Incidence of polysaccharide storage myopathy in draft horse-related breeds: a necropsy study of 37 horses and a mule

Skeletal muscle samples from 38 draft horse-related animals 1-23 years of a ge were evaluated for evidence of aggregates of glycogen and complex polysa ccharide characteristic of equine polysaccharide storage myopathy (EPSSM). Cardiac muscle from 12 of these horses was also examined. Antemortem serum levels of creatine kinase (CK) and aspartate aminotransferase (AST) from 9 horses with EPSSM and 5 horses without EPSSM were compared. Skeletal muscle from 17 horses contained inclusions of periodic acid-Schiff (PAS)-positive , amylase-resistant complex polysaccharide. Similar inclusions were also pr esent in the cardiac muscle of 1 horse. A vacuolar myopathy with aggregates of PAS-positive, amylase-sensitive glycogen was seen in 8 other horses, an d these findings are also considered diagnostic for EPSSM. Antemortem serum activities of CK and AST were often higher in EPSSM horses than in horses without EPSSM. Using the presence of amylase-resistant complex polysacchari de as the criterion for diagnosis of EPSSM, the incidence in this populatio n was 45%. Inclusion of horses with aggregates of glycogen but no amylase-r esistant complex polysaccharide as representative of the range of pathologi c findings in horses with EPSSM resulted in a 66% incidence in this populat ion.