Polysaccharide storage myopathy (PSSM)

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Polysaccharide Storage Myopathy (PSSM)

Equine Polysaccharide Storage Myopathy (EPSM Or EPSSM)

Polysaccharide Storage Myopathy(PSSM) is a type of muscular disorder of horses. It is associated with two different forms---designated as PSSM Type 1, which is caused by a glycogen synthase 1 (GYS1) gene mutation, and PSSM Type 2, are not caused by the GYS1 mutation and whose origin is yet unknown.

PSSM1 vs PSSM2 Comparison

Type 1Type 2
Muscle biopsy
Positive genetic test results
Cause is unknown
Muscle biopsy
Often seen in halter horsesQuarter Horses and Warmbloods
Often seen in horses used for barrel racing, reining and cutting
Muscle stiffness
Reluctance to move
Horses seem lazy
Shifting lameness
Tense abdomen
Tremors in flank area
Stretch out as if to urinate as soon as they stop moving.
Sweat profusely
Firm, hard muscles, particularly over their hindquarters
Paw and roll immediately following exercise
History of numerous episodes of muscle stiffness
Coffee colored urine
Muscle atrophy
Typing up (episodes of muscle pain, stiffness, reluctance to move)
inability to rise
stiff hind limb gait
poor performance
Gait abnormality
Sore muscles
Drop in energy level
painful firm back and hindquarter muscles
reluctance to collect and engage the hindquarters
poor rounding over fences
slow onset of atrophy especially when out of work
Signs are most often seen in horses after 10-20 minutes of light exercise upon initial start of training or following a lay-up period when they receive little active turn-out. Horses can also show symptoms without getting exercised.Horses usually show signs of unwillingness to perform after 5 - 10 min of exercise.
Both forms are diagnosed through a conducting a muscle biopsy, which shows clumping of muscle glycogen. However, a false positive diagnosis can occur if the muscle biopsy is crushed with forceps resulting in abnormal glycogen and a false negative diagnosis can occur if samples are not kept chilled and shipped quickly to the laboratory because glycogen is degraded while the muscle biopsy is in transport. In addition, muscle biopsies for horses with PSSM2 and recurrent exertional rhabdomyoloysis (RER) can look very similar. The mean age of onset of clinical signs of PSSM2 in Warmbloods is between 8 and 11 years of age.


Poor performance
Reluctance to engage hindquarters
Shifting lameness
Subtle stride changes
Tense abdomen
Difficulty backing up
Difficulty picking up hindlimbs
Slow onset of atrophy, especially when out of work
Gait abnormalities
Sore muscles
Painful firm back and hindquarter muscles
Poor rounding over fences
unwillingness to perform after 5 -10 min of exercise
Coffee colored urine
Sweating profusely
Paw and roll after exercise
Tremors in flank area
Muscle pain


  • History
  • Clinical signs
  • Genetic testing
  • Muscle biopsy

While waiting for your veterinarian

  • If the horse is being ridden: Stop exercising the horse.
    Move horse (if willing) into a stall.
    Evaluate how dehydrated the horse is by pinching the skin or feeling the saliva.
  • What Not to Do: Do not feed the horse any grain.
  • What to do: Make sure there is fresh, clean water available for the horse to drink.
    Provide electrolytes in a separate bucket of water and offer to the horse.
    If its cold, blanket the horse to keep warm.
    If its warm weather, remove any sweat and provide access to a fan to keep cool.



Diet modificationSwitch to a low starch (less than 10%), high fat (13%) diet feed or low calorie ration balancer and a low NSC (12% or less) hay
Exercise managementTurned out as much as possible, limiting time spent in stall, provide daily controlled exercise
Grazing muzzlesWhen horses are turned out on pastures with lish grass


  • Provide regular turnout with other horses
  • Daily exercise
  • Base diet composed of a high quality grass or oat hay
  • Eliminate grain and sweet feed from the diet
  • Soaking hay


If both diet and exercise are altered, than the majority of horses no longer have episodes of tying up or significantly reduce the number of episodes that occur.

Scientific Research

General Overviews

Risk Factors

  • genetics
  • high starch diet
  • lack of exercise

Commonly Affected Breeds

Quarter Horse iconAppaloosa iconPercheron iconMorgan iconPaint Horse icon