Subpulmonic effusions (also known as subpulmonary effusions) are pleural effusions 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 infrapulmonary fluid. The peak of this pseudodiaphragm will lie lateral to the normal position. When located on the left, an increased distance may be seen between the pseudodiaphragm and the gastric bubble.
Subpulmonic effusions will not be present on supine films, as the fluid should lie dependently if not loculated and thus will be too shallow to be appreciated. 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: