Developmental onset of polysaccharide storage myopathy in 4 Quarter Horse foals.

Abstract

Polysaccharide storage myopathy (PSSM) is characterized by the accumulation of glycogen and an abnormal polysaccharide in the skeletal muscle fibers of Quarter Horse–related breeds. Glycogen storage disorders have been identified in human beings and other animal species that are due to single gene mutations causing deficiencies in the enzyme activities of the glycogenolytic or glycolytic pathways.1 Biochemical studies have demonstrated that horses with PSSM have functional glycogenolytic and glycolytic pathways.2 More recently, muscle glycogen and abnormal polysaccharide accumulation in PSSM horses have been suggested to be due to a novel defect in glucose transport in skeletal muscle. Results of IV and oral glucose tolerance tests in adult PSSM horses indicated that affected individuals have enhanced glucose clearance from the bloodstream, which occurs at lower insulin concentrations than in controls.3 Clinical signs of PSSM include muscle stiffness, pain, and reluctance to exercise and are usually 1st noticed in adult horses at the commencement of training or when exercise resumes after a lay-up period.4 Serum creatine kinase (CK) activity may be high at rest in PSSM horses and can increase up to 80,000 IU/L after a 15-minute submaximal exercise test.5 Although pedigree analysis suggests that PSSM is probably transmitted as an autosomal recessive trait, there is little information about the expression of the disease from birth to the beginning of training at 2–3 years of age.6 Recently, 2 foals with pneumonia developed severe rhabdomyolysis and were diagnosed with PSSM, suggesting that, under certain circumstances, this disorder may be expressed in young animals.7 The purpose of this longitudinal study was to follow the offspring from PSSM mares bred to a closely related stallion to determine the age of onset of skeletal muscle glycogen and abnormal polysaccharide accumulation, enhanced glucose clearance, and exertional rhabdomyolysis that are indicative of PSSM. The