Myocardial hamartoma of the right atrium in a dog

A myocardial hamartoma of the right atrium is described in an 8-year-old dog that died from pneumonia. At necropsy, a firm, mottled, dark-brown right atrial appendage, of normal shape but slightly enlarged, was found incidentally. On section, the right atrial appendage was composed of a grey-tan, solid mass. Histological features of the mass were as follows: the component cells were mature cardiac muscle cells; the mass contained all of the components of the normal heart wall (i.e., epicardium, myocardium and endocardium), but the arrangement of the component tissues was disorganized; growth of the mass was non-invasive, and continuity of the component cells with adjacent normal myocardial cells was evident, suggesting a congenital origin. This appears to be the first report of congenital myocardial hamartoma in any animal other than man.