Mystery Quiz- The Answer

July 9, 2010

Vivian Hayashi MD and Robert Smith MD, Mystery Quiz Section Editors

The answer to the mystery quiz is pleural effusion that is loculated in both the horizontal and right oblique fissures. Pleural effusion is seen as blunting of the right costophrenic angle and tracking of fluid laterally (Image 3, arrowhead). The horizontal fissure thickens due to fluid which becomes an ovoid density more medially (Image 3, arrow). This ovoid density, representing loculated fluid in the horizontal fissure, is often referred to as a pseudotumor. It appears as a homogenous shadow suggesting fluid, rather than parenchymal disease which, when consolidated and airless, will appear denser. The second overlying shadow (Image 3, open arrows) represents loculated fluid in the oblique fissure. The opacity appears denser at its inferior margin where there is greater volume of fluid and fades as it tracks superiorly where there is less fluid. These fissural fluid collections and their relationship to each other can be further visualized in the transverse (Images 5-10) and coronal (Images 11-16) CT images.

In order to confirm that the opacification is fluid density, the region of interest can be quantified in Hounsfield units (HU) and compared to that of a soft tissue such as the liver (Image 17). The Hounsfield scale is a measure of the attenuation of radiation by different densities , wherein water is scaled at 0 HU, air at -1000 HU, fat at -120HU, bone at +400HU, and muscle at +40. Within an average transverse CT slice of 3mm thickness, a mixture of densities will yield a HU measure that will differ from these scaled values. Image 17 shows the density in the fissure as -16.59 HU while the liver is + 42.14HU indicating the presence of fissural fluid.

The most common reason for loculation of pleural fluid is inflammation of the pleural space with resulting fibrous stranding. In these cases, the pleura may appear thickened, diffusely or focally, and sometimes the area of thickening may be quite subtle. Alternatively, when the pleura appears normal and there is no history to suggest a past inflammatory process, loculation may be due to differing pressure gradients in the pleural space. In the latter instance, positional changes, such as the decubitus position, may shift the fluid and cause the typical appearance of layering pleural fluid.

Our patient had pleural effusions due to congestive heart failure related to severe mitral regurgitation and non-ischemic cardiomyopathy. His valvular heart disease had worsened considerably in the year prior to admission and was associated with a moderate reduction in left ventricular function. Twelve days after admission, the patient underwent a mitral valve repair and radiofrequency ablation of the atrial fibrillation.

This case illustrates how a common condition, pleural effusion, can present in an uncommon fashion, as a mass lesion. Knowing the fissural anatomy and the distinctive texture of pleural fluid can help identify the lesion.