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Polysaccharide Storage Myopathy (PSSM)
Equine Polysaccharide Storage Myopathy (EPSM Or EPSSM)
Polysaccharide Storage Myopathy(PSSM) is a type of muscular disorder in horses. It is associated with two different forms---designated as Type 1, which is caused by a glycogen synthase 1 (GYS1) gene mutation, and Type 2, are not caused by the GYS1 mutation and whose origin is yet unknown.
PSSM1 vs PSSM2 Comparison
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.
If the horse is being ridden: Stop exercising the horse. Move horse (if willing) into a stall. Evaluate how dehydrated the horse is by pinching the skin or feeling the saliva.
What Not to Do: Do not feed the horse any grain.
What to do: Make sure there is fresh, clean water available for the horse to drink. Provide electrolytes in a separate bucket of water and offer to the horse. If its cold, blanket the horse to keep warm. If its warm weather, remove any sweat and provide access to a fan to keep cool.