Managing PSSM in Horses
Polysaccharide Storage Myopathy (PSSM), aka Equine Polysaccharide Storage Myopathy (EPSM or EPSSM), is a muscle disease which primarily affects horses with Quarter Horse bloodlines, but it can also occur in Drafts breeds, Warmbloods, and occasionally other breeds.
Here are the basics of what occurs with PSSM: When carbohydrates are consumed, the body changes them into a form of sugar called glucose that can be used for energy. The glucose is then changed to glycogen, a form of sugar that can be easily stored by the muscles and liver. With PSSM horses, muscle tissue accumulates an abnormal amount glycogen and polysaccharides (another type of sugar) which, in turn, leads to muscle pain and often, tying-up symptoms. In fact, PSSM horses can have anywhere from 1.8 to 4 times the normal amount of glycogen in their muscles.
There are two types of PSSM: Both are the result of the abnormal accumulation of muscle glycogen, but Type 1 is caused by a gene mutation whereas the cause of Type 2 has not yet been identified. Type 1 can be diagnosed with a genetic blood or hair test, but Type 2 can only be diagnosed through a muscle biopsy.
Symptoms of PSSM
Symptoms of PSSM can range from the classic signs of tying up, including stiffness, sweating, reluctance to move, and firm and painful muscles to more subtle symptoms such as:
- changes in stride
- shifting lameness
- tense abdomen
- seeming lazy during work
- reluctance to engage the hindquarters
- difficulty backing or picking up hind limbs
- loss of muscle mass (atrophy)
- muscle tremors in flank area
- dark-colored urine
PSSM episodes usually begin after very light exercise such as walking and trotting, but horses with Type 1 PSSM can show symptom even without exercise. Episodes are also more likely to occur when horses are exercised after a lay-up period.
Note: Though tying up is a symptom of PSSM, not all horses who tie up have PSSM since tying up can have other causes as well.
PSSM is managed primarily through diet and exercise. Since muscle cells in PSSM horses remove sugar from the blood stream, transporting it into the muscle at a faster rate and also produce excess glycogen, these horses are very sensitive to insulin, the hormone released by the pancreas into the bloodstream in response to a carbohydrate meal. Therefore, high starch feeds such as sweet feed, corn, wheat, oats, barley, and any feed with molasses should be limited or avoided. Instead, a low NSC diet with quality grass hay (preferably warm-season grass hays or any hay with an NSC level of 12% or less) is preferable. Some alfalfa is usually fine to feed as well.
Most Type 1 PSSM horses are easy keepers and do well on forage (and a ration balancer or mineral supplement) alone, but if your PSSM horse needs more calories, adding fat such as flaxseed oil or camelina oil to the diet is recommended.
Since horses with PSSM are also dealing with oxidative stress, supplementing Vitamin E and other antioxidants like selenium, Vitamin C, and grape seed extract can be helpful. Magnesium may also be beneficial.
Daily exercise is very important for PSSM horses. This will help the horse to use up glycogen stores and will also improve metabolism in skeletal muscle. Even 10 minutes of exercise can make a difference. It is best for these horses to be turned out 24/7 or or kept in corrals and not stalled. However, since pasture access may need to be limited, utilizing a dry lot or a grazing muzzle may be necessary.
When re-introducing exercise after several days of rest, take it slow. Start with slow longe line work for 3-5 minutes and gradually increase the amount of work each day.
Interestingly enough, research shows that 50% of PSSM horses improve with dietary changes alone, but 90% will improve with both dietary and exercise changes.
Sources and Further Reading