Polysaccharide Storage Myopathy (PSSM): Search for Underlying Causes Continues


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.

In addition, no difference in enzyme activity (i.e., enzymes that breakdown or synthesize glycogen from glucose) or the proportion of type 1, type 2a, or type 2b muscle fibers were noted in Belgians with and without PSSM.

Compared to the 10 Quarter Horses also included in this study for comparison (five with PSSM and five without PSSM), the Belgians had more type 2a muscle fibers and less type 2b fibers. This difference in the structure of the muscle was evident regardless of PSSM status.

"In Belgians, PSSM appears to be caused by excessive glycogen synthesis as opposed to a decrease in glycogen utilization by skeletal muscles or due to an increase in glucose uptake by the muscles," said Firshman.

According to Firshman, "This is the first study that has looked at these findings in Belgians and will help veterinarians in understanding the underlying mechanisms of PSSM in Belgians."

This study also serves to highlight some differences and similarities of PSSM compared to other breeds of horses that commonly get PSSM, such as Quarter Horses.

The study, "Insulin sensitivity in Belgian horses with polysaccharide storage myopathy," was published in the June 2008 edition of the American Journal of Veterinary Research.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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