What is Polysaccharide Storage Myopathy
Polysaccharide storage myopathy (PSSM) is a chronic muscular condition that results in an abnormal accumulation of glycogen (sugar) in the muscles. This can cause episodes of muscle stiffness and pain after exercise, also known as “tying up.” It is seen in a number of breeds and the clinical signs may include reluctance to move, sweating, and muscle tremors with the onset of exercise. There are currently two identified types of PSSM: Type 1 and Type 2.
Type 1
Type 1 is caused by the genetic mutation that causes muscle cells to produce glycogen continually. PSSM1 is an autosomal dominate trait, meaning only one copy of the mutation is needed for a horse to be affected. PSSM1 is common in Quarter Horses, Paints, Appaloosas, and draft breeds and can be determined by a DNA test with a hair or blood sample.
Signs of PSSM 1 have been associated with exertional rhabdomyolysis or “tying up”. Most common signs include pain, stiffness, sweating and reluctance to move in conjunction with exercise. Signs are most often seen after a horse is put into initial training or after time off with limited turn-out. However, these signs can all occur without exercise too. During a tying-up episode, horses act lazy, have sudden shifts in lameness, tense up in their abdomen. When horses stop moving, they may stretch out looking as if they need to pee. Muscles are hard, stiff, and painful, particularly over the hindquarters.
Type 2
PSSM Type 2 may also be genetic, however the exact cause is unknown and there may actually be multiple causes. PSSM2 represents those horses in which a muscle biopsy shows clumping of muscle glycogen yet do not have Type 1 PSSM based on genetic testing. PSSM2 is also commonly seen in Quarter Horses however warmbloods, arabians, thoroughbreds, and standardbreds can also be affected. Per Michigan State University College of Veterinary Medicine, there is no scientifically validated DNA test for PSSM2, but a muscle biopsy can be performed for diagnosis.
Management of PSSM Horses
There is no cure for PSSM, however with diet and exercise it can be managed successfully. For those that follow dietary and exercise management recommendations, over 75% of horses had no episodes of tying up. For the most part, recommendations for type 2 PSSM have been the same as PSSM1.
Exercise
Daily exercise is crucial for managing horses with PSSM1 and PSSM2. University of Michigan Collage of Veterinary Science states that even 10 min of exercise has been shown to be extremely beneficial in reducing muscle damage with exercise. Once conditioned, some PSSM horses thrive with 4 days of exercise as long as they receive daily turn out. For riding horses with PSSM2, a prolonged warm-up with adequate stretching is recommended. Rest periods that allow horses to relax and stretch their muscles between 2 – 5 min periods of collection under saddle may be of benefit.
Diet
Nonstructural carbohydrates (NSC) should be 12% or less to keep blood insulin levels low and reduce glycogen storage in muscle. PSSM horses are often easy keepers and a diet with a low-NSC grass hay and a quality ration balancer is usually adequate. If additional calories are needed, a low-NSC and/or high fat feed source should be incorporated. Since small management changes can have a big impact, fine-tuning the diet over time may be necessary.
Ingredients that May Lend Support
Vitamin E is considered the most important antioxidant and works closely with selenium to protect the body from the oxidative stress of exercise and illness. Found in high amounts in fresh pasture, levels begin to decay the moment pasture is cut for hay. That is why any horse that does not have access to grass, regardless of its activity level or health, would be a candidate for vitamin E supplementation. Horses are not very efficient in storing vitamin E and deficiency may be accelerated if the diet is deficient in selenium.
Selenium is a trace mineral that along with vitamin E function in a partnership that helps to protect body tissues from free radical damage that occurs during oxidation (the conversion of feedstuffs into energy). While some parts of the country have high levels of selenium in their soil and therefore the plants that grow there, selenium deficiency has been reported in 46 states. Therefore, most horses will need supplementation to meet the NRC requirement of 1 mg/day for maintenance.
Magnesium (Mg) is a micromineral that helps to maintain normal muscle and nerve function, aids to regulate blood sugar levels and promotes normal blood pressure. Since a symptom of magnesium deficiency is agitation/anxiety, it is included in many calming supplements.
DMG (Dimethylglycine) is a naturally occurring substance in the body and in many foods, but in low levels. Supplementing with this ingredient makes additional DMG available to cells throughout the body, where it is involved in energy production processes that use oxygen. DMG is used to help support the immune system, metabolism and serve as an antioxidant.
For more information on PSSM, please visit the University of Michigan College of Veterinary Science website: https://cvm.msu.edu/research/faculty-research/comparative-medical-genetics/valberg-laboratory