Polysaccharide Storage Myopathy(PSSM) is a type of muscular disorder of horses. It is associated with two different forms---designated as PSSM Type 1, which is caused by a glycogen synthase 1 (GYS1) gene mutation, and PSSM Type 2, are not caused by the GYS1 mutation and whose origin is yet unknown.

PSSM1 vs PSSM2 Comparison

Type 1	Type 2
Inherited Muscle biopsy Positive genetic test results	Cause is unknown Muscle biopsy
Often seen in halter horses	Quarter Horses and Warmbloods Often seen in horses used for barrel racing, reining and cutting
Muscle stiffness Sweating Reluctance to move Horses seem lazy Shifting lameness Tense abdomen Tremors in flank area Stretch out as if to urinate as soon as they stop moving. Sweat profusely Firm, hard muscles, particularly over their hindquarters Paw and roll immediately following exercise History of numerous episodes of muscle stiffness Coffee colored urine	Muscle atrophy Typing up (episodes of muscle pain, stiffness, reluctance to move) inability to rise stiff hind limb gait poor performance Gait abnormality Sore muscles Drop in energy level painful firm back and hindquarter muscles reluctance to collect and engage the hindquarters poor rounding over fences slow onset of atrophy especially when out of work
Signs are most often seen in horses after 10-20 minutes of light exercise upon initial start of training or following a lay-up period when they receive little active turn-out. Horses can also show symptoms without getting exercised.	Horses usually show signs of unwillingness to perform after 5 - 10 min of exercise.

Both forms are diagnosed through a conducting a muscle biopsy, which shows clumping of muscle glycogen. However, a false positive diagnosis can occur if the muscle biopsy is crushed with forceps resulting in abnormal glycogen and a false negative diagnosis can occur if samples are not kept chilled and shipped quickly to the laboratory because glycogen is degraded while the muscle biopsy is in transport. In addition, muscle biopsies for horses with PSSM2 and recurrent exertional rhabdomyoloysis (RER) can look very similar. The mean age of onset of clinical signs of PSSM2 in Warmbloods is between 8 and 11 years of age.