skip to main content

Polysaccharide Storage Myopathy

What Is It?

  • PSSM is a muscle disease and one form of tying up. When tying up becomes chronic, it is termed RER or recurrent exertional rhabdomyolysis.

  • Polysaccharide Storage Myopathy (type 1 PSSM) is caused by a genetic mutation that results in excessive glycogen storage in skeletal muscle.

  • A second form of PSSM, not caused by the mutation, has been coined type 2 PSSM. There is a more severe form of type 2 PSSM called MFM (myofibrillar myopathy).

  • It is proposed that affected horses have an abnormality of glycogen synthesis regulation that leads to storage of unmetabolized carbohydrate in the muscles.

  • Horses with type 1 PSSM are unable to normally store and use sugar in skeletal muscle. Causes of type 2 PSSM are currently unknown.

  • Clinical signs are those typical of tying up, including muscle stiffness, pain, profuse sweating and reluctance to move. Rest for a few days prior to exercise is a common triggering factor. Some horses with type 1 PSSM are asymptomatic. Type 2 PSSM may have similar signs although many common clinical signs included those of poor performance, including a drop in energy level and unwillingness to perform after 5-10 minutes of work. Undiagnosed gait abnormality, sore muscles, and slow onset of muscle atrophy are also described.

Dietary and Management Recommendations for PSSM1

  • On a total diet basis (hay and feed), provide less than 15% of the daily digestible energy intake as dietary starch and at least 15% of the daily digestible energy intake from fat sources.

  • In practical terms, the majority of the diet should consist of high quality forages such as alfalfa hay or grass-alfalfa mix, with a total NSC (WSC+Starch) lower than 12%.

  • Affected horses should have minimum contact with lush pasture grass as high levels of fructans (plant sugars) can exacerbate problem.

  • Select a feed with high quality protein content, containing high quality amino acids.

  • The majority of horses with PSSM1 are easy keepers, and should be fed a low NSC forage; 30% protein diet balancer (to manufacturer’s recommendations) with high quality amino acids; and 1-2 pounds of fat supplement per day.

  • If additional calories are warranted, they should be provided in the form of fat.

  • Ensure the total diet includes 2000 IU/day of vitamin E. Additional supplementation may be needed for specific individual horses, based on serum vitamin E level.

  • Access to salt and fresh water at all times.

  • Minimize stress and provide regular routine with exercise, turnout and feeding.

  • PSSM1 horses respond most favorably to both diet and exercise change. Typically, after 3-6 months of therapy, post-exercise serum concentrations of CK are within reference limits.

Dietary and Management Recommendations for PSSM2/MFM

  • Minimize calories sourced from dietary fat (fat can act as a pro-oxidant and exacerbate oxidative cell damage), and instead utilize digestible fiber and moderate levels of starch and sugar. Typical concentrates should contain 25-30% NSC.

  • Select a feed with high quality protein content, containing high quality amino acids.

  • PSSM2/MFM horses lacking in muscle mass or strength, benefit from a branch-chain amino acid supplement, given within 45 minutes post-exercise.


Recommended Products

for Type 1 PSSM:

for Type 2 PSSM: