E.P.S.M. / P.S.S.M. (Tying Up)

Equine Polysaccharide Storage Myopathy is seen mainly in draft horses, Quarter Horses, Warmbloods, Appaloosas, Andalusians, Arabians, and Morgan horses but not Thoroughbreds. The condition causes an abnormally high level of the storage sugar Glycogen in muscles. Clinical signs include muscular pain and stiffness, especially in hind limb and rump muscles during or after exercise. Dark coloured urine containing the muscle protein myoglobin released from damaged muscle cells may be apparent.

E.P.S.M has been classified into Types 1 and 2 according to the presence (Type 1) or absence (Type 2) of a mutation in the glycogen synthetase 1 gene GYS1. The cause of Type 2 is unknown. In Quarter Horses the condition may be associated with Insulin Resistance and abnormal insulin response to starch intake, including storing more glucose as muscle glycogen than normal horses.

Presentation of an attack of P.S.S.M (Tying Up) can vary from very mild with no obvious symptoms other than elevated muscle enzymes found on routine blood tests, to extreme, with horses in severe cramping muscle pain and unable to move or even recumbent. Generally, it exhibits as discomfort and stiffness with reluctance to go forward, sweating, and tense painful muscles over the rump and hindquarters.

Diagnosis may be confirmed by genetic testing of hair roots for the GYS1 gene or by muscle biopsy.

Immediate treatment may involve mild sedation, NSAIDS for pain relief and short- term box rest. Dietary management of carbohydrate intake, particularly starch & sugars (NSC) is essential. This means low starch non grain diets with adequate good quality protein, low NSC forages like soaked Lucerne Hay, alternative energy sources such as fibre from sugar beet and soy hulls and vegetable oil. Avoid pasture where the fructan (sugar) content is likely to be high such as in the afternoon on sunny days or following frost. Avoid molassed or sweetfeeds. Use of a balancer pellet or supplement to ensure adequate mineral and vitamin intake may be appropriate.

Management. Daily exercise is important to reduce muscle glycogen loading. Rest days should be avoided if possible as this promotes glycogen accumulation. The disease is not curable but with care and attention to diet and exercise it can be managed effectively in most horses. The early training period or shortly after a training break or rest period are the riskiest times.

Historical Note. The old- fashioned name for the condition was ‘’Monday Morning Leg’’ which was highly descriptive and referred to the condition in draft horses and Hackneys. These were usually worked six days per week and rested on Sundays on full rations, accumulating glycogen in their muscles. On Monday morning they were put back to work and often ‘’Tied Up’’ within a few minutes and “Set Fast’’ in the shafts of the cart, giving yet another name to the malady.

Note. See also R.E.R. Tying Up

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