What causes the Oreo cookie sign on the lateral chest radiograph?
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Answer:
The Oreo cookie sign is the appearance on the lateral chest radiograph of a vertical soft tissue stripe sandwiched between two lucent lines behind the sternum: the anterior mediastinal fat and the epicardial fat form the pale outer layers and the intervening fluid-filled pericardium forms the dark filling. It indicates a pericardial effusion and requires a stripe thicker than about 2 mm, and preferably above 4 mm, to be called with confidence, since the normal pericardium is only 1 to 2 mm thick. It is one of the few reliable radiographic clues to effusion, because the classical frontal appearance of a globular, water-bottle-shaped cardiac silhouette requires a large volume, generally more than 200 to 250 mL, and does not appear at all in rapidly accumulating effusions where tamponade can occur with as little as 150 mL. For that reason a normal-sized heart never excludes tamponade, and echocardiography is the definitive test, showing the effusion, right atrial and right ventricular diastolic collapse, respiratory variation in inflow velocities and a plethoric inferior vena cava. CT and MRI characterise the fluid, distinguish haemopericardium and identify pericardial thickening, nodularity and calcification when constriction or malignant involvement is suspected.
Why is it called so?
The dark pericardial fluid between two pale layers of fat resembles the cream filling between the biscuits of an Oreo cookie.
Pathophysiology
The pericardium consists of a visceral layer adherent to the epicardium and a fibrous parietal layer, separated normally by 15 to 50 mL of serous fluid. Fat lies immediately outside the parietal layer in the anterior mediastinum and immediately inside the visceral layer on the epicardial surface. When fluid accumulates it separates these two fat planes, and because fat is radiolucent and fluid is of soft tissue density, the lateral radiograph resolves the three layers as a sandwich. The haemodynamic consequence depends on the rate of accumulation rather than the volume, because the fibrous parietal pericardium stretches only slowly; rapid filling reaches the steep part of the pressure-volume curve quickly and causes tamponade at small volumes.
Alternative names: Pericardial fat pad sign; sandwich sign of the pericardium
Other associated named signs: Water bottle heart on the frontal radiograph and the epicardial fat pad sign
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