Pleural thickening

On the chest radiograph the changes predominantly affect the dependent areas with blunting of the costophrenic angle. Extensive pleural thickening produces volume loss and a veil-like reduction in transradiancy of the hemithorax. There is a soft tissue density medial to and paralleling the chest wall with a well-defined inner margin. There may be extension into the fissures which appear thickened. In contradistinction to pleural fluid the appearance remains constant on a decubitus radiograph.

Chest ultrasonography can be used to detect pleural thickening and distinguish it from pleural fluid but is only reliable if the pleura is more than 1 cm thick. The appearances are usually of a homogeneous echogenic layer between the chest wall and underlying lung.

CT is much more sensitive in detecting pleural thickening which is seen as a layer of soft tissue density between the chest wall and lungs (Fig.1). Thickening of as little as 1 - 2 mm can be detected with HRCT. Pleural thickening is best assessed immediately internal to the ribs where there is normally no detectable soft tissue. Between the ribs the normal pleura and innermost intercostal muscle produce a thin line which should not be mistaken for pleural thickening. In the paravertebral region, any thickening of the pleural line is abnormal. In many diseases causing pleural thickening the layer of extrapleural fat between parietal pleura and chest wall is increased, allowing a subtle degree of thickening to be detected, especially with HRCT. The distribution and shape of pleural thickening as determined by CT are useful for differentiating between benign and malignant causes of pleural thickening. The most useful signs in predicting the presence of malignancy are circumferential thickening, nodularity, thickening of greater than 1 cm and involvement of the mediastinal pleura.

Localized pleural thickening often occurs at the lung apices with increasing age, forming an apical cap. This may be uni- or bilateral and is usually of homogeneous, soft tissue density, usually less than 5 mm thick, with a well-defined inferior margin. It should be distinguished from a superior sulcus neoplasm.

CF - HM