Polysaccharide Storage Myopathy (PSSM) in Horses
PSSM is a disease that causes an abnormal accumulation of glycogen, the form of sugar stored in the muscle. This excess sugar causes mild to severe muscle cramps, sore muscles and/or muscle weakness. Horses that are managed properly can generally go on to have successful performance careers.
Two types of PSSM have been classified: PSSM1 and PSSM2. The genetic test used by AQHA identifies PSSM1 mutation. Type 2 PSSM refers to PSSM symptoms that occur in horses without the known PSSM1 variant. At this time, aside from PSSM1, there is not a genetic test for other forms of PSSM. PSSM2 can, however, be diagnosed with a muscle biopsy. PSSM2 is most commonly found in warmbloods and Arabians.
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.
Polysaccharide storage myopathy (PSSM) and equine polysaccharide myopathy (EPSM) are glycogen storage disease of horses that lead to exertional rhabdomyolysis, a condition in which exercise brings about muscle pain and cramping. The disease arises in horses of many different breeds. Generally called polysaccharide storage myopathy, or PSSM, the disease is called equine polysaccharide storage myopathy, or EPSM, when it occurs in draft horses. EPSM most commonly occurs in draft horses and Quarter Horses, but is also found in Warmbloods, Fjords, Friesians, Haflingers, Arabians and Icelandics. In Quarter Horses, EPSM has been proven to be an inherited disease, yet this has not yet been confirmed as such in all other breeds.
At this time, there is not documented sex predilection to the disease. No matter the type, the mechanisms of the disease are complex. Glycogen is a polysaccharide molecule that is stored in animal cells along with water and then used as a source of energy. Glycogen is essential in animals as it provides energy for the benefit of skeletal muscles, as well as cardiac muscles.Polysaccharide storage myopathy (PSSM or EPSM) is a glycogen storage disease of horses that leads to exertional rhabdomyolysis, a condition in which exercise brings about muscle pain and cramping.
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.
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).
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
Image-Courtesy-Google
Reference-On Request.
