Polysaccharid storage myopathy type 1 (PSSM)

Polysaccharide storage myopathy (PSSM) is characterized by the abnormal accumulation of the normal form of sugar stored in muscle (glycogen) as well as an abnormal form of sugar (polysaccharide) in muscle tissue. Thousands of horses have been identified with tying-up associated with polysaccharide accumulation in muscles. Horses with PSSM have signs typically associated with tying-up. Most commonly these signs are muscle stiffness, sweating and reluctance to move. The signs are most often seen in horses when they are put into initial training or after a lay-up period when they receive little active turn-out. Episodes usually begin after very light exercise such as 10-20 minutes of walking and trotting. Horses with PSSM can exhibit symptoms without exercise. Most horses with PSSM have a history of numerous episodes of muscle stiffness at the commencement of training; however, mildly affected horses may have only one or two episodes/year.