We recently discussed the problem of tying up (rhabdomyolysis) in performance horses and options when it comes to preventing and treating that condition, but in some instances horses can have recurring episodes of tying up due to a condition known as Polysaccharide Storage Myopathy (PSSM).
While PSSM is incurable, the good news is that this genetic muscle disease can often be managed with diet and exercise.
What is PSSM in horses?
According to equine internal veterinary medical specialist and Fullbucket co-founder, Dr. Robert Franklin, PSSM is a problem where the horse’s body has difficulty processing the stored muscle energy source, glycogen.
When a horse consumes carbohydrates from forage or feed, their body breaks those nutrients down into a sugar called glucose that can be used for energy. Some glucose that is not immediately used by the body is further processed to glycogen, a form of sugar that can be easily stored by the muscles and liver.
But in horses with PSSM, muscle tissue accumulates an abnormal amount of glycogen and polysaccharides (another type of sugar) which, in turn, leads to muscle pain and often, tying-up symptoms.
In fact, affected horses can have anywhere from 1.8 to 4 times the normal amount of glycogen stored in their muscles.
You can think of glycogen like a CLIF energy bar, Franklin says.
“Glycogen is simply a CLIF bar for the muscle cells. Whenever the horse is exercising hard, they’ve got those CLIF bars to chew on and give them plenty of energy,” he says. “If there is a storage problem, such as in a PSSM horse, then basically all the CLIF bars just get piled into the cell, but the muscle can’t actually unwrap the wrapper and get to the bar. Basically, it becomes a problem where the muscle cells can’t utilize the energy that they need, causing them to fail.”
Researchers have discovered two types of PSSM in horses. Both are the result of an abnormal accumulation of muscle glycogen, but type 1 PSSM is caused by a genetic mutation whereas the cause of type 2 PSSM has not yet been identified. It’s also important to note that PSSM affects many different breeds, but according to Franklin, it’s most commonly seen in Quarter Horse and Draft breeds.
Symptoms of PSSM in horses
While not every horse with a PSSM diagnosis shows symptoms, many do, and those symptoms can include more than just the typical tying up episodes that many people tend to associate with the condition. Other clinical signs of PSSM in horses can include:
- Skin twitching
- Stiffness
- Firm, painful muscles
- Seeming lazy during work
- Reluctance to move and/or engage the hindquarters
- Difficulty backing up or picking up hind limbs
- Abnormal or excessive sweating
- Weakness
- Dark colored urine
Some of the less common clinical signs include:
- Gait abnormalities
- Mild colic
- Muscle wasting
- Inability to rise
- Roaring (abnormal breathing sounds due to laryngeal hemiplegia)
One way that PSSM differs from sporadic exertional rhabdomyolysis is that symptoms usually begin after very light exercise such as walking and trotting. However, horses with Type 1 PSSM can show symptoms even without exercise. Tying up episodes are also more likely to occur when horses are exercised after a lay-up period.
PSSM testing in horses
If you suspect your horse may have PSSM, getting him tested by your veterinarian is important. Type 1 PSSM can be diagnosed with a genetic blood or hair test. However, type 2 PSSM can only be diagnosed through a muscle biopsy.
Once you have a diagnosis though, PSSM must be managed primarily through diet and exercise. Since these horses are very sensitive to insulin, the hormone released by the pancreas into the bloodstream in response to a carbohydrate meal, high starch feeds such as sweet feed, corn, wheat, oats, barley, and any feed with molasses should be limited or avoided.
Instead, a low non-structural carbohydrate (NSC) diet with quality grass hay (preferably warm-season grass hays or any hay with an NSC level of 12% or less) is best. Some alfalfa is usually fine to feed as well.
Most type 1 PSSM horses are easy keepers and do well on a forage-based diet, but vitamin, mineral, and most notably, vitamin E supplementation, are all important for these horses.
However, if your PSSM horse needs more calories, go for a diet higher in fat instead of carbohydrates.
“You will see many feed companies marketing high-fat, low-carbohydrate feeds for horses that have muscle problems,” notes Franklin. “That’s because a lot of muscle diseases are genetic and related to the storage of glycogen (or the CLIF bar). Think about a marathon runner who is going to “carb load.” The whole point of “carb loading” is to get those CLIF bars inserted into the muscle cells. If the athlete has a genetic problem dealing with carbohydrates, then they actually need to use different energy substrates.”
How do you treat horses with PSSM?
Aside from diet, routine exercise is also extremely important for PSSM horses as it improves energy metabolism.
In fact, excess time off can actually predispose PSSM horses to additional episodes of tying up. Therefore it’s recommended that these horses receive regular turnout and conditioning should consist of short duration workouts (no more than 20 minutes), gradually increasing to high-intensity exercise.
Consistency is key with your horse’s workouts, as is including a warmup and cool down period.
Additionally, certain supplements may be helpful for horses with Polysaccharide Storage Myopathy.
Franklin and his business partner at Fullbucket, equine surgeon, Keith Latson, helped to develop a product called Medical Muscle, an all-natural, antioxidant-rich supplement which supports muscle recovery and endurance in not only PSSM horses, but any horse needing additional muscle support.
“Not every antioxidant is built with the same efficiencies and capabilities, and the unique one that we use in Medical Muscle, called astaxanthin, has potency unlike any other antioxidant,” notes Franklin. “Elite human athletes, especially endurance athletes, use astaxanthin because it accumulates in the muscle cell and staves off the incidence of tying up. It also increases endurance, allowing each muscle cell to perform at a high level for a longer period of time.”
While PSSM can be a debilitating and potentially career ending condition, research has shown that strict management practices centered on a low starch, high fat diet, a muscle supplement such as Medical Muscle, and routine exercise allows approximately 90% of horses to return to a normal performance level.
If you are looking for the best supplements for horses with PSSM, we have you covered! Check out our veterinarians’ favorite right here.“Good nutrition, sound training, ensuring those horses are getting exercise on a regular basis and you’re not seeing these unusual spikes of intense exercise followed by nothing but pasture; All of these management practices really matter for treating equine PSSM,” says Franklin. “As veterinarians, and certainly as supplement manufacturers, we want to steer towards total wellness and widen that band of resiliency and exercise tolerance that these horses have.”
