P.S.S.M. / E.P.S.M. (PSSM / EPSM) Equine Polysaccharide Storage Myopathy, Exertional Rhabdomyolysis, Ty Up / Tying Up

P.S.S.M is caused by a genetic mutation which results in the abnormal accumulation of the sugar storage compound glycogen in skeletal muscle as well as a build up of an abnormal sugar called amylase resistant polysaccharide. Amylase is an enzyme which breaks down large sugar molecules into smaller ones.

There are two types of PSSM, Type 1 is caused by a mutation defect in the glycogen synthase gene GYS1 which causes continual glycogen production in the muscle cells. Though about 20 breeds have been shown to be affected the condition is most common in Quarter horses and related Appaloosa’s & Paints and some draft horses, but rare in Thoroughbreds and Arabians. In P.S.S.M Type 2 there is also an accumulation of glycogen in muscle but the mutation in GYS1 is absent and the cause is as yet unknown. Cases of PSSM2 have been reported in Quarter horses, Arabians, and Warmbloods.

Symptoms are those of muscle pain and dysfunction including sweating, reluctance to move, muscle cramping, tremors, stiff gait, tense muscles, especially in the hindquarters (tying up) or in mild cases poor performance or unwillingness to go forward when ridden. The urine of an affected horse may be stained brown, even dark like Guinness beer in severe cases, by myoglobin lost from damaged muscle cells. Usually, attacks are exercise related but can occur without exercise on occasion. Diagnosis is from the clinical signs plus clinical pathology tests for elevated muscle enzymes in the blood and in some cases muscle biopsy or genetic testing for the GYS1 gene defect. In some horses, P.S.S.M may occur in conjunction with Insulin Resistance which also drives glycogen deposition in muscle cells and requires similar management strategies.

Treatment of the acute attack normally entails box rest overnight and sometimes pain medication. Management relies on reducing NSC intake to ideally ~ 12% of the dietary energy supply by means of avoiding sugars and starchy grains and maximizing energy supply from fibre and fat. Soaked Lucerne hay, soy hulls, sugar beet pulp, rice bran etc. combined with vegetable oil and a ration balancer for minerals and trace elements. Regular exercise with ration reduction on rest days is appropriate. With good control of exercise and diet to minimize environmental triggering, further attacks can be avoided or minimized, but there is no cure for the condition because of its genetic origin.

See also: Recurrent Exertional Rhabdomyolysis R.E.R. Tying Up

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