PSSM

PSSM

Polysaccharide Storage Myopathy (PSSM) is an inherited, debilitating condition related to tying up. It is caused by a sugar imbalance in which even light exercise can lead to cramping.

Normally after exercise, a horse will slowly absorb glucose from the bowel to replenish metabolized glycogen (sugar in the muscle) until normal resting levels are achieved. Horses with PSSM experience an increased sensitivity to insulin, resulting in overcompensated storage of glycogen in their muscles, up to twice the normal level.

As glycogen builds up, pain is brought on as a result of disrupted muscle fiber function. Frequently, this pain is found in the back, both along the epaxial muscles (muscles on both sides of the back) and in the longissimus dorsi, found in the sacrum.

Signs of PSSM will generally occur early in the training session. In addition to pain, there may be stiffness and cramping, poor performance, muscle weakness and atrophy, or even behavioral changes. To alleviate cramping, male horses may stretch as if to urinate.

The typical PSSM horse will be calm and well muscled but usually not fit. PSSM is found more frequently in Quarter Horses, warmbloods, and drafts, with Quarter Horses appearing to have earlier onset of PSSM in comparison to other breeds.

Conformation is made through a muscle biopsy. The biopsy will grade as mild, moderate, or severe. There is currently research being conducted on testing which will be able to utilize genetic markers to diagnose PSSM, but for the time being muscle biopsy is the only channel of diagnostics. Muscle enzymes may be evaluated but do not always indicate the presence of PSSM.

Horses diagnosed with PSSM will be placed on a strict diet consisting of low sugar, low starch, and high fat. Fat, such as rice bran, helps muscle cells take up less glucose. It is best to feed these horses small meals several times a day to alleviate surges of blood glucose.

In addition, the horse should be turned out as much as possible. Confinement may lead to elevated levels of creatine kinase, especially when the horse is exercised, placing the horse at risk for a full-scale episode of ER. Exercise should be gradually increased to allow the horse to adapt to the demands and the horse should be exercised often, even if only for 10 minutes per day.

Any performance horse with back pain should be evaluated for PSSM. There is a favorable improvement rate (around 80%) seen in horses with mild to moderate PSSM and many of them return to full competition. Owners need to realize there may always be some risk for muscle soreness but with a vigilant management program this disease is quite controllable. As with ER, is may take weeks or even months to discover the success of a treatment plan.