PSSM & Muscular Health

By: Dr. Lydia Gray

SmartPak strongly encourages you to consult your veterinarian regarding specific questions about your horse's health. This information is not intended to diagnose or treat any disease, and is purely educational.

Brief Description

Polysaccharide Storage Myopathy (PSSM), also called Equine Polysaccharide Storage Myopathy (EPSM or EPSSM), is likely an inherited condition, occurring most commonly in Quarter Horses, draft horses, and warmbloods, but also showing up in other breeds. The muscles of a horse with PSSM are unable to properly store glucose (sugar) so it is unavailable when needed for energy. Two forms have now been identified: Type 1, which is caused by a genetic mutation, and Type 2, the cause of which has yet to be determined. PSSM is related to “tying up”, but “tying up” can have many causes. For more information on the causes of “tying up” not related to PSSM, click here.

Supplements that May Lend Support

Because of the oxidative stress associated with this disorder, as well as the free radicals generated from the high fat diet often recommended by horses with PSSM (see ‘Diet’ below), experts advise supplementing with Vitamin E for its antioxidant properties. Other antioxidants include Selenium, Vitamin C, and Grape Seed Extract. Anecdotal evidence also suggests that magnesium, branched chain amino acids, and Dimethylglycine (DMG) may also be beneficial for horses with muscle disorders like PSSM.

Possible Diagnostic Tests

Signs of PSSM can range from the classic stiffness, sweating, reluctance to move, and firm and painful muscles associated with traditional “tying up” to:
  • subtle 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).
Exercise should be stopped and a veterinarian called immediately if any of these signs occur. The usual blood tests for “tying up” may confirm damage to the muscles, but additional testing will need to be performed in order to establish a diagnosis of PSSM. Type 1 can be diagnosed with a genetic test on hair or blood while Type 2 can only be diagnosed through muscle biopsy.

Prescription Medications Available

At this time, there are no prescription medications approved by the FDA for PSSM. Depending on the severity of signs, a veterinarian may administer pain relievers, fluids, sedatives or other medication to provide comfort and assist in recovery after an episode.

Other Management Suggestions

Both diet and the exercise/turnout program for a horse with PSSM must be adjusted in order to properly manage the condition.


Because PSSM causes horses to be more sensitive to insulin and therefore store more glucose in their muscles than normal, it is important to restrict the amount of sugars and starches in their diet. Nutritionists refer to the easily digestible sugar and starch portion of feeds and forages as non-structural carbohydrates (NSC). Since grain such as sweet feed is high in NSC, it should be removed from the diet and replaced with either a multi-vitamin/mineral supplement or a ration balancer. Also, hay that tests less than 10-12% NSC is more appropriate for PSSM horses, with soaking as one method to reduce sugars and starches in forage that tests higher. Access to fresh grass should be limited either with a grazing muzzle or by turning out onto a dry lot for exercise. Fat can gradually be added into the diet for those horses that require additional calories or energy.

Housing & Exercise

Ample turnout is essential for horses with PSSM, as they should not be stalled for more than 12 hours at a time. Ideally these horses should be kept outside 24 hours per day. Another key to managing this condition is keeping the horse in a consistent exercise program – meaning controlled exercise such as hand walking, lunging, riding, or driving. When returning the horse to work after a bout of PSSM, start gradually. Always provide these horses with a long warm-up and cool-down, and offer frequent walk breaks.

About Dr. Lydia Gray

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