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Polysaccharide Storage Myopathy

PSSM, or polysaccharide storage myopathy, is a muscular disorder that occurs primarily in horses with Quarter Horse blood, including purebred Quarter Horses, Paints, and Appaloosas.  However, it has also been observed in draft horses and warmbloods, and in any sport horse that may have some appendix ancestry. PSSM is also known as EPSM and EPSSM. PSSM causes tying-up in affected horses, which is colloquial term for muscle cramps and tremors. While tying-up can occur in other horse breeds, typically this is very different from tying-up episodes in horses affected with PSSM. There is no cure for PSSM, but with careful attention it can be readily managed. The mechanism for PSSM involves glucose metabolism and insulin sensitivity. In normal horses, insulin causes glucose to travel into muscle tissue and liver cells where the molecule will be transformed into glycogen and stored for later use. In horses affected by PSSM, glycogen accumulates at much higher rates along with another abnormal form of sugar called polysaccharide. PSSM muscle tissue draws significantly more glucose from the blood stream and creates much higher amounts of glycogen due to an enzyme mutation. Horses with PSSM can have up to four times the muscle glycogen concentration than those in unaffected horses. The most common type of PSSM, Type 1 PSSMis heritable through both mares and stallions. Type 2 PSSM is less common. It is unknown if it is heritable or not.

Tying-up is a primary symptom in horses with PSSM. This may occur after 10-20 minutes of light exercise and often is observed in horses that are just resuming training. During an episode horses will appear tense with shifting lameness, muscle tremors, a tense abdomen and hardness palpable in the large muscle groups (haunches, shoulders, etc). In severe cases the muscle pain can be extremely evident with horses becoming unable to stand. The discomfort may cause the horse to lay down. In the most extreme cases urine can become coffee-colored and cloudy -- if this is observed, call your veterinarian ASAP.

Feeds that are high in starch and sugar contribute to the severity and development in both types of PSSM. For this reason it is important to decrease sugars in feeds fed to PSSM horses and to limit levels of starch as much as possible. Ingredients that are often high in starch and sugar include molasses, oats, barley, corn, wheat, and other grains. Traditional sweet feeds tend to be heavily grain-based and flavored with molasses. It is important that you add fats in the form of oil or a high-fat feed instead of feeding high sugar or starch grains. Many feed stores carry brands that create low-starch feeds specially for PSSM or insulin sensitive horses and your veterinarian can help you pick an appropriate feed. Exercise is extremely important in managing horses with PSSM. In chronic cases rest can actually be counter-productive. It is important that affected horses return to turn-out as soon as reluctance to move dissipates. Daily exercise is crucial, as this can improve glucose utilization and energy metabolism in muscles. Fortunately with regular exercise, turn-out, and proper feeding, many PSSM horses can experience long, useful lives.
Note the camped under position, sweating, and tenseness through the abdomen of the first horse.  It is even more extreme in the second photo.