Introduction

A pleural effusion is an abnormal condition in which fluid has accumulated in the pleural space. The plain film signs of pleural effusion are commonly seen and readily identified on plain film chest images.

Pleural fluid can accumulate in isolation between the lung base and diaphragm. This is known as a subpulmonic effusion and presents a more difficult diagnostic challenge. When a chest X-ray is reported by a radiologist as "normal", there is potential for embarrassment when that slightly elevated-looking diaphragm turns out to be a large subpulmonic effusion. The radiographer can help prevent such embarrassment with the selective application of the decubitus chest projection on patients with suspected subpulmonic effusion.

The Pattern Recognition

Sign	Comment
Abnormally large distance between fundus of stomach and lung base	- erect chest only - must have air-filled fundus of stomach - only identifies pathology on left
"abrupt termination of vascular shadows at the level of the diaphragm" (1)	- may be somewhat exposure dependent (you may see nothing under the diaphragms on a slightly underexposed radiograph for example)
Blunting of affected costophrenic angle (PA)	- usually only seen with larger effusions
A blunted posterior costophrenic sulcus may be seen on the lateral film
Pseudodiaphragm can appear to "peak" more laterally
Pseudodiaphragm can appear more horizontal medially than you would typically see with a normal diaphragm	the medial pseudodiaphragm almost always looks abnormal
Crowding of lung parenchyma on affected side

Subpulmonic effusion

Normal Chest

Note the flat looking "hemidiaphragm", particularly medially. When you compare this appearance with the normal chest, the differences may appear subtle. I would argue that the fact that the patient has a raised right hemidiaphragm, the flattened look of the diaphragm and the concave looking horizontal fissure would be enough to suggest a subpulmonic effusion http://www.med.yale.edu/intmed/cardio/imaging/cases/pleural_effusion_subpulmonic/index.html

Compare this normal right hemidiaphragm shape to that of the patient with the right sided subpulmonic effusion.

A subpulmonic effusion can be mimicked by "subdiaphragmatic abscess, enlarged liver, paralysis and eventration of the diaphragm, and ascites" (2)


Case 1

This is a PA chest image. The radiographer has positioned the patient with the shoulders rolled well forward affording a good view of the lungfields. The linear opacity above the right hemidiaphragm is subsegmental atelectasis (lower white arrow). There are multiple pleural plaques associated with previous asbestos exposure (black arrow). Conveniently for this case study, there is a diaphragmatic plaque on the left (white arrow). The grey arrows identify the normal space between the fundus of the stomach and the diaphragm.

This is a PA chest image on the same patient taken after cardiac surgery. Note the increased distance between the fundus of the stomach (black arrow) and what appears to be the diaphragm (white arrow). The diaphragmatic plaque is marked by the grey arrow. Question: If the diaphragmatic plaque identifies the position of the left hemidiaphragm.... what is the structure marked by the white arrow? Answer: The white arrow identifies the visceral pleural surface of the left lung base (so called pseudodiaphragm). The appearance is complicated somewhat by the apparent air fluid level (not marked), suggesting that there is air as well as fluid in the pleural space. Where it is clinically indicated, the presence of a subpulmonic effusion can be proven with the aid of a lateral decubitus projection.

Case 2

This is a PA chest image. The patient is post cardiac surgery. Note the increased distance between the air-filled fundus of the stomach and the left "hemidiaphragm" (arrowed). The radiographer noted this finding and suspected a subpulmonic effusion. He/she performed a left lateral decubitus view to see if the pleural fluid would track along the left chest wall within the pleural space.

The left lateral decubitus chest image demonstrates fluid in the pleural space (arrow). Note that this is termed a left lateral decubitus view even though it is marked "R decubitus".

Discussion

Not every patient who has a subpulmonic effusion warrants a lateral decubitus chest view. It would not be unusual, for example, for a patient to have a subpulmonic effusion post thoracic surgery. Subjecting a patient to the rigors of adopting the lateral decubitus position unnecessarily is undesirable for a variety of reasons.

Where a subpulmonic effusion is suspected on the PA chest image, and is likely to be a significant finding, a lateral decubitus chest view may assist the radiologist, referring doctor and ultimately the patient.

References

1. MI Schwarz and BL Marmorstein A new radiologic sign of subpulmonic effusion. http://www.chestjournal.org/cgi/reprint/67/2/176.pdf
http://chestjournal.org/cgi/content/abstract/67/2/176
2. Rothstein E, Landis FB: Infrapulmonary pleural effusion simulating elevation of the hemidiaphragm. Am J Med 8:4852, 1950

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