Subpulmonic effusions are a pleural effusion that can be seen only on an erect projection. Rather than layering laterally and blunting the costophrenic angle, the pleural fluid lies almost exclusively between the lung base and the diaphragm.
The fluid causes apparent elevation and flattening of the diaphragm. What appears to be the diaphragm actually represents the visceral pleura, and the true diaphragm is obscured by the presence of intrapulmonary fluid. The peak of the pseudo-diaphragm will lie lateral to the normal position. When located on the left, an increased distance may be seen between the pseudo-diaphragm and the gastric bubble.
Subpulmonic effusions will not be present on supine films, as the fluid should layer dependently if not loculated. If required, a decubitus projection can be performed to clarify the definite presence of a subpulmonic effusion.
The differential on plain film includes causes of an elevated hemidiaphragm:
- 1. Federle MP, Mark AS, Guillaumin ES. CT of subpulmonic pleural effusions and atelectasis: criteria for differentiation from subphrenic fluid. AJR Am J Roentgenol. 1986;146 (4): 685-9. doi:10.2214/ajr.146.4.685 - Pubmed citation
- 2. Kafura PJ, Barnhard HJ. Ascites simulating subpulmonary pleural effusion. Radiology. 1971;101 (3): 525-6. doi:10.1148/101.3.525 - Pubmed citation
- 3. Schwarz MI, Marmorstein BL. A new radiologic sign of subpulmonic effusion. Chest. 1975;67 (2): 176-8. Pubmed citation
- 4. Mueller PR. In evaluating fluid collections, how does one differentiate subpulmonic from subphrenic collections?. AJR Am J Roentgenol. 1994;163 (3): 739-40. doi:10.2214/ajr.163.3.8079881 - Pubmed citation