Chest ImagingSubpulmonary pleural effusion
a collection of
pleural fluid beneath the lung. Although
pleural fluid usually collects initially at this site when the patient is erect, it is unusual for fluid to remain localized under the lung when more than 200 -300 ml is present. Occasionally, however, large volumes of
pleural fluid accumulate in the subpulmonary area without fluid collecting in the rest of the
pleural space. The cause is unknown; theories such as distorsion of
pleural pressures due to abnormality of the lower lobe have not been substantiated. The fluid is not loculated by
pleural adhesions and is freely mobile when the patient's posture is changed. Subpulmonary effusions may be bilateral but are more commonly unilateral and right-sided and due to transudates rather than exudates. They are most frequently seen in
cardiac failure.
On the frontal chest radiograph there is apparent elevation of the hemidiaphragm as the superior margin of the fluid collection is well defined, mimicking the diaphragmatic contour. The correct diagnosis is suggested by the contour of the collection which peaks more laterally than a raised hemidiaphragm; on expiration, this appearance is more pronounced. The adjacent costophrenic angle may be clear but it is often slightly blunted. On the left side, the distance between the gas-filled stomach bubble and lung is usually greater than 2 cm. Bilateral subpulmonary effusions are particularly easily missed. On the lateral radiograph the superior margin of the fluid collection tends to be relatively horizontal, particularly posteriorly, mimicking a flattened hemidiaphragm. Occasionally fluid can be seen extending into the oblique fissure.
A suspected subpulmonary effusion may be confirmed by means of a chest radiograph in the supine or decubitus position or by chest ultrasonography.
CF - HM