Despite having recently identified a genetic defect that results in polysaccharide storage myopathy (PSSM) in many horses (see articles 11654 and 12725), the underlying cause of PSSM remains to be determined in others. According to a multi-institutional study on PSSM in Belgian horses, the overproduction of glycogen (rather than a decrease in glycogen utilization or excessive glucose uptake by muscle cells) is thought to be the underlying cause of PSSM in this breed.

"Studies evaluating how skeletal muscles in draft horses utilize glucose and breakdown glycogen (the storage form of glucose), are limited," explained co-author Anna Firshman, BVSc, PhD, Dipl. ACVIM, assistant professor in internal medicine at Oregon State University’s College of Veterinary Medicine. "Therefore, the purpose of our study was to evaluate insulin sensitivity, the types of muscle fibers, and the activity of the enzymes involved in glycogen synthesis and breakdown in skeletal muscle samples from Belgians with or without PSSM."

Ten Belgian horses were included in this study. Five of these had PSSM, while the other five did not. The diagnosis of PSSM was based on muscle biopsy examination. No significant difference in insulin sensitivity was noted between these two groups, suggesting that increased glucose uptake by muscle cells is not responsible for abnormal accumulation of polysaccharide