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→ Here you’ll find the Equine Myopathy/PSSM2 information for downloading and printing:

→ Exercise intolerance due to muscle wasting, stiffness, and pain.


  • 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)

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)” and “Recurrent Exertional Rhabdomyolysis (RER)” are subtypes of PSSM2.
  • Several variants (P2, P3, P4, Px) in the genes MYOT, FLNC, MYOZ3 and CACNA2D3 have been shown to cause these subtypes.
  • A horse can have more than one P-variant, e.g. n/P3 + n/P4 or n/P2 + n/P3, 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.

Inheritance: incomplete autosomal dominant/semidominant
→ Horses with one or two copies of the variant (n/P or P/P) are affected. Incomplete or semidominant means that animals with one copy (n/P) may show milder symptoms and a later age of onset than animals with two copies (P/P).

Possible genotypes: The following genotypes and effects apply for the variants P2, P3, P4 und Px.

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.
n/P 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.

More information about the disease, specific mutations, and inheritance can be found on our PSSM2 Information Page.


  • 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 (n/P 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.

Test information: This Panel test identifies the changes in the MYOT, FLNC, MYOZ3 and CACNA2D3 genes.

The Equine Myopathy/PSSM2 panel is offered under license from EquiSeq Inc. and is based upon their unpublished research. 

Further information:
McCue ME et al. (2008). “Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis.” Genomics. 91(5):458-66. PMID: 18358695.

McCue ME et al. (2008). “Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy.” Journal of Veterinary Internal Medicine. 22(0):1228–1233. PMID: 18691366.

McCue ME et al. (2009). “Polysaccharide storage myopathy phenotype in quarter horse-related breeds is modified by the presence of an RYR1 mutation.” Neuromuscular Disorders. 19(0):37–43. PMID: 19056269.

McCue ME et al. (2009). “Comparative skeletal muscle histopathologic and ultrastructural features in two forms of polysaccharide storage myopathy in horses.” Vet Pathol. 46(6):1281-1291. PMID: 19605906.

Maile CA et al. (2017). “A highly prevalent equine glycogen storage disease is explained by constitutive activation of a mutant glycogen synthase.” Biochim Biophys Acta.. 1861(1):3388-3398. PMID: 27592162.

Valberg SJ et al. (2016). “Suspected myofibrillar myopathy in Arabian horses with a history of exertional rhabdomyolysis.” Equine Vet J.. 48(5):548-556. PMID: 26234161.

Lewis SS et al. (2017). “Clinical characteristics and muscle glycogen concentrations in warmblood horses with polysaccharide storage myopathy” Am J Vet Res. 78(11):1305-1312. PMID: 29076373.

Further information is available at EquiSeq .

Breeds affected:

Most breeds

Test #: H108

Description: Myopathy Panel (P2, P3, P4, Px)

Samples: Hair, blood

Cost: 238 € (200 € + 19% VAT)

Time: 10 - 14 business days


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