The abnormality is that of proliferation of fat in the interatrial space. This fat proliferation is epicardial within the pericardial space and does not arise from the septum itself. This causes separation of the walls of the left and right atrium except where they are inseparable, namely the fossa ovalis. This is a characteristic appearance. The lesion is not a genuine lipoma or neoplasm, simply fat proliferation that can confuse the unwary. The preservation of the fossa ovalis often results in a dumbbell like shape with an additional component medial to the fossa. This entity is an incidental finding sometimes found in obese patients or patients on steroid therapy or with mediastinal lipomatosis, although commonly none of these are present. It is best considered a benign incidental finding of no consequence. Although occasional cases have been described as causing mild dysrrythmias these links may be coincidental. Rare occurrences of superior vena cava compression have been described, I would imagine these were huge as for the most part these can be very large and yet asymptomatic. The most common association is that there may be proliferation of fat in the remainder of the epicardial space too - again inconsequential. On one occasion I have seen pericardial neoplastic infiltration spread through the space of the fat. This is an entity to recognise but to offer reassurance in dismissing it.
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