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Polysaccharide storage myopathy (PSSM) is characterized by recurrent episodes of tying-up or muscle stiffness, colic-like signs, and accumulation of abnormal complex polysaccharide in muscle fibers.

Type 1 PSSM is an autosomal dominant trait that affects the glycogen synthase enzyme1(GSY-1) gene. This results in overly active glycogen synthase activity, which produces less branched, abnormal glycogen in muscle cells that is resistant to amylase digestion (which breaks down normal glycogen) and an exaggerated response to high blood glucose and insulin concentrations. Persistently increased glycogen synthase activity somehow disturbs muscle energy metabolism.

Type 1 PSSM can be diagnosed using the genetic test developed by the Neuromuscular Diagnostic Laboratory at the University of Minnesota. Horses with PSSM type 2 do not have the GSY-1 mutation but have accumulations of abnormal complex polysaccharide that may be sensitive to amylase. Polysaccharide storage myopathy type 2 is diagnosed by muscle biopsy.

Horses with PSSM must be managed nutritionally, but a regular and frequent exercise program is equally important. Affected horses should be fed at least 1% of their body weight in low-NSC hay per day. Less than 10% of digestible energy (DE) should come from NSC and 15 to 20% of DE should be supplied by fat. All grain and molasses-containing concentrates should be removed from the diet. Ideally, affected horses are turned out all the time. It is very important that horses in work remain in  frequent regular exercise and any changes are made gradually.

A small number of horses with PSSM may also be affected with malignant hyperthermia (MH), which is usually associated with high body temperature and muscle rigidity under general anesthesia. However, MH has also been found with PSSM and affected horses develop high body temperature with signs of tying-up that may be fatal. Horses that are affected with both PSSM and MH are less likely to respond to dietary and exercise changes. Genetic testing for MH also can be performed by the Neuromuscular Diagnostic Laboratory at the University of Minnesota.

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