VCU Department of Radiology

Differential Diagnosis

Elevated or paralyzed diaphragm

Background

Discussion

Pleural effusion is a common manifestation of many local and systemic diseases affecting the thorax. There are two layers of pleura, the inner visceral layer adherent to the lung surface, and the outer parietal layer adherent to the chest wall, mediastinum, and diaphragm. A potential space exits between these layers which contains a small volume of transudative fluid (approximately 5-15 mL) that allows for frictionless lung excursion. The net flow of normal pleura fluid is from the parietal to the visceral pleural surface. The rate of pleural fluid “influx” and “efflux” are governed by Starling’s equation, which basically states that the net filtration and resorption of water and solutes across the pleura (i.e., semi-permeable membrane) is determined by balances between hydrostatic and osmotic pressures. Anything that disrupts this balance results in the excess accumulation of pleural fluid, which may then declare itself radiographically (increase in hydrostatic pressure; decrease in osmotic pressure; decrease in pleural space pressure; increase in membrane permeability; etc.)

Etiology

In the case of subpulmonic pleural effusion, fluid accumulates between the lung base and the diaphragm (i.e., subpulmonic space) and often fails to spill out into the costophrenic recesses and sulci. As the fluid further accumulates in volume, the lung base is pushed up as the diaphragm is pushed down and inverted by the intervening fluid, creating the radiographic appearance of an elevated diaphragm. Subpulmonic effusions most often form in the setting of heart failure, renal failure, and ascites.

Clinical Findings

Patients may be asymptomatic or may present with dull aching chest pain, cough, and or dyspnea. Dullness to percussion and decreased or absent breath sounds may be present. Additional clinical features may be present depending on the etiology of the pleural effusion.

Chest Radiography Findings

Bilateral Subpulmonic effusions

• Diagnosis unlikely to be made; mimic hypoventilation

Frontal Radiography

• Lateral displacement of apex of apparent diaphragm (i.e., pseudodiaphragm) (Fig. A)
• Lower lobe basilar vessels below affected hemidiaphragm inconspicuous or silhouetted (Fig. A)
• +/- Small juxtaphrenic spur from fluid extending into an inferior accessory fissure
• Downward displacement of gastric air bubble from pseudodiaphragm ≥ 2.0 cm when left-sided
• Recognition requires patient being imaged upright
• Recognition requires change from antecedent radiographs

Lateral Radiography

• Small amount of fluid may be seen in the inferior aspect of the oblique fissure
• Convex upper margin of fluid flattens as it contacts the oblique fissure and descends to anterior costophrenic sulcus (“rock of Gibraltar” sign) (Fig. B)

Caveats:

• Subpulmonic effusions are invariably localized (not loculated), transudative pleural fluid collections between the lung base and diaphragm. A lateral Decubitus exam can confirm the presence of a subpulmonic effusion in those clinical cases where differentiation from an elevated diaphragm may be more problematic.
• There may be only minimal costophrenic angle blunting even in the setting of rather large subpulmonic effusions.

Selected Readings:

Parker MS, Rosado-de-Christenson ML, Abbott GF. Pleura, Chest Wall, and Diaphragm In: Teaching Atlas of Chest Imaging 2006; Thieme, New York: 705-710.