calories, sugars, and starches.
Because turnout on grass pasture provides much-needed movement but could result in too many calories, starch, and sugar, owners are urged to use a grazing muzzle to restrict intake, especially when grass is lush.
In general, many horses with PSSM tend to be easy keepers and prone to weight gain. However, horses with an ideal (or less-than-ideal) body condition score and in heavy work may require additional calories, which are best supplied from fat. Choosing a fat source such as flax seed or camelina oil with a significant amount of omega 3 fatty acids, or at least more O3s than O6s, supports a healthy inflammatory response at the same time as providing energy for weight gain and exercise.
In addition, nutritionists recommend horses be supplemented with 600 IU (International Units) of vitamin E per cup of oil. The Type 2 PSSM horse – which may have muscle loss in the topline and/or hindquarters -- may also benefit from the addition of high quality amino acids through a protein supplement. Additional supplements with anecdotal support are magnesium and ALCAR (acetyl-L-carnitine).
Exercise and Riding PSSM Horses
The second component of successfully managing a PSSM horse is ensuring daily movement. Activity not only helps “train” the mitochondrial system responsible for metabolizing fat for fuel, but it may also help flush the excess and abnormal glycogen from muscle tissue. “Activity” includes structured exercise such as hand walking, lunging, or riding as well as unstructured exercise i.e., turnout.
While it has long been recommended that PSSM horses spend no more than 12 hours at a stretch in a stall, experts agree that the more time in a paddock or pasture the better, with 24/7 turnout being the gold standard.
Complete rest should be avoided. Work does not need to be hard, but it should be regular, consistent, and daily if possible, gradually built up over time. In addition, the Type 2 PSSM horse will benefit from a prolonged warm-up with plenty of stretching in a long and low frame as well as frequent breaks from collected work.
Some experts recommend thoughtful use of training equipment such as the Pessoa Lunging System. Used properly, it encourages horses to reach forward with the head and neck while rounding and lifting the back and activating the hind legs.
PSSM horses that are recovering from a recent episode of tying up should be carefully returned to exercise under the direction of a veterinarian.
Ask the Vet Video on Management of Horses with PSSM Types 1 and 2
Key Takeaways on PSSM
Polysaccharide Storage Myopathy can be a frustrating condition for many owners who do not understand why a horse with a certain pedigree, conformation, or talent is not living up to their expectations.
As breakthroughs in research enhance our understanding of PSSM – what causes it and what can be done about it – more horses are able, through appropriate diet and exercise, to achieve a better degree of comfort and a higher level of performance.
While horses with classic tying up and high CK in blood tests respond to low starch-high fat diets and regular exercise, we now know that what was previously referred to as PSSM 2 in poorly performing Warmbloods, is actually a separate disorder, myofibrillar myopathy, which requires a different diet and training approach from PSSM.
Article originally published June 2012