Exertional myopathy in horses is a syndrome of muscle fatigue, pain, or cramping associated with exercise. Less common exertional myopathies that cause exercise intolerance without muscle necrosis include mitochondrial myopathies and forms of polysaccharide storage myopathy in Warmblood horses. Most commonly, exertional myopathies produce necrosis of striated skeletal muscle and are termed exertional rhabdomyolysis. Although exertional rhabdomyolysis was previously considered a single disease described as azoturia, tying-up, or cording up, it is now known to comprise several different myopathies, which, despite similarities in clinical presentation, differ significantly in etiopathology. Clinical signs usually are seen shortly after onset of exercise.
Equine Neuromuscular Diagnostic Laboratory. The American Quarter Horse Association AQHA has funded research into this disease since and has provided us with the opportunity to learn much about the diagnosis, cause and treatment for this disease. Prior to the development of a genetic test, PSSM was diagnosed by muscle biopsy. The variety of acronyms used are in part related to preferences of different laboratories, as well as to differences in the criteria used to diagnose polysaccharide storage myopathy. Type 2 PSSM represents one or more other forms of a muscle disease that are characterized by abnormal staining for muscle glycogen in microscopic examination of muscle biopsies. The original research performed by Dr. Valberg on PSSM centered around Quarter Horses with clinical signs of tying-up and abnormal amylase-resistant polysaccharide in their muscle biopsies.
Glycogen is a required energy source in the rapidly growing fetus and neonate and is synthesized by glycogen synthase, which creates straight chains of glucose with alpha 1,4-glycosidic linkages, and by glycogen branching enzyme, which creates a branched structure through alpha 1,6-linkages. It is likely that the GBE1 mutation and lack of GBE enzymatic activity results in cardiac and skeletal muscle, liver, and brain being unable to store and mobilize glycogen to maintain normal glucose homeostasis. Carrier frequency estimates of 7. However, clinically, the number of GBED foals born alive appears less than expected.
It is most commonly associated with heavy horse breeds and the American Quarter Horse. While incurable, PSSM can be managed with appropriate diet and exercise. Glycogen is a molecular polymer of glucose a polysaccharide used to store energy, and is important for maintaining glucose homeostasis in the blood, as well as for providing energy for skeletal muscle and cardiac muscle contraction. This bond may be broken by amylase when the body wishes to break down glycogen into glucose for energy.