Type 2 Polysaccharide Storage Myopathy (PSSM2) is a type of muscle disease and glycogen storage disorder characterized by the abnormal accumulation of the normal form of sugar stored in muscle (glycogen), as well as an abnormal form of sugar (polysaccharide) in muscle tissue. Muscle glycogen concentrations in affected horses are up to 4 times greater than in normal horses. The unique feature of PSSM is that the muscle cells in PSSM horses remove sugar from the blood stream and transport it to their muscles at a faster rate and make more glycogen than normal horses. As opposed to PSSM1, which is known to be caused by a mutation in the GYS1 gene, the cause of PSSM2 is not known.

Signs of PSSM2 in Horses

Clinical signs of PSSM2 in horses differ somewhat depending on the breed of horse affected. The primary clinical sign of this disease is muscle cramping or tying-up. Clinical signs in Arabians and Quarter Horses with PSSM2 is typing up episodes which consist of muscle pain, stiffness and reluctance to move. When Quarter Horses are less than one year of age, the most common presentation of horses affected with PSSM2 is the inability to rise or a stiff hind limb gait. When Warmblood horses are experiencing typing up episodes the most commonly presented signs include:

• Poor performance
• An undiagnosed gait abnormality, sore muscles and drop in energy level and unwillingness to perform after 5 -10 min of exercise
• Painful, firm back and hindquarter muscles
• Reluctance to collect and engage the hindquarters
• Poor rounding over fences
• Slow onset of muscle atrophy especially when out of work

Most Warmbloods with PSSM2 develop initial onset of clinical signs when they are between 8 to 11 years of age.

Methods of Managing Horses with PSSM2

PSSM2 can be managed through diet modification and specific exercise routines.

• Diet changes - The diet is altered to provide a moderate starch and sugar content, a slightly higher protein content with high quality amino acids and, if needed for energy, fat supplementation. This means reducing or replacing sweet feed, corn, wheat, oats, barley, and molasses with a ration balancer that contains vitamins, minerals and at least 20% protein.
• Regular Daily Exercise - This is extremely important for managing horses with PSSM. Consistent exercise enhances glycogen utilization, increases turnover of structural proteins in the muscle and builds enzymes needed to burn energy as fuel. Once conditioned, some PSSM horses thrive with 4 days of exercise as long as they receive daily turn out. A prolonged warm-up with adequate stretching is recommended. he collected work should be performed in intervals lasting no more than 5 min with a period of stretching provided between intervals. The time of active collection can be gradually increased as the horse works more underneath himself and in balance.