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. PSSM has traditionally been found in Quarter horses, Paint horses and American Appaloosas, however it is now also found in draft horses, draft horse crossbreds, Haflingers and warm-blood horses.

The AQHA (American Quarter Horse Associations) has been funding research into this disease since 1995. A lot of the research has been performed by Stephanie Valberg DVM PhD who is associated with the University of Minnesota, USA.

In healthy horses, consumption of carbohydrate rich foods (food rich in sugar, e.g. grass or grains) stimulates the pancreas to release the hormone insulin. Insulin stimulates 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 also store glycogen from the bloodstream at a much larger rate than in normal horses and in much larger quantities. This results in thestorage of much larger amounts of glycogen, up to double or triple the amounts that are found in healthy horses. The muscles of a PSSM afflicted horse are highly sensitive to insulin from the age of approximately 6 months.