Radiological manifestations and small volumes
On a standard erect posteroanterior chest radiograph, pleural fluid produces an opacity with a concave upper border. The outer (lateral) margin of this opacity lies higher than the inner margin, forming the so-called meniscus sign. It is considered that when the fluid level reaches the anterior segment of the 4th rib, the effusion volume is approximately 1000 mL.
Sometimes fluid accumulates in an isolated manner beneath the base of the lung, which is a subpulmonic effusion. On images, it mimics an elevated diaphragm dome; however, the peak of such a "dome" is shifted more laterally than normal, and the contour drops off sharply towards the costophrenic angle. On the left side, diagnosis is facilitated by a specific sign: the distance between the gastric air bubble and the lower border of the lung exceeds 2 cm.
In supine patients (supine patient), fluid spreads evenly across the posterior surface of the cavity. The classic meniscus disappears, and instead, a hazy opacity (hazy opacity) of half of the thoracic cavity appears against the background of a preserved pulmonary pattern. With a volume of 175 mL or more, fluid may extend to the apex, forming an "apical cap" (apical cap).
Massive effusion and total opacification of the hemithorax
A significant accumulation of fluid causes dense opacification of the entire hemithorax. Normally, such a volume acts as a mass effect and displaces the mediastinum in the opposite (contralateral) direction. In addition, fluid pressure can cause inversion and paradoxical movement of the diaphragmatic dome. This occurs more frequently on the left, as the right diaphragm is supported by the liver.
If a massive effusion is visualized on the radiograph, but there is no mediastinal shift, this is a serious diagnostic marker. Such a picture indicates that the fluid volume is compensated by:
- Obstructive atelectasis (collapse) of the lung on the affected side.
- Extensive malignant pleural infiltration (e.g., mesothelioma or metastatic carcinoma), which rigidly fixes the thoracic structures.
Role of ultrasound and computed tomography (CT)
Previously, small volumes of fluid were sought using lateral decubitus radiography, but today ultrasound has become the method of choice.
On ultrasound, transudate typically appears anechoic, whereas exudate and hemorrhagic effusion are more often echogenic, contain septations, and are accompanied by pleural thickening. Ultrasound excellently differentiates between fluid and solid structures: during breathing, fluid and strands within it change shape. In addition, pleural lesions form an obtuse angle (obtuse angle) with the chest wall, while intrapulmonary lesions form an acute angle (acute angle).
Computed tomography (CT) has high sensitivity, but it is important to remember: X-ray attenuation on CT does not reliably differentiate between transudate and exudate. CT is used for:
- Localization of a loculated effusion (loculated effusion), which has a lenticular shape.
- Detection of the "split-pleura sign" (split-pleura sign)—marked thickening and contrast enhancement of both pleural layers, which is typical for empyema or cancer.
- Differentiation of empyema from a lung abscess.
Specific variants: empyema and chylothorax
Pleural empyema (Empyema) is an accumulation of pus passing through three stages: exudative, fibrinopurulent, and the organizational stage (a thick pleural peel forms). On images, empyema causes compression of the adjacent lung, whereas an abscess is characterized by destruction of the lung parenchyma itself.
Chylothorax (Chylothorax) is an accumulation of milky-colored lymph (containing triglycerides) due to injury of the thoracic duct. Right-sided trauma indicates a rupture of the lower third of the duct, and left-sided trauma indicates the upper two thirds. On a plain chest radiograph, chylothorax is indistinguishable from a standard effusion.
Effusions associated with abdominal and cardiac pathologies have their own features:
- Acute pancreatitis: causes a left-sided exudative effusion (due to contact of the diaphragm with the tail of the pancreas) with an elevated amylase level.
- Left ventricular failure: the effusion is usually bilateral, but its volume is significantly larger on the right side.
- Subphrenic abscess: accompanied by basal atelectasis of the adjacent lung.
Briefly: answers to topical questions
Which diseases are the most common causes of bilateral pleural effusion?
- Left ventricular failure (Left ventricular failure) is a frequent cause; in approximately 90% of cases, the effusion is bilateral, although it occurs more frequently and reaches larger volumes on the right side.
- Nephrotic syndrome (Nephrotic syndrome)—bilateral effusion develops in about 20% of patients, as well as in other hypoproteinemic states.
Ultrasound angle: pleural lesions = obtuse angle with the chest wall; intrapulmonary = acute angle.
Class and exam questions
What does an effusion look like in a supine patient?
In the supine position, fluid spreads along the posterior surface of the cavity. The classic meniscus is not detected; instead, a uniform hazy opacity of the entire hemithorax is visible without the air bronchogram sign.
Why is the absence of mediastinal shift in a total effusion a bad sign?
This indicates that the fluid pressure is offset by the loss of lung volume (obstructive collapse) or that the organs are rigidly fixed by an extensive malignant pleural tumor.
Can CT be used to accurately distinguish transudate from exudate?
No, assessing the X-ray density of the fluid (CT number) does not make it possible to reliably differentiate between transudate and exudate. This requires clinical analysis of the punctate or specific MRI sequences.
How does empyema differ from a lung abscess on images?
An abscess is a focus of destruction (necrosis) of the lung tissue itself with irregular thick walls. Empyema, on the other hand, is an accumulation of pus in the pleural cavity that causes compression of the adjacent lung.
What else to study on this topic
- Magnetic resonance imaging (MRI) of the pleura and diffusion-weighted imaging (DWI)
- Causes of total opacification of a hemithorax
- Loculated pleural effusion and its differentiation from lung tumors
- Drug-induced effusions (effects of cytotoxics, amiodarone, etc.)
- Specifics of effusions in end-stage renal disease (uremia) and HIV infection
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