Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Polysaccharide Storage Myopathy, PSSM

Synonyms: Equine Polysaccharide Storage Myopathy, EPSM, PSSM 1, PSSM 2


PSSM is a muscle disease that results from a disorder of sugar storage in the muscle cell. There are 2 types of the disorder determined to date:

PSSM Type 1 - Horses with the mutation of the glycogen synthase 1 (GYS1) enxyme. A genetic basis for PSSM Type 1 has been discovered.

PSSM Type 2- A suspected genetic disorder that affects QH and QH crosses, Thoroughbreds, Morgans, Warmbloods, Arabians and others. Horses affected with PSSM2 have similar clinical signs but without the GYS1 mutation.

Dr. Stephanie Valberg at the University of Minnesota has spent a large part of her career researching equine muscle disorders. She has excellent horse owner resources at the University of Minnesota website. See the Outside Resources tab linked to this record for more information.

my vet's role


The prognosis is surprisingly good if these horses are diagnosed and managed with the proper diet and exercise.

Author: Doug Thal DVM Dipl. ABVP