However, with the exception of a small sub-population of horses, these ideas have been unable to explain why, with all management factors identical, one horse will be affected while the rest will not. The one exception to this is a small subpopulation of horses that have been found to be affected by a condition called Polysaccharide Storage Myopathy. It has been suggested, through in vitro studies and treadmill exercises, that these horses have abnormal glycolytic metabolism resulting in an insufficient amount of pyruvate for oxidation or anaerobic conversion to lactate. This condition has yet to be identified in any bloodlines other than the Quarterhorse with mares more commonly incriminated than geldings. The clinical picture is typically one of a calm and sedate horse that has had multiple episodes usually commencing with the start of training.
Signs can range from mild to severe with some deaths being reported.
Varying amounts of exercise intolerance has been seen, usually being more pronounced at higher speeds. Muscle fasciculations, dimpling of the flank, and stretching out after exercise may also be seen followed by persistent elevations in the plasma of creatine kinase. This elevation in creatine kinase is seen shortly after exercise and may remain elevated between episodes. Along with clinical signs, diagnosis is made through careful examination of a muscle biopsy. Prevention is best achieved through gradual training and the provision of a high fat diet.