This patient was ten years post orthotopic cardiac transplant, doing well but with a large cardiomediastinal shadow. The heart sounds were muffled and the ECG showed low voltages so we suspected a pericardial effusion. However, the echo was not clear-cut, with what looked like a large but partly echogenic pericardial collection. Before venturing into aspiration , we did a CT.
There was gross epicardial lipomatosis, plus some paracardiac fat, doubtless due to steroid therapy. It was the cause of the trachea being deviated to the right (hope you noticed it!).
Fat like this can be surprisingly echolucent. We had a few patients on long term steroids with bladder compression symptoms from similarly excessive pelvic fat.
Though steroid induced lipomatosis is not uncommon, this case is unusual because the CT shows lipomatosis of part of the interatrial septum. In the normal this type of lipomatosis mostly affects the posterior half of the atrial septum: here, the fat is actually in the posterior septum of the donor heart, with the posterior half of the recipient atrial septum posterior to it. Because parts of both donor and recipient atria are used for the transplant, the composite atria are larger than normal.