Lipomatous Hypertrophy of Atrial Septum : Pathology
Macroscopic Features: The thickened atrial septum extends on both sides of fossa ovalis, sparing the valve of fossa ovalis, producing a dumbbell or hourglass shaped mass bulging into the right atrium. The cephalad portion is thicker than caudal portion. The septal thickness ranges from 2 to 8 cm. The cut surface looks different than normal epicardial fat, being firmer in consistency and brown in color (due to brown fat). Microscopically, lipomatous hypertrophy of atrial septum is not encapsulated and consists of an admixture of mature fat and brown fat (vacuolated adipocytes with central nuclei) with entrapped large myocytes (left half of the image). Due to the presence of brown fat, it is considered to be a form of hamartoma. Brown fat clusters should not be mistaken for lipoblasts. Primary liposarcomas of the heart are exceedingly rare.