California Riding Magazine • June, 2010

PSSM and Muscle Health

Dr. Lydia F. Gray

Brief Description
Polysaccharide Storage Myopathy (PSSM), also called Equine Polysaccharide Storage Myopathy (EPSM), is an inherited condition that occurs most commonly in Quarter Horses (Type I), draft horses and warmbloods (Type II), but can also show 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. PSSM is related to “tying up,” but the traditional forms of “tying up,” or exertional rhabdomyolysis, has different causes.

Possible Diagnostic Tests
Signs of PSSM can range from the classic stiffening up and reluctance to move associated with “tying up,” to subtle changes in stride, difficulty backing or picking up hind limbs, loss of muscle mass and others. The usual blood tests for “tying up” may or may not be helpful. Muscle biopsy has been the only means of truly diagnosing PSSM until very recently, when a DNA test that can be performed on mane or tail hair became available.  However, the genetic test is only accurate for western breeds such as Quarter Horses, Paints, Appaloosas or crosses.  Draft horses, Warmbloods, and gaited breeds such as The Rocky Mountain Horse, Tennessee Walking Horse, Morgan and Haflinger usually also need a muscle biopsy to confirm the diagnosis.

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

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 used in horses with PSSM (see Diet below), experts recommend supplementing with Vitamin E for its antioxidant properties. Other antioxidants include Vitamin C, Grape Seed Extract and Super Oxide Dismutase. Anecdotal evidence suggests that the anti-inflammatory/antioxidant Dimethylglycine (DMG) may be beneficial for horses with muscle disorders such as PSSM.

Other Management Suggestions
Since horses with PSSM are unable to process sugars and starches properly, fat should be used as their main source of energy. Feed companies have created several low-sugar, high-fat commercial feeds that are intended for horses with metabolic conditions such as PSSM. Other dietary suggestions include feeding little to no grain, limiting access to pasture, providing low-sugar hay (less than 15% non-structural carbohydrates or NSC), and supplementing with additional sources of fat. These extra fat sources can include fortified rice bran, vegetable or fish oils, and powdered fat supplements. For horses receiving little to no grain, a multi-vitamin/mineral supplement is recommended.

Housing and Exercise
Ample turnout is essential for horses with PSSM, as they should not be stalled for more than 12 hours at a time. Another key to managing this condition is keeping the horse in a consistent work program – meaning controlled exercise such as hand walking, lunging or riding. 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 on a long rein.

Questions to Ask Your Veterinarian

  • How is this different from regular “tying up?”
  • Does my horse have to be diagnosed with PSSM or can I just begin feeding the recommended diet?
  • Will my horse return to his former level of performance?
  • How is this related to the condition “Shivers?”

For more information on this topic or general horse care information, contact a SmartPak Product Specialist at 1-800-461-8898 or visit the website at

Article provided by SmartPak.
Dr. Lydia Gray is the Medical Director/Staff Veterinarian for SmartPak, where she guides research and new product development, answers questions on her Ask the Vet blog, and speaks around the country at various events such as Equine Affaire, Dressage at Devon, and the USHJA Trainers’ Symposiums.