Polysaccharide storage myopathy (PSSM)
Polysaccharide storage myopathy (PSSM) is a glycogen storage disorder that affects numerous breeds. Clinical signs vary, but can range from muscle atrophy and progressive weakness commonly seen in Draft breeds, to muscle soreness and gait abnormalities in Warmbloods, and acute exertional rhabdomyolysis (commonly referred to as “tying up”) in Quarter Horses and related breeds.
A dominant (one copy of the mutation is sufficient to produce disease) mutation in a gene responsible for glucose storage in the muscle was identified in Quarter Horses. This same mutation in the GYS1 gene has been shown to cause PSSM in greater than 20 horse breeds to date, and is likely a very old genetic mutation that occurred prior to the foundation of most modern horse breeds. Therefore this mutation cannot be attributed to any given stallion or pedigree with in a breed.
There are forms of PSSM also characterized by an increased content of glycogen in skeletal muscle that are nor attributable to the GYS1 gene mutation. Therefore, PSSM is subdivided into type 1 and type 2. The form of PSSM due to the GYS1 gene mutation is termed type 1 PSSM. The horses with a glycogen storage disorder that do not have the identified gene mutation are classified as having type 2 PSSM.
Genetic testing is available at the University of Minnesota for type 1 PSSM.