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PSSM2 (Muscle Integrity Myopathy) horses

Muscle Integrity Myopathy (PSSM2) is form of Exertional Myopathy (link in those words), in which the structure and/or function of the muscle is disrupted.  It is caused by hereditary predisposition and various environmental factors (age, feeding, husbandry). Typical symptoms can include unexplained lameness, muscle stiffness, difficulties with gait changes/coordination, reluctance to move, muscle atrophy and/or difficulty building muscle. Almost any breed of horse can be affected.
Six genetic variants that can disrupt muscle structure and/or function have been identified in horses. These predispose a horse to developing symptoms of exertional myopathy.


Muscle Integrity Myopathy (PSSM2) Test: available in Shop


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PSSM2 Symptoms

  • Pain-related changes in temperament/behavior
  • Shifting lameness
  • a horse with pssm 2 often has Atactic gait/coordination problems (cross firing or disunited canter/bunny hopping/rope walking)
  • Frequent muscle tremors/tension
  • Stiff hindquarters and topline
  • Difficulties with collection
  • Sudden, explosive behaviour
  • High muscle tension, often worsened by cold
  • Muscle wasting (most obvious in hindquarters and shoulder)
  • Local muscle atrophy (small divots that could look like kick marks or "washboard" appearance)
  • Most horses with PSSM2 do not have changes in muscle enzyme values


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Breeds affected of Muscle Integrity Myopathy (PSSM2)

Which breeds can be affected from Muscle Integrity Myopathy (PSSM2)?


Most breeds can be affected with the symptoms of MIM (PSSM 2).

The P2, P3, P4, Px, P8, and K1 variants have been found in nearly all breeds of horses; in Icelandic horses only the P8, K1 and (rarely) P3 variants have been found.


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Which Genes are involved with Muscle Integrity Myopathy (PSSM2) /Test Information

Six semi-dominant gene variants (P2, P3, P4, P8, Px and K1) are currently being investigated as risk factors for the occurrence of Muscle Integrity Myopathy (PSSM2)  symptoms.


This Panel test identifies the changes in the MYOT (P2), FLNC (P3), MYOZ3 (P4), PYROXD1 (P8), COL6A3 (K1) and CACNA2D3 (Px) genes.


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


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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.


Weitere Informationen sind auf der Webseite EquiSeq verfügbar.

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