Two types of PSSM have been identified: PSSM type 1 and PSSM type 2. The term "PSSM type 2"  goes back to the time where the PSSM research team found that a number of horses that exhibited PSSM symptoms did not have the mutation of the GYS1 gene that causes true Polysaccharide Storage Myopathy (PSSM). These horses were classified as having "PSSM type 2" and research into the genetic cause(s) continued. Since then, several different researchers have found several different genetic mutations that can cause symptoms of PSSM. EquiSeq, an American company, has identified a number of genes that they have termed P2, P3, P4, P8, Px and K1. More information on these variants can be found under PSSM type 2 - EquiSeq panel. Dr Valberg and her team, responsible for the discovery of the mutation of the GYS1 gene that causes PSSM type 1, have identified a mutation on the MYH1 gene that can cause Immune Mediated Myositis. More information can be found under PSSM type 2 - IMM. 


PSSM type 1 is found in all breeds that have been influended by cold blooded or draft horses, such as Quarter Horses, Paint Horses, Appaloosa's , Tinkers, Gypsy Horses, Haflingers, Morgans and a few warmblood breeds. Variants of PSSM type 2 are believed to have originated in thoroughbreds (some going back to Arabian horses) and as a consequence are found in almost all modern horse breeds. The symptoms of PSSM type 1 and type 2 are similar. IMM only occurs in Quarter horses and breeds derived from Quarter Horses. 

PSSM type 1, which is sometimes also referred to as PSSM1, is a sugar storage problem in muscles that is seen in horse breeds such as Quarter Horses, Paint Horses, Haflingers, Tinkers, Morgans, draft horses and other breeds from cold blooded ancestry.

Horses that have PSSM type 1 have a mutation of the GYS1 gene, which is responsible for the storage of the glycogen that is present in skeletal muscles. In horses that have PSSM type 1, the enzyme that regulates this storage is hyperactive, causing abnormal glycogen accumulation in muscles and a disrupted energy release, causing horses to tie up when they exercise. The exact dynamics of this process are still being researched.

PSSM type 2, also referred to as PSSM2 is a muscle disease that occurs in in a wide variety of horse breeds.