Polysaccharide storage myopathy (PSSM) is a muscle disorder that primarily affects horses with Quarter Horse bloodlines, such as Quarter Horses, Paints, and Appaloosas. It is characterized by the abnormal accumulation of glycogen (sugar) in the muscles, which can lead to muscle cramping, stiffness, and weakness. PSSM is a form of tying-up, a condition that causes painful muscle cramping in horses.
There are two types of PSSM:
- Type 1: This type is caused by a known genetic mutation in the GYS1 gene. A horse only needs one copy of the GYS1 gene mutation to be affected by Type 1 PSSM. The GYS1 gene is responsible for glycogen production, and a mutation in this gene causes the muscle cells to continually make glycogen, which the horse is unable to use for energy during exercise. Type 1 PSSM is more commonly observed in Quarter Horses, related breeds such as Paints and Appaloosas, and draft breeds.
- Type 2: The exact cause of Type 2 PSSM is unknown, although it may also have a genetic component. There are currently no scientifically verified DNA tests for PSSM2, but a muscle biopsy can be performed for diagnosis.
There is no cure for PSSM, but it can often be managed successfully through diet and exercise. Feeding hay with less than 10 percent nonstructural carbohydrates (NSC) and providing regular exercise are good care practices for horses with PSSM. Approximately 50% of affected horses with PSSM1 show improvement under dietary management alone, and of those that adhere to dietary and exercise management, 90% have few to no episodes of tying-up. However, clinical signs may resume if there are disruptions to the horses management routine.
