Horses with both forms of PSSM have signs typically associated with tying-up. These signs are most commonly muscle stiffness, sweating, and reluctance to move. The signs are most often seen in horses when they are put into initial training or after a lay-up period when they receive little active turn-out. Episodes usually begin after very light exercise such as 10-20 minutes of walking and trotting. Horses with PSSM can exhibit symptoms without exercise.
During an episode, horses seem lazy, have a shifting lameness, tense up their abdomen, and develop tremors in their flank area. When horses stop moving they may stretch out as if to urinate. They are painful, stiff, sweat profusely, and have firm hard muscles, particularly over their hindquarters. Some horses will try pawing and rolling immediately after exercise. Most horses with PSSM have a history of numerous episodes of muscle stiffness at the commencement of training; however, mildly affected horses may have only one or two episodes/year.
Rarely, episodes of muscle pain and stiffness can be quite severe, resulting in a horse being unable to stand and being uncomfortable even when lying down. The urine in such horses is often coffee colored, due to muscle proteins being released into the bloodstream and passed into the urine. This is a serious situation, as it can damage the horse’s kidneys if they become dehydrated.
Very young foals with PSSM occasionally show signs of severe muscle pain and weakness. This occurs more often if they have a concurrent infection such as pneumonia or diarrhea. Some weanlings and yearlings, particularly those with type 2 PSSM can develop muscle stiffness with daily activities and difficulty rising.
What Causes PSSM in Horses?
Polysaccharide storage myopathy (PSSM) is 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. Thousands of horses have been identified with tying-up associated with polysaccharide accumulation in muscles. There are two forms Type 1 and Type 2 PSSM. We know that both are the result of the accumulation of muscle glycogen which is the storage form of glucose in muscles.
Type 1 PSSM is caused by 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 has yet to be identified. Both types have an abnormal type of glycogen staining in muscle biopsies, and the types can be distinguished by 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 they should be avoided and extra calories can be provided in the form of fat. 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 is changed, we found that approximately 50% of horses improve. If both diet and exercise are altered, then 90% of horses have had no or few episodes of tying-up.
An old theory about tying-up is that it is due to too much lactic acid in the muscle. Many exercise studies have proven that this is absolutely not the case with PSSM. PSSM is actually a glycogen storage disease and there are several diseases in other species and in human beings that also result in the storage of too much glycogen in skeletal muscle. In these other diseases, glycogen accumulates because the muscle lacks an enzyme (protein) necessary to burn glycogen as an energy source. These similarities led us to test PSSM horses for the disorders in glycogen metabolism identified in human beings. We found that PSSM is a unique glycogen storage disease because the PSSM horses have all the necessary enzymes to burn glycogen as a fuel in their muscles. With exercise, PSSM horses show the expected decrease in muscle glycogen as it is burned as fuel.
The unique feature of PSSM is that the muscle cells in PSSM horses remove sugar from the blood stream and transported into their muscle at a faster rate, and make more glycogen than normal horses. Our recent research shows that the reason for this is that PSSM muscles are very sensitive to insulin beginning as early as 6 months of age. Insulin is a hormone released by the pancreas into the bloodstream in response to a carbohydrate meal. It stimulates the muscle to take up sugar from the bloodstream. Once inside the cell the muscle’s of PSSM horses make much more glycogen than a normal horse due to an overactive enzyme called glycogen synthase in the case of type 1 PSSM.
What Should I Do If I Have A Horse That Is Reluctant To Move?
- Stop exercising the horse and move it to a box stall. Do not force the horse to walk.
- Call your veterinarian.
- Blanket the horse if weather is cool. Hose the horse to remove sweat if the weather is warm.
- Determine if the horse is dehydrated, due to excessive sweating. Pinched skin will normally spring back and saliva should be wet, not tacky.
- Provide fluids: small frequent sips of water. Electrolytes (potassium, sodium, and chlorine) may be added to drinking water, if palatable to the horse. Plain water should always be available as an alternative. If the horse is dehydrated, intravenous fluids may be needed. Once cool, the horse may have free access to water.
- Relieve anxiety and pain. Your veterinarian may administer drugs such as acepromazine and flunixin meglumine (Banamine).
- Remove grain. Feed only hay until symptoms subside.
- Provide small paddock turnout once the horse walks freely, usually in 12-24 hours.
How Do I Know If My Horse Is Having A Tying Up Episode?
A blood sample can be used to determine what is happening inside your horse’s muscles. Proteins measured in a blood sample can determine the extent of the muscle damage.
Two muscle proteins often used as a measure of muscle damage include creatine kinase (CK) and aspartate transaminase (AST). When muscle cells are damaged, they release these proteins into the bloodstream within hours. A blood sample taken to measure these proteins can determine how much muscle damage has occurred. This is not a specific test for PSSM as the presence of CK and AST in the blood occurs with any muscle damage. With many forms of tying-up, blood CK activity returns to normal within days if horses are rested. It is very common for horses with PSSM to have high CK activities even if they are rested for weeks after an episode of tying-up.
What If The Episode Occurred Weeks Ago And I Want To Know If My Horse Is Susceptible To Tying-up?
One way to see if tying-up occurs is to do an exercise test. This is useful if a horse shows no signs of muscle stiffness when a veterinarian performs a physical exam but still suspects the horse suffers from a form of tying-up. The horse should be worked in a round pen at a walk and trot for 15 minutes. If the horse shows any signs of persistent muscle stiffness, the test should stop. The idea here is not to produce signs of stiffness, but to see if this mild form of exercise cases slight muscle damage. In cooperation with your veterinarian, have a blood sample drawn 4-6 hours after the exercise test. A significant result indicating sub clinical muscle damage would be a serum CK activity greater than 800 U/L 4 hours after 15 minutes of exercise at a trot. The blood sample is not informative if taken right after exercise, as the CK protein has not had enough time to leak out of the muscle cells. This test is not specific for PSSM, but does indicate muscle damage is occurring with exercise. For more details visit with your veterinarian about testing.
How Is PSSM Diagnosed?
PSSM can be diagnosed with genetic testing if it is the inheritable type or muscle biopsies can be taken.
For More Information:
For more information on testing, rehab of a PSSM episode and PSSM diets, visit the University of Minnesota’s website.