APolysaccharide storage myopathy is a muscle disease affecting horses of several breeds. Horses with PSSM store excess abnormal glycogen (sugar) in their muscles. When these horses are asked to exercise they often have painful muscle cramping, sweating, and reluctance to move, which is often termed "tying-up." Other less common signs in Quarter Horses with PSSM include muscle wasting (atrophy), muscle tremors (fasciculations), muscle soreness, and other unexplained gait abnormalities or lameness.

The clinical signs you describe could be due to PSSM, although they are not typical. The signs could also be due to an orthopedic or neurologic problem. I suggest you have your mare evaluated to help determine if she has an orthopedic, neurologic, or muscle problem. A veterinarian should perform neurologic and lameness exams, including nerve blocks and radiographs (X rays) if necessary. If the these exams are normal, then evaluate for muscle disease.

This includes several diagnostic steps. First, your veterinarian should evaluate your mare’s musculature, looking for any signs of atrophy or asymmetrical muscle mass, and identifying any sore areas with palpation. Second, your mare should have the concentration of two muscle enzymes measured in her blood, creatine kinase (CK) and aspartate aminotransferase (AST). These enzymes are indicators of muscle damage that can be routinely evaluated. Ideally, CK and AST levels would be measured, then your mare would be exercised for 15 minutes at a trot, and four hours after she completed the exercise, the CK value would be repeated. A rise in the CK value after exercise indicates muscle damage, even if we cannot see overt signs of tying-up when asked to trot. It is also important to evaluate your mare’s blood levels of vitamin E and selenium. Horses deficient in either of these nutrients can have chronic muscle problems