Endocardial fibroelastosis, a disease of unknown cause, is characterized by diffuse thickening of the left atrial, left ventricular, and/or mitral valve endocardium. It is a rare cause of myocardial failure in young dogs and cats. Affected animals are usually < 6 months old and present with clinical signs of left heart failure Congestive Heart Failure (Left Heart Failure) The three primary functions of the cardiovascular system are to maintain 1) normal blood pressure and 2) normal cardiac output, both at a 3) normal venous/capillary pressure. Heart failure is... read more . Breeds reported to be affected include Labrador Retrievers, Great Danes, Bulldogs, English Springer Spaniels, Boxers, pit bulls, and Siamese and Burmese cats (in which the disease is believed to be inherited). Echocardiography demonstrates dilation of the left ventricular and atrial chambers, decreased left ventricular fractional shortening due to an increased left ventricular end-systolic diameter, and possibly diffuse endocardial thickening. Clinical signs, treatment, and prognosis are similar to those of dilated cardiomyopathy Dilated Cardiomyopathy in Dogs and Cats Dilated cardiomyopathy (DCM) is characterized by the progressive loss of myocyte number and/or function, along with a decrease in cardiac contractility. DCM is most prevalent in dogs and is... read more .