PSSM – Polysaccharide Storage Myopathy

What causes PSSM in horses?

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, characterizes Polysaccharide storage myopathy (PSSM). Tying-up associates with polysaccharide accumulation in muscle effects in thousands of horses. The two forms of PSSM are both the result of the accumulation of muscle glycogen which is the storage form of glucose in muscles.

Types of PSSM

Type 1 PSSM is a result of a mutation in the GYS1 gene. The mutation causing PSSM is a point mutation on the GYS1 gene which codes for the skeletal muscle form of the glycogen synthase enzyme. The cause of Type 2 PSSM is not yet identified. Both types have an abnormal type of glycogen staining in muscle biopsies, and Type 1 is findable through genetic testing. Horses with Type 2 PSSM lack the mutation that is specific for Type 1 PSSM. At present, there is not a specific genetic test for Type 2 PSSM and we do not have conclusive evidence that it is inherited.

Carbohydrates that are high in starch, such as sweet feed, corn, wheat, oats, barley, and molasses, appear to exacerbate Type 1 and Type 2 PSSM. That is why horses suffering from PSSM should avoid them and intake of extra calories can come from fats. An important part of the management of PSSM horses is daily exercise. This enhances glucose utilization and improves energy metabolism in skeletal muscle. If only the diet changes, it has been found that approximately 50% of horses improve. When altering both diet and exercise, 90% of horses have had no or few episodes of tying-up.

What is safe to feed my horse?

Because of the oxidative stress associated with this disorder, as well as the free radicals generated from the high-fat diet often required by horses with PSSM, experts recommend supplementing with vitamin E for its antioxidant properties. Other antioxidants include vitamin C, Grape Seed Extract and Super Oxide Dismutase. Anecdotal evidence also suggests that the anti-inflammatory/antioxidant Dimethylglycine (DMG) may be beneficial for horses with muscle disorders like PSSM.

Beth Valentine, DVM, proposed the first successful therapy to halt and reverse the progression of PSSM, except in advanced cases. This is a diet change. Due to the fact that PSSM is a genetic condition, there can be no ‘cure’ in the symptomatic horse, but many affected horses can be restored to full health and work status.

The approach considers that the horse gains no nutrition from grains, other than the calories. Since the starches and sugars in grains cause specific digestive problems, the approach is to replace starch and sugar calories with fat-based calories. The result is the ‘added fats’ diet.

In basic outline one divides the daily ration into two feedings:

• 1.2% to 2.1% body weight in good quality forage (hay)
• 2 cups vegetable oil (corn, soy, canola) per 1,000 lb body weight (1-liter vegetable oil per 1,000 kg bodyweight)
• 1-2 IU (International Units) vitamin E per pound (2 to 5 IU/kg)
• Selenium supplement
• Plenty of fresh water
• Exercise to the horse’s comfort level
• Usual salt and mineral supplements
• Improvement of EPSM symptoms may take 4–6 months.

The Importance of Fat Calories

Adding the fat calories (vegetable oil) to the diet is as important as removing the starches and sugars from grain sources. Oils that provide balanced omega 3 to omega 6 fatty acids, like ExcelEQ ProElite can be a healthy alternative energy source that can aid in the stabilization of blood sugar. Depending on the severity of the condition, the horse may have comfort restrictions in exercising or working, but some exercise needs to happen to begin rebuilding damaged muscles. As the recovery proceeds, exercise may increase to best occupy the horse’s attention and maintain recovery.

Watch Out For Molasses

You can find molasses in many feeds and grains to control dust and to sweeten (sugar) the feed to entice the appetite. The sugars and starches in molasses, however, are a problem for EPSM-symptomatic horses. The horse’s appetite will often decline when molasses is withdrawn. This will pass as the horse relearns the taste of feed without molasses.

Do You Know Your Feeds Selenium Level?

Some regions are naturally selenium-sufficient, and one needs to take caution with additional supplementation. A local veterinarian can advise about local selenium levels.

Transport of Important Nutrients

Part of the consumption of the hay ration can be beet pulp shreds, alfalfa pellets, or a combination, to provide a carrier to pour the oil over for feeding.

Know Your Numbers

The specified level of vegetable oil, 2 c./1,000 lb bodyweight, targets a dietary level of 24% calories from fat sources. Several commercial feed companies now provide feeds with reduced sugars and starches and added fat sources. These feeds can reduce the amount of additional vegetable oil needed to meet the dietary goal ((2 c./1,000 lb)/day), or in some cases will meet the goal without additional fat calories. At least one company manufactures a dry fat supplement that can substitute liquid vegetable oil.