PSSM is the abbreviation of Poly-Saccharide Storage Myopathy, a muscle disorder (myopathy) involving a problem in the storage of sugar (polysaccharides). Other abbreviations that refer to the same disease are EPSM and EPSSM in which E refers to Equine. Originally thought to be a problem unique to Quarter Horses and related breeds, PSSM has turned out to have been entered into these breeds by introducing heavy (draft) horse blood into the gene pool. The gene that causes PSSM type 1 is hundreds, if not thousands of years old and can be found in draft, cold blooded or mixed breeds. It should therefore come as no surprise that PSSM type 1 is frequently found in not only the typical draft horse breeds, but also in Tinkers/Gypsy Horses, Haflingers and warmblood horses that at some point had draft horse blood added to their gene pool.

The AQHA (American Quarter Horse Association) has been funding research into this disease since 1995. A lot of the research has been performed by Stephanie Valberg DVM PhD .

In healthy horses, consumption of carbohydrate rich foods (food rich in sugar, e.g. grass or grains) stimulates the pancreas to release a hormone called insulin. Insulin stimulates the storage of sugar (in the form of glycogen) into the muscle from the bloodstream. Glycogen is necessary for adequate functioning of the muscle; without glycogen the muscle doesn’t have the fuel to perform.

The muscle cells in horses afflicted with PSSM store glycogen from the bloodstream at a much larger rate than normal horses and in much larger quantities. This results in the storage of much larger amounts of glycogen, up to double or triple the amounts that are found in healthy horses. 


During the research into PSSM type 1, it was found that not all horses with PSSM symptoms carried the gene that causes PSSM type 1. They clustered these horses together into a category they called "PSSM type 2". Research into what is causing PSSM type 2 is ongoing, but progress has been made identifying genes that cause Myofibrillar Myopathy (MFM), Limb Girdle Muscular Distrophy (LGMD), Immune Mediated Myositis (IMM) and  Exertional Rhabdomyolysis (RER), all of which can cause  PSSM-like symptoms.