Polysaccharide Storage Myopathy (PSSM)

PSSM is a disease that causes an abnormal accumulation of glycogen, the form of sugar stored in the muscle. This excess sugar causes mild to severe muscle cramps, sore muscles and/or muscle weakness. Horses that are managed properly can generally go on to have successful performance careers.

Two types of PSSM have been classified: PSSM1 and PSSM2. The genetic test used by AQHA identifies PSSM1 mutation. Type 2 PSSM refers to PSSM symptoms that occur in horses without the known PSSM1 variant. At this time, aside from PSSM1, there is not a genetic test for other forms of PSSM. PSSM2 can, however, be diagnosed with a muscle biopsy. PSSM2 is most commonly found in warmbloods and Arabians.

PSSM is a dominant mutation, which means horses with just one copy will experience effects, though likely less severe than those that have two copies.

PSSM Type 1 and 2 can present the same symptoms but are caused by different issues. The cause of PSSM2 remains unknown, though multiple causes are suspected.

Approximately 11% of quarter horses are affected by PSSM1.