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Equine-Myopathy panel/PSSM2


Polysaccharide Storage Myopathy (PSSM) is the name of a group of hereditary muscle diseases with similar symptoms. The variants categorized under the umbrella term PSSM2 (P2, P3, P4, Px) lead to exercise intolerance due to muscle wasting, stiffness, and pain.

Typical Symptoms include:

  • Changes in behavior and temperament (caused by pain)
  • Frequent tying-up/tension/muscle tremors
  • Shifting lameness
  • Stiff hindquarters
  • Gait abnormalities
  • Muscle wasting (especially in the hindquarters)

PSSM2 is not curable and can greatly impact a horse’s quality of life. An accurate diagnosis allows owners to optimize feeding and handling/exercise regimens to support the health of their horses.

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Product information

Affected breeds: Most breeds
Test #: H108
Description Myopathy-Panel (P2, P3, P4, Px)
Samples: Hair, blood
Time: 7 – 10 days

General information

  • Two different types of Polysaccharide Storage Myopathy (PSSM) are known so far: Type 1 and Type 2.
  • The diseases have similar symptoms; in the past horses with PSSM1 symptoms that were negative for the PSSM1 GYS1 gene mutation were classified as PSSM2.
  • Research has shown that PSSM2 is not a polysaccharide storage disease, but is actually caused by defects in muscle fibers themselves. The name “PSSM2” is still used because people are familiar with it.
  • “Myofibrillar Myopathy (MFM)” und “Recurrent Exertional Rhabdomyolysis (RER)” are subtypes of PSSM2.
  • Several variants (P2, P3, P4, Px) in the genes  MYOTFLNC, MYOZ3 and CACNA2D3 have been shown to cause these subtypes.
  • A horse can have more than one P-variant, e.g. P3/n + P4/n or P2/n + P3/n, etc. The combination of different variants leads to more severe symptoms and an earlier age of onset in the affected horse.
  • Research is on-going to identify more genetic variants that may be responsible for other subtypes of PSSM2.


  • Changes in temperament/behavior (because of pain)
  • Atactic gait/coordination problems
  • Shifting lameness
  • Muscle wasting (most obvious in hindquarters and shoulder)
  • Local muscle wasting (small divots that could look like kick marks)
  • Frequent tying up/tension
    • Stiff hindquarters
    • Muscle tremors
    • Gait changes (cross firing or disunited canter/bunny hopping/rope walking)

Possible genotypes

Genotype: The horse is: Effects:
n/n normal. The horse does not have any P-variants and therefore cannot pass them on to any offspring.
P/n affected (heterozygous). The horse has one copy of the P-variant and will pass it on to approximately 50% of its offspring. These 50% are at risk of developing PSSM2.
P/P affected (homozygous). The horse has two copies of the P-variant and will pass it on to 100% of its offspring. All offspring will be at risk of developing PSSM2.


  • Althought PSSM2 diseases cannot be cured, most affected horses benefit from a diet with high fat and protein content and/or supplementation of the amino acids lysine, threonine, and methionine.  Consult with your veterinarian or equine nutritionist about the appropriate feeding recommendations for your horse.
  • Affected horses (P/n or P/P) should only be bred after careful consideration and with the advise of a genetic expert and veterinarian. Please contact us for consultation and support.
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