The author and her horse, Murphy.

Polysaccharide Storage Myopathy (PSSM)

is a condition in horses where there is abnormal glycogen storage occurring in muscles resulting in the accumulation of excessive amounts of glycogen and an unusable polysaccharide. The accumulation of these compounds can result in symptoms that might range from reluctance to “move out,” in its mildest form, to an acute episode of “tying up” or rhabdomyolysis. Another term for this condition is “Equine Polysaccharide Storage Myopathy” or “EPSM.” The term “PSSM” was initially used by Dr. Stephanie Valberg (University of Minnesota) when she first described the condition in Quarter horses while Dr. Beth Valentine (Oregon State University) used the term “EPSM” when she described a similar condition in draft horses. While symptoms may vary depending on the breed, there is so much similarity that the two terms probably represent two ends of a spectrum of a single metabolic muscle disorder. A recent article from Spain documented PSSM in 9 show jumpers and dressage horses showing back pain. Those horses included “warmbloods,” Anglo-Arabs and Andalusian breeds. Genetically, the Morgan breed has common ancestry with both Quarter horses (the Morgan breed contributed significantly to the Quarter horse gene pool) and draft breeds. Since there is the suggestion that the condition has a familial basis (mode of inheritance is not known and is most likely complex), it would make sense that Morgans could have PSSM / EPSM. (For simplicity, the condition will be referred to as PSSM in the rest of this article). While the incidence of the condition in Morgans is not known, researchers studying PSSM have documented multiple Morgan cases.

The purpose of this article is to describe our current knowledge of the metabolic abnormality involved, the resulting symptoms exhibited by horses with the condition, how to diagnose the condition and the diet and husbandry used to successfully manage the condition. In addition, the author would like to share some anecdotal insight gained by personally dealing with the condition in my own Morgan.

Metabolic Abnormalities

Glucose is normally stored as an energy source in the form of polysaccharide (“polysaccharide” means “many molecules of glucose”) called glycogen in the liver and muscles. Horses with PSSM actually have an excessive accumulation of glycogen molecules in muscle cell cytoplasm. Over time, this glycogen combines with certain proteins present in the cytoplasm to form abnormal glycogen-protein complexes. How these abnormal polysaccharides damage the muscle cell is not exactly clear but may involve interfering with intracellular pathways for generating energy and thus result in cell necrosis or death. There are several “glycogen storage diseases” recognized in humans and other species but they are usually characterized by accumulation of excessive glycogen due to deficiencies in the enzymes that break down glycogen for energy utilization. In horses with PSSM, the enzymes to break down glycogen are present and work appropriately. Instead, the problem seems to be that these horses are too efficient in storing glucose as glycogen in muscle cells. Muscle cells are one of the cell types in the body that utilize the hormone insulin to help glucose enter the cell. Insulin serves to aid cell membrane proteins called “Glucose Transporters” (GLUTs) in transporting glucose into the muscle cells. Studies have shown that horses with PSSM actually have increased insulin sensitivity so that they are more efficient in moving glucose into the cells. (This would be the opposite of Type 2 diabetes where cells have decreased insulin sensitivity which is also called insulin resistance) Preliminary studies suggest that this increased insulin sensitivity may involve abnormalies involving the GLUTs.

Symptoms Associated with Excessive Polysaccharide Storage

With the excessive accumulation of these polysaccharides, the muscle cell can be damaged and die. When this occurs, depending on the number of muscles involved, duration of the process and other stress factors, multiple symptoms can be shown by the affected horse. While symptoms can occur in 6 month or younger foals, most often signs are not apparent until the horse goes into training at two to four years of age. Depending on the severity of involvement, symptoms can include any of the following:

Generally stiff, difficulty rising, reluctant to “move out,” tires easily, saddle issues (sore back), bucking, resistance to holding up the hindfeet for shoeing, subtle lamenesses, abnormal gaits, cranky attitude, muscle tremors or sensitivity, kicks at flies that aren’t there (muscle cramps), swishes tail excessively (muscle pain), looks at belly or flanks as if colicky (muscle pain), having a preference for rubbing or rolling a lot, or the opposite, with resistance to brushing/grooming, especially over the back and rump. Affected horses may show a stiff, tense gait with difficulty getting that nice relaxed rhythmic swing that is so desirable in the dressage horse or any other athletic, sport horse. Symptoms of “shivers” where there is an abnormal hindleg action and muscle quivering have been described in draft breeds. If left unmanaged, over time the muscles can atrophy and the horse can show severe muscle wasting. The horse may eventually be unable to rise from a lying down position.

In its most severe manifestation, an affected horse can “tie up”(also known as exertional rhabdomyolysis or Monday morning disease or azoturia). While the horse is exercising, they will stop moving, often quite abruptly, or sometimes just slow down. They may come to a complete standstill, refusing to move. Their muscles tremble all over their body and they break out in profuse sweating due to extreme pain as the muscles, especially their rump muscles, go into severe cramps. As these affected muscles are severely damaged, they will release a cellular pigment called myoglobin which is normally involved with oxygen utilization within the muscle cell. Released myoglobin is then transported in the bloodstream and is filtered by the kidneys, appearing in the urine as a dark brown color. Unfortunately, myoglobin can be toxic to the kidneys and result in renal failure if not treated aggressively. It can take weeks to recover from an episode of rhabdomyolysis.

Symptoms can be exacerbated by nervousness and can occur after being rested from exercise for a couple of days or longer. A history of recent respiratory infection may serve as a precipitating event. Not gradually warming up in cold weather may also precipitate symptoms.

Diagnosis of PSSM

A presumptive diagnosis can be made based on a typical history of symptoms described as above, as well as the presence of typical physical findings such as sore back or rump muscles, resistance to pick up hind limbs or in extreme cases muscle wasting. To support the diagnosis, it is helpful to document elevations of muscle enzyme concentrations in the blood. The two most commonly evaluated muscle enzymes are creatine kinase (CK) (also known as creatine phosphokinase, CPK) and aspartate aminotransaminase (AST) (also known as serum glutamate transaminase, SGOT). CK has a shorter half life and therefore will become elevated sooner and return to normal range sooner after an acute episode of muscle damage and is therefore the preferred muscle enzyme to evaluate when diagnosing and monitoring muscle recovery. Horses with PSSM may have normal or elevated CK concentrations prior to exercise. After exercise the concentrations of CK will rise to higher levels than normal horses. One recommended protocol for diagnosis is to measure CK concentrations 4 hours after after mild walk/trot exercise for 15 minutes. A CK concentration greater than 800U/L would suggest significant muscle damage. While elevated CK concentrations are not diagnostic for PSSM, since any condition that damages muscles can elevate CK, elevations would be supportive of the diagnosis and warrant pursuing a definitive diagnosis by muscle biopsy. It is recommended that the veterinarian performing the biopsy contact the laboratory prior to doing the biopsy to learn the best way to collect, handle and transport the tissue to the lab. Because the histopath diagnosis is very dependent on tissue handling (temperature [freezing/thawing], duration of time from biopsy to processing), staining and processing, it is recommended that only labs knowledgeable in diagnosing PSSM be used (2 contacts are listed at the end of the article). The muscle tissue is stained with PAS (periodic acid Schiff) stain to show glycogen within the muscle cells. In addition, the tissue is then treated with amylase to differentiate glycogen from the abnormal polysaccharide complexes (glycogen is digested by the amylase whereas the abnormal polysaccharide complexes are resistant to digestion by amylase).The amount of PAS positive muscle cells that are and are not digested by amylase help in making the diagnosis. While greater than normal amounts of PAS positive muscle cells are usually present, increased amounts of amylase resistant PAS positive tissue may or may not be present, depending on the duration of the condition in the horse (probably increases with increasing duration). In addition, there may be other intracellular changes that suggest muscle damage that can be visualized in the evaluation.

Management of PSSM

The successful management of PSSM is based on a combination of changes in dietary and exercise programs, tailored to each individual horse. Since these horses have an increased sensitivity to insulin and therefore are more efficient at taking in glucose circulating in the bloodstream into muscle cells, dietary modification is oriented at minimizing the amount of glucose and therefore carbohydrates in the diet (the so called “Horsekins Diet”). An attempt to eliminate all grains should be made. Since carbohydrates are a major source of calories, this energy source needs to be replaced.

A very good source of calories is fat in the diet since fat has twice as many calories as carbohydrate or protein on a per gram basis. Sources of fat that can be used in horses include rice bran and corn oil. In addition to being a good source of calories, fat has been shown to be “muscle sparing” in horses used for athletic competition. That is, when fat is incorporated in the diet in greater quantities, studies have suggested that muscle cells are somewhat protected from some of the negative oxidative processes that occur with high levels of exercise and the muscles seem to recover better after an athletic event even in normal horses. In diets high in fat, there may be an increase in the need for fat soluble vitamins such as Vitamin E. Since both Vitamin E and selenium are important for muscle health, it is important to make sure adequate levels are available. However, excessive levels of selenium (and other nutrients) can be toxic; therefore, care should be taken in formulating an individual diet. Also attention to electrolyte needs that are vital to proper muscle and other cell functions should be addressed, especially in hot, humid climates.

When electing to use fat sources other nutritional factors need to be considered. While rice bran is an excellent source of fat (10-20% fat, Dry Matter Basis, depending on the source), it is also high in phosphorus so that a source of calcium must be fed to maintain an ideal Ca:P ratio. Good sources of calcium include alfalfa and beet pulp. Caution should be used in just feeding alfalfa as your only source of hay since it can be a source of higher available carbohydrate than grass hays. The importance of adequate fiber cannot be over emphasized in any horse’s diet and hay at volumes of at least 1.5-2% of body weight should be fed or have plenty of high quality grass available. Sugar content of grass is also a consideration in certain grasses at certain times of the year.

Another good source of fiber and “safe calories” is beet pulp. Since it is also high in calcium it can be used to balance the high phosphorus in rice bran if fed in a 3:1 ratio of beet pulp to rice bran on a weight basis. When shopping for sources of beet pulp, be sure to purchase beet pulp that has not had molasses added! When feeding corn oil, the limiting factor can often be palatability. As much as 16 ounces per day can be fed. Recent suggestions that corn oil (1.5 oz /day) can be protective against gastric ulcers would be an additional plus to using corn oil.

If trying to figure out your own balanced diet seems a little overwhelming, the good news is that there are several feed companies that have diets available that could be used for PSSM. Manufacturers include Kentucky Equine Research (ker.com) – Equi-Jewel, balanced rice bran and other protein mineral supplements; Hallway Feeds (hallwayfeeds.com) – Releve, a complete, balanced diet specifically developed for PSSM; Moorman’s Natural Glo Rice Bran, Buckeye Ultimate Finish and others. In addition, Purina has recently marketed their Ultium which has rice bran and beet pulp; however, there is some grain/molasses content to that diet. Be sure to work with a good nutritionist to make sure that your diet is well balanced in calories, protein, electrolytes, minerals and vitamins. The other good news is that a high fat, low carbohydrate diet high in fiber is also optimum for preventing other equine diseases including colic, laminitis, gastric ulcers and recurrent rhabdomyolysis in race horses which is due to another mechanism than PSSM. It is helpful in managing these disorders as well as in the management of equine pars intermedia dysfunction (Equine Cushings) as well as in the management of Peripheral Cushings or so called Metabolic Disorder, if calories are appropriately regulated.

Studies have suggested that optimum response to management must not only utilize a dietary change but must incorporate daily, regular exercise so that muscle gycogen is used daily and not allowed to accumulate. Horses should get as many hours as possible of turn out to exercise, even if you have to use a grazing muzzle to limit caloric intake in quality pastures for fat horses. Workouts should be done daily on a consistent basis with “days off” limited to one or two a week. When prolonged lay offs cannot be avoided due to schedules, illness or injury, starting back to work should be gradual.

Prognosis for a horse diagnosed with PSSM

While PSSM could be fatal if a horse has a severe episode of tying up or if he/she becomes debilitated by muscle wasting, most horses can do very well if diagnosed early and managed successfully. In fact, according to Dr. Valentine, there is the suggestion that horses with PSSM, when managed correctly, are superior in temperament, conformation and performance. And in fact, while breeding for these characteristics, we may have been unknowingly breeding for this type of “over-efficient” muscle metabolism.

An anecdotal case of PSSM

My personal experience with PSSM began on a chilly November evening 2 years ago. I had been to a 2 day show the weekend before and given my horse 2 days rest before starting back to work. I was riding my then 5 year-old Morgan gelding at the canter when he came to an abrupt stop and started shivering and sweating all over. We were some 100 yards from the barn and it took me 15 minutes to walk him back to his stall. With each step I began to realize that my horse had just “tied-up.” As I instituted IV fluids, Banamine and acepromazine therapy to stabilize him, I mentally went through my differentials for rhabdomyolysis. In retrospect, I realized that he had shown a lot of those subtle symptoms like the sore back (I had to have a custom made saddle built for him), resistance to the farrier lifting his hind feet, ticklish when being brushed, loving to roll after any exercise, swishing his tail a lot when being ridden, and kicking at non existent flies. He would often get mad when you asked him to do certain tasks while riding. While we had only shown in 2 recognized shows at training level, the judges’ comments consistently included “seems tense,” “resistant,” and “needs to relax.” So I decided to confirm my suspicions (his CK was >35,000!) by sending a muscle biopsy to Dr. Valberg at Minnesota.

While my gelding had to have two weeks of complete rest to recover from the severe muscle damage, I didn’t wait for biopsy results before I started instituting dietary changes. I did my homework on commercial diets available and finally came up with a diet that has seemed to work for us. I formulated the diet, consulted a feed company that ran it through a nutrition software program to make sure it was well balanced and gradually introduced it to my horse. The diet basically consists of Ca:P balanced rice bran, beet pulp (non-molassed), a protein/mineral supplement, corn oil, a Vit E/Selenium supplement (grass and hay in my area is selenium deficient), an electrolyte supplement, one flake of alfalfa hay, one to three flakes of grass hay, and 8-12 hours of grazing (quantities depend on time of year and thus grass quality).

My exercise program did not have to change since he was already getting 8-12 hours of grazing and when stalled he has free access to a 50x50 paddock. He is exercised 5-6 days a week being either ridden, driven or longed. When I go out of town for several days, if I can’t get friends to exercise him, I start exercise back very gradually. When I go to horse shows, I take a portable electric paddock so that he can have some turn out time even at shows. When the weather is cold, he gets ridden or at least warmed up with a rump rug on his back. He loves getting massages from our local equine massage therapist as often as I can afford it. Slow warmups are very important to our daily training. After about 4 months, I could start to see a difference in how he moved. It took at least a year before I could say that he had a lovely relaxed swishy trot.

Realizing that I am a relatively new dressage rider working with a trainer, we have made slow steady progress. We are consistently scoring in the mid 60s at training level, and are strongly schooling 1st level with aspirations to show first level at our next recognized show. In addition, we have recently started to drive and hope to do some dressage driving as well as pleasure driving. We have competed in a 17 mile “fun” endurance race, a 6 mile Hunter Pace and trail ride weekly. We do have to be careful about long rides and chilly, damp weather. Since I have always felt that good things always come from bad, I feel fortunate to have made the diagnosis in my horse because I now have a better knowledge of good nutrition as well as a better feel for exercise physiology and muscle movement after all the research I have done to understand and manage his condition. His attitude is that of a typical, happy Morgan that loves to work. I think I have a happier, better athlete and we hope to ride and compete for many years to come!

Conclusions

As Morgan enthusiasts, it is important that we educate ourselves about this possibly under-diagnosed condition. What else can we do besides educating ourselves and our colleagues, looking for it in our own horses and adjusting dietary/exercise management? We can contribute to the advancement of knowledge about the condition by helping researchers. We can help them by offering blood and tissue from our own horses for biochemical and genetic studies so we can have a better understanding of the incidence within our breed. We can support research by donating dollars to those investigators that are producing quality research data to better inform us. Recognition is the first step towards the elimination of the negative aspects of this unique metabolic condition.

Investigators currently doing research and muscle biopsies:

Dr. Stephanie Valberg
Department of Clinical and Population Sciences
College of Veterinary Medicine
University of Minnesota
St.Paul, Minnesota 55108
Phone 612-625-5768 (Neuromuscular Laboratory)
Fax 612-625-6241
E-mail: Valbe001@umn.edu

Dr .Beth A. Valentine
College of Veterinary Medicine
Oregon State University
Corvallis, Oregon 97331
Phone 541-737-3261
Fax 541-737 6817
E-mail: Beth.Valentine@oregonstate.edu

References