Polysaccharide storage myopathy (PSSM) was first identified in 1992 as one of the causes of exertional rhabdomyolysis (ER) in horses, which is muscle damage from exercise, commonly known as tying up.
Tying up was first described in heavy draft horses and officers’ riding horses as early as 1840 by a French army veterinarian. The term “Monday Morning Disease” was coined due to the frequency of symptoms appearing in working draft and carriage horses on the Monday following a weekend of rest, while consuming a full ration of oats or sweet feed.
An acute episode of tying up, which often begins within 20 minutes of light exercise, is characterized by reluctance to move, tucked-up abdomen, quivering in the flank, significant sweating and muscle stiffness. Hindquarters are most often affected but the back, abdomen and forelimbs may also be involved. Horses will have markedly elevated serum creatine kinase (CK) of more than 35,000 U/L and may have myoglobin in the urine, giving it a reddish-brown color. In 1932, a study1
by Carlström reported that in some horses with Monday Morning Disease, rations high in grains resulted in an accumulation of muscle glycogen.
What is glycogen?
Glycogen is a form of sugar called a polysaccharide, which is stored in the muscle to burn as fuel for work. Muscle biopsy studies from Dr. Stephanie Valberg in 1992 revealed that horses affected with PSSM store 1.5 to 4 times the muscle glycogen as normal horses. In addition, an abnormal polysaccharide that doesn’t break down properly during exercise was found in the muscle fibers of some PSSM horses. This led to the conclusion that there were possibly multiple forms of PSSM.
Currently, horses with both elevated glycogen and the abnormal polysaccharide are identified as type 1 PSSM
, whereas horses with elevated glycogen stores but no abnormal polysaccharide are identified as type 2 PSSM
Genetic mutation in horses
In 2008, researchers at the University of Minnesota, led by Dr. Valberg and supported by the American Quarter Horse Association (AQHA), found a genetic mutation was responsible for the abnormal polysaccharide in type 1 PSSM horses. A genetic test is commercially available through the University of Minnesota for diagnosis of type 1 PSSM and requires whole blood or hair root samples to be submitted. Since this is less invasive than the muscle biopsy, many horse owners opt for the genetic testing.
PSSM horses can remain asymptomatic for years (average age of first appearance of symptoms is 6 years) until there is some change in their schedule, such as being laid off due to injury or not getting regular exercise due to weather. Owners of PSSM horses report that they generally lack energy when ridden, show reluctance to move, and may stop and stretch as if to urinate without actually urinating. They often show a sour attitude about being brushed, saddled and exercised. Many of these symptoms are not specific to PSSM, and horses exhibiting any of these symptoms should be examined by a veterinarian to determine the cause. An initial blood workup can be helpful, since PSSM horses often have elevated CK even when rested with no recent tying up episode.
It is important to note that feeding a normal horse oats or sweet feed will not cause PSSM. This is a genetic condition a horse is born with. A PSSM horse will always have the condition and may experience episodes of tying up, but they can be better managed through diet manipulation and regular exercise. Both diet and exercise are equally important in the management of this condition.
Horse diet modifications
Significantly reducing starch and sugar, and increasing the number of calories supplied by dietary fat will decrease uptake of glucose into the muscle and reduce glycogen stores. Many PSSM horses tend to be easy keepers, making high-fat diets hard to feed without resulting in obesity. For horses with high calorie requirements that need higher fat, Purina® Ultium® Competition
horse feed has been helpful in managing many PSSM horses. For those with lower calorie requirements that cannot eat 3 to 4 pounds or more of Ultium®
Competition feed without gaining excessive weight, Purina® Strategy® Healthy Edge® horse feed
has proven to be a great option.
These low-starch feeds should be fed with good-quality grass hay or a maximum of 50 percent alfalfa hay. Regular turnout for as much time as possible is critical to successful management of PSSM horses. They do not do well confined to stalls or missing days of exercise. Regular daily exercise for horses with PSSM has been shown to produce a dramatic decrease in serum CK following exercise. Adjusting to a lower-starch, higher-fat diet will further support these efforts. For some horses, improvement may be evident within a few days of the new diet and exercise program. Others will take longer to metabolically adjust and show a change for the better.