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Muscle Integrity Myopathy (formerly known as "PSSM2") in horses

Muscle Integrity Myopathy(formerly known as "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.


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What are the possible symptoms of Muscle Integrity Myopathy?

Common symptoms include:
• Pain-related changes in temperament and behaviour
• Difficulty with gait changes, coordination
• Lack of power from the hindquarters
• Difficulty building muscle (especially hindquarters   and topline/shoulders)
• Shifting, unexplained lameness
• Muscle stiffness in the hindquarters or topline, often requiring a long warm-up time
• High muscle tension or muscle tremors/cramping, often aggravated by cold weather or stress
• Muscle loss in the hindquarters and topline, and/or focal muscle atrophy (appearing like kick marks)
• Ataxic gait/coordination problems
• Blood levels of Creatinine Kinase (CK) and Aspartate Aminotransferase (AST) are usually within normal limits, even with obvious symptoms of muscle disease


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Breeds affected of Muscle Integrity Myopathy (formerly known as 'PSSM2')

Which breeds can be affected from Muscle Integrity Myopathy (formerly known as ('PSSM2')?


Most breeds can be affected with the symptoms of MIM.

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 (formerly known as '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 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 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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