Polysaccharide Storage Myopathy (PSSM)

Polysaccharide Storage Myopathy (PSSM)

Polysaccharide Storage Myopathy (PSSM) is a dominant autosomal hereditary condition that causes a genetic form of tying-up with muscle damage and inability to move. PSSM Type1 is causes by a single base pair substitution in GYS1 gene, thereby changing the amino acid sequence of the glycogen synthase enzyme. At least 20 breeds have been identified with Type 1 PSSM. The prevalence of the GYS1 mutation in Belgians is as much as 50% of and 8% of the Quarter Horse-related breeds. Horses make and store abnormal muscle glycogen and cannot tolerate dietary starches and sugars. Horses with PSSM can be maintained with low-starch and low-sugar rations and precise exercise protocols. Symptoms usually begin by 2 to 3 years of age. Some horses are subclinical. Clinical signs include skin twitching, stiffness, firm painful muscles, sweating, weakness, and reluctance to move with light exercise. Ocationally gait abnormalities, mild colic, and muscle wasting may occur An additional genetic mutation in RYR1 gene (MH) influences PSSM and can increase the severity of the symptoms of PSSM in Quarter Horses and related breeds. The combination of PSSM and MH genetic tests are available at Animal Genetics. Not all cases of tying up are caused by the PSSM mutation. Horse that test N/N for Type 1 PSSM but exibit signs of tying-up or muscle pain, may be suffering from another muscle disorder.

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