Polysaccharide Storage Myopathy (PSSM)

horses looking out of stalls in barn

What is polysaccharide storage myopathy?

Polysaccharide storage myopathy results in an abnormal accumulation of glycogen, the form of sugar stored in muscle. This can cause episodes of muscle stiffness and pain after exercise, also known as “tying up” or exertional rhabdomyolysis. Affected horses may be reluctant to move and may display sweating, lameness, and muscle tremors.

There are two types of PSSM. A genetic mutation in the glycogen synthase 1 (GYS1) gene causes PSSM Type 1 (PSSM1). The mutation causes muscle cells to produce glycogen continually. Since it is an autosomal dominant trait, only one copy of the mutation is needed for a horse to be affected. However, environmental factors, namely diet and exercise, play important roles in the onset of clinical signs. PSSM1 is more commonly observed in Quarter Horses, related breeds such as Paints and Appaloosas, and draft breeds, although cases have been reported in more than 20 breeds.

Polysaccharide storage myopathy type 2 (PSSM2) also results in abnormal glycogen storage in muscle, but horses do not have the GYS1 mutation. The cause of PSSM2 remains unknown; there may actually be multiple causes. PSSM2 is commonly seen in Quarter horses and warmbloods; however, Arabians, Thoroughbreds and Standardbreds can also be affected.

What are the clinical signs of polysaccharide storage myopathy?

Clinical signs of PSSM range from mild to severe. They include sweating, lameness, and muscle tremors (“tying up”) and may occur with or without exercise. Some affected horses, however, do not exhibit any clinical signs.

How is polysaccharide storage myopathy diagnosed?

PSSM1 is diagnosed through the genetic test for the GYS1 mutation. This can be performed on hair or blood samples. There is currently no scientifically validated diagnostic test for PSSM2. A muscle biopsy may be taken to evaluate muscle damage and measure the amount of glycogen in the muscle. There are other causes for tying-up other than PSSM, so it is important to rule these out to ensure the horse is treated appropriately.

How is polysaccharide storage myopathy treated?

Dietary nonstructural carbohydrate (NSC) intake in affected horses should be limited to less than 12% of the diet to keep blood insulin levels low and reduce glycogen storage in muscle. Horses must exercise daily to maximize the muscles’ ability to burn glycogen. Affected horses are often easy keepers and management through a low-NSC grass hay and a good-quality ration balancer is usually sufficient. If additional calories are needed, a low-NSC and/or high fat feed source should be incorporated.

Since small management changes can have a big impact, fine-tuning the diet over time may be necessary. Ensuring an adequate source of quality protein in the diet appears to be important.

What is the prognosis for polysaccharide storage myopathy?

There is no cure for PSSM, but it can be managed successfully. Approximately 50% of affected horses with Type 1 PSSM show improvement under dietary management alone. Of those that adhere to dietary and exercise management, 90% have few to no episodes of tying-up. However, clinical signs will likely resume if there are disruptions to the management program.

How can polysaccharide storage myopathy be prevented?

Genetic testing can be utilized to avoid breeding PSSM1-affected horses. Since PSSM1 is an autosomal dominant trait, there is at least a 50% chance of passing the causative mutation to each offspring. Although the cause of PSSM2 is currently unknown, horses that are diagnosed as affected by muscle biopsy should not be bred since it may have a genetic basis.

For more information:

UC Davis Veterinary Genetics Laboratory PSSM1 testing

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to cehadmin@ucdavis.edu