Polysaccharide Storage Myopathy

By: Dr. Lydia Gray

Most horse owners are familiar with the term "tying up," which describes the pain and cramping of a horse’s muscles with exercise. Veterinarians use the term "chronic exertional rhabdomyolysis" to describe repeated episodes of the condition, and now differentiate between two types, according to the fifth edition of Current Therapy in Equine Medicine.

Recurrent Exertional Rhabdomyolysis typically occurs in the thoroughbred racehorse that "ties up" when stressed and is most likely due to a calcium regulation problem. Polysaccharide Storage Myopathy (PSSM) is an inherited condition that occurs in western-type breeds such as quarter horses, paints and appaloosas. It is due to a dysfunction in the way muscles store glycogen (glucose or sugar).

The classic PSSM horse has a calm demeanor, good body condition, and may have been off work for a few days before showing signs of "tying up" after only a few minutes of exercise. These signs include:

• Stiffness, reluctance to move
• Posturing to urinate, stretching-out
• Muscle twitches
• Pawing, rolling

Horses that "tie up" may also have firm and painful muscles, increased heart and respiratory rates, and sweat excessively. Exercise should be stopped and a veterinarian called immediately if any of these signs occur. Diagnosis is confirmed with bloodwork and muscle biopsy.

Changing the diet and scheduling regular exercise and turnout are the main ways to control the condition. The best diet for a horse with PSSM contains less non-structural carbohydrates and more fat. Non-structural carbohydrates (NSC) are the easily digestible sugar and starch portion of the feed and forage. Structural carbohydrates are the fiber portion. Since grains are highest in NSC, they should be removed from the diet and replaced with a multi-vitamin/mineral supplement or ration balancer. It is especially important to meet or exceed the vitamin E/Se and electrolyte requirements of any horse that "ties up."

Because forage makes up most of a horse’s diet, significant amounts of NSC can be eaten if hay and pasture are overlooked as source of sugars and starches. Typically, grass pasture is higher in NSC than grass hay, and grass hay is higher in NSC than alfalfa hay. However, the NSC content of forage cannot be guessed just by looking at it, so it is best to have the forage analyzed for NSC content and feed “low carb” hay (under 10% NSC) or "moderate carb" hay (under 20% NSC).

Reducing simple sugars and starches in the diet is just one step. Adding fat to the diet as a replacement energy source is also important. Up to one pound of fat per 1000 lbs of body weight per day should be gradually added to the diet, as long as this concentrated source of calories doesn’t cause too much weight gain or GI disturbance. There are a number of powdered fat supplements available that are accepted by horses, are nearly 100% fat, and contain no sugars or starches.

Finally, turnout and exercise must be regularly scheduled for PSSM horses. Ideally, these horses should be kept outside 24 hours per day. Because pasture grass may at times contain high NSC levels though, this turnout may need to be on a dry lot. If stalling is necessary, it should be limited to no more than 12 hours per day. Controlled exercise should also be provided on a daily basis, whether lunging, riding, driving or other activity. Horses that have suffered a bout of "tying up" should return to light exercise upon the advice of a veterinarian.

About Dr. Lydia Gray