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PSSM in Horses; Thoughts and Considerations on Therapy Options

Polysaccharide storage myopathy (PSSM) is an increasing diagnosis in the equine community and is linked back to a group of conditions, referred to as ‘myopathies’.  The problem of PSSM is becoming more commonplace, leaving many owners unsure as to what the condition is or how to best manage it. Like many other conditions, PSSM is not one to be cured, but more so managed in order to improve long-term quality of life and performance for that horse.  Let’s dig into the condition of PSSM and take a look at physiology, diet, and herbal options that may be available to us.

Anyone that has been in the horse industry long enough is familiar with the term ‘tying-up’ and many have also either owned a horse or knew of a horse that seemed to be affected by this condition.  We discuss tying-up or Rhabdomyolysis in another article.  If you need a refresher or further information on that topic, please visit that article for a review.  Tying-up is general term used in horses that present with overall acute-like symptoms of stiffness in the muscle groups, referring to a myopathy.  There are many potential causes for a horse to present with tying-up symptoms, and PSSM is just one of those causes.  It is a myopathy, leading to improper muscle function in the affected patient.

What is PSSM?

PSSM or polysaccharide storage myopathy is a disorder of carbohydrate metabolism in the horse.  Carbohydrates are the various forms of sugars and starches that are present in almost every food source. The body uses carbohydrates for energy production at a cellular level. A polysaccharide is a carbohydrate molecule consisting of several sugar molecules bound together.  So, by definition of the disease, PSSM is a disorder in which polysaccharides (carbohydrates) build up within the cell – a storage problem of sorts.

There has been two types of PSSM determined: type 1 and 2.  Type 1 (PSSM1) is associated with a mutation in the glycogen-synthase enzyme gene (GSY1) and is the most common form presented.  Type 2  (PSSM2) is associated with a mutation in the RYR1 gene and is less common, but more severe in presentation when clinically present usually associated with malignant hyperthermia.  Genetic testing is routinely available at some universities, including the University of Minnesota College of Veterinary Medicine.  Dr. Stephanie Valberg is one of the leading authorities and researchers on the subject of PSSM.

Pulling from one of Michigan State’s articles, “The mutation causing PSSM1 is a point mutation in the gene that codes for the skeletal muscle form of the glycogen synthase enzyme. The mutation causes this glycogen synthase enzyme to be overactive, increased in activity especially in the presence of insulin resulting in constant production of glycogen. When glycogen is being produced the reciprocal breakdown of glycogen is impaired potentially resulting in a deficit of energy in the muscle cell.”

Type 2 (PSSM2) is similar to PSSM1, but on biopsy examination, there are differences. The main issue is that this group of horses also demonstrate glycogen clumping or accumulation within the cells, but upon genetic testing, they do not have the GSY1 gene mutation.

In reality, the bottom line is that with PSSM, we have a reduced ability to utilize carbohydrates properly within the muscle tissue.  This leads to accumulation of glycogen within the cells and further reduces energy production.

For more detailed information, please evaluate Michigan State’s articles on Type I and Type II PSSM.

What are the clinical signs of PSSM in the horse?

PSSM, both types, present very similar in horses and are really hard to discriminate grossly from any other form of tying-up or Rhabdomyolysis.  Almost any breed can be impacted, but originally it was thought to only involve the Quarter Horse or Draft breeds.  Horses afflicted with PSSM can vary in their clinical signs and in some cases, only start to demonstrate problems later in life and career.  Some have the condition, but are asymptomatic, meaning they have no clinical problems, which is often reflected back to some form of management via diet, exercise or supplementation.  Some owners don’t realize they are controlling the problem, but they are, and only find this out when something changes.

Common clinical signs include:

  • Stiffness
  • Unwilling to work/move
  • Sweating / Anxiety
  • Shifting lameness
  • Muscle tremors
  • Tight / hard muscle groups
  • Painful muscle groups on palpation

Most of these clinical signs come on rather quickly when starting an exercise routine.  The pain levels can be mild or severe.  The more painful horses may paw at the ground, roll, stretch out to urinate, almost like a colicky horse.

Diagnosis of PSSM

PSSM is very similar in presentation to other forms of tying-up in the horse.  We cannot simply look at a horse that is tying-up and state a diagnosis.  In most cases, laboratory testing is done which includes bloodwork and even genetic testing via a muscle biopsy.  The bloodwork may indicate dehydration of variable forms and elevations in muscle enzymes, i.e., Creatine kinase (CK) and Lactate dehydrogenase (LDH).  The CK elevation can vary from one patient to another, dependent on the severity of the condition and in some cases of PSSM2, there is no elevation of CK.  One can work off of a suspicion of PSSM in any patient, but in order to verify the condition is present, a muscle biopsy is warranted.

Management of PSSM

In all cases of a horse that is tying-up, it is highly recommended to stop work immediately as to not induce any further cellular damage or pain to the animal.  In horses that are cold, applying a blanket to keep them warm would be ideal.  In those horses that are overtly hot and sweating, cold or neutral water application via a hose would be beneficial…if the weather permits.  In all cases, contact your veterinarian and alert them to the situation.  Some of these cases can be very critical and thus, veterinary attention is needed to prevent or minimize further muscle tissue damage, pain, organ failure and rehydrate the patient.  In most cases, bloodwork will be performed, pain medications administered and rehydration attempted by oral fluids with electrolytes or intravenous fluids.

If we look at the disease for what it is, a carbohydrate metabolism disorder, we can see not just the problem but also a solution for better management.  In this condition, PSSM, the horses have a dysfunction of carbohydrate utilization.  Their body needs to produce energy, but in most of them, carbohydrates are not the preferred source, at least in the muscle tissue.  We need to turn to other forms of energy production, which may include fats and even protein.

There is no tried and true means of dietary management for a PSSM patient.  What we need to do in these cases is evaluate the level of carbohydrates going into the horse via the diet.  Given today’s equine diet, there is a high use of commercial and even whole-grains, when this is not ideal for any horse.  We have discussed ‘grains and impact on health‘ in another article.  I think one of the biggest issues in many of these cases is that the horse is simply consuming too many carbohydrates via grains.  If we reduce this intake, relative to their prior consumption, then we are moving in a positive direction.  In most, at least in the short term during recovery, it is recommended to discontinue all grains and rely only on hay sources.  This reduces the carbohydrate load upon the body, at least through grains.

We have to remember that almost every food source is going to contain carbohydrates, so they are next to impossible to eliminate from a diet.  Grains are the highest sources of carbohydrates, but hay still contains them, but at lower levels.

In most cases, our goal is to reduce the intake of carbohydrates (usually in the form of all grains), but focus more on whole-food nutrition via high quality hay and also the application of increased fats.  We want to move away from high glycemic index diets and towards lower ones.  This implies less simple carbohydrates and more complex ones.  Complex carbohydrates tend to produce less of a spike in blood sugar and insulin levels. Some PSSM horses do respond to a higher fat diet, which can be done through several methods. However, we need to keep some things in mind.

Further Thoughts

PSSM, despite being a genetic condition, results in inflammatory by-products in that patient, especially during episodes of attacks.  It is a carbohydrate metabolism disorder, but the process of inflammation is intertwined and we shouldn’t forget this fact.

What does this mean in regards to management?

Most PSSM horses in my experience, are easy-keeper types.  They are overweight, bigger boned and have a tendency to put on weight easily.  To me, based on our research, this ties in the gastrointestinal tract, and inflammation not just at that level but in the entire body.  Simply put, I think these horses are ‘inflammatory vats’, just waiting for the opportunity to emerge.

When we look at the high use of grain in the horse industry, we are contributing to inflammatory events.  Grains are not the preferred food source for horses and in too high of a quantity, we contribute to health ailments, usually starting at a gut level.  Is it possible that we are greatly contributing to this PSSM condition through our feeding practices?  Is is possible that many horses are asymptomatic but through our feeding practices we are bringing out the condition?  Likely.

Our goal is to manage the inflammatory process in that PSSM horse.  Reducing carbohydrates down to an acceptable level is one way.  Adding fats to the diet is potentially another, as some fats are anti-inflammatory in nature.  Providing a high quality hay source, such as alfalfa, may be another mainly due to the fact that that food source if very rich in macro- and micro-nutrients that benefit cell health.

I think we do need to be careful, though, in regards to which fats we provide for those horses, if that route is chosen.  Fats can be both anti- and pro-inflammatory in nature, dependent on their omega-6 to 3 ratios.  Ideally, we want one that is balanced, possibly Flax as an example.  Most oils are very concentrated, which can quickly raise the caloric content.  We don’t want to add more fat and calories to an already overweight horse.  This equates to more inflammation, as fat produces its own inflammatory proteins (adipokines).  The other problem with many oils is that they are heavily processed, so we create more harmful fats and higher levels of omega-6 in relation to omega-3.  Hemp and Flax oils are both exceptions, as the balance is much better than corn oil or vegetable oil, but we still need to watch caloric content.

Herbs are to me, the key to managing these patients, in addition to dietary modifications.  We have to remember that in most, we have inflammatory problems, which may be the result of PSSM or contributing to the condition.  In most, my opinion is that the inflammatory problem stems back to the gut and gastrointestinal function.  Take a look at many of these patients and we will see concurrent ulcers, gas production, loose stools, anxiety, and laminitis.  These are all tied back to gut health and inflammation on some level.

My goal is not just to manage the PSSM, but to make that horse a healthier and more productive animal. Diet, in terms of reduction in carbohydrates, is one that I use in every patient, not just PSSM.  Every horse can benefit!  Then we manage or attempt to balance the inflammatory status in that patient by using herbs.  Herbs that I find extremely beneficial are curcumin, turmeric, dandelion, parsley, marshmalllow, boswellia, aloe, ashwaghanda and many, many others.  I do use some vitamin E and even vitamin C in almost all of our patients, but not in huge volumes as people are led to believe.  I supplement based on need, taking into consideration that most of the herbs are potent antioxidants themselves, so they work together.  I think that with excessive use of isolated antioxidants, we can create more harm and imbalance in that patient.  Everything in moderation!

Two of the main supplements that I will use specifically in myopathy horses, especially if they are the easy-keeper type, are Cur-OST EQ Total Support and Cur-OST EQ Rejuvenate.  Our goals with these supplements are to manage the inflammatory process, provide antioxidant support, balance gastrointestinal health and provide a natural nutrient base.

The bottom line is that PSSM is a prevalent condition, impacting many horses and breeds.  Despite its level of occurrence, I think that it may be more manageable than what most perceive.  I believe that if we step back and ‘see the whole horse‘, we may see things differently.  We gain a better idea as to how our current dietary practices and even housing conditions are creating problems in that animal.  When we get a broad view of the problem, we quickly realize that just adding fats to a PSSM horse’s diet is likely not going to provide the answers we desire.

PSSM may be a genetic condition, but just like many other equine ailments, there are often many contributors that need to be addressed.


Author:  Tom Schell, D.V.M., CVCH, CHN




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