Polysaccharide storage myopathy (PSSM) is a disorder that causes muscle cramping in horses from abnormal glycogen (sugar) storage in the muscles. Sore muscles, muscle weakness, and tying-up (equine exertional rhabdomyolysis or ER) are other clinical signs of PSSM.

A mutation in the GYS1 gene causes PSSM type 1. However, the cause of PSSM type 2 has not yet been identified. Current research suggests that one form of PSSM-2 is a glycogen storage disease that causes muscle damage characterized by high serum CK activity (PSSM2-ER).

Recent PSSM Research

Stephanie Valberg, DVM, PhD, DACVIM, ACVSMR, did research1 on this disease. She is a professor emeritus at Michigan State University’s McPhail Equine Performance Center. Multiple colleagues joined her in this research. They looked at exertional rhabdomyolysis in Quarter Horse-related breeds (Quarter Horses, Paints, Appaloosas) with type 2 polysaccharide storage myopathy (PSSM2-QH).  

PSSM2-QH is a glycogen storage disorder causing exertional rhabdomyolysis. It has a less severe histopathology than PSSM1-QH. Quarter Horse-related breeds with PSSM2-QH have a 1.6-fold higher muscle glycogen concentration than normal control horses. However, the concentration is lower than is seen in horses affected with PSSM type 1. 

There are differences in the abnormal polysaccharide in PSSM2-QH compared to that seen in PSSM1-QH. Therefore, PSSM2-QH “appears to be a novel gycogenosis that at present can only be diagnosed by muscle histopathology,” noted the researchers

Those researchers had some good news. They found that low nonstructural carbohydrate, fat-supplemented diets and regular exercise appear to be effective in managing PSSM2-QH. This was based on the experiences of 13 owners who followed diet and exercise recommendations for affected horses.

Reference

  1. Type 2 polysaccharide storage myopathy in Quarter Horses is a novel glycogen storage disease causing exertional rhabdomyolysis. Open Access. Stephanie J. Valberg, Zoë J. Williams, Carrie J. Finno, Abigail Schultz, Deborah Velez-Irizarry, Marisa L. Henry, Keri Gardner, Jessica L. Petersen. Equine Veterinary Journal. 2022.

Further Reading

Dry Lot for PSSM Horse. MySeniorHorse.com

  • Kimberly S. Brown
    Kimberly S. Brown is an award-winning writer and publisher. She is the Editorial Director for My Senior Horse. Brown spent 10 years at Equine Network, parent company of My Senior Horse. Prior to that she worked for three years in equine nutrition after she retired from nearly 30 years working at The Blood-Horse. Brown spent the last 15 years of her time at that organization creating and developing The Horse and TheHorse.com.
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