How to diagnose pleural effusion by ultrasound in a patient with ascites?

From the Guidelines

Diagnosing pleural effusion by ultrasound in a patient with ascites is best achieved by using a microconvex transducer and scanning at the posterior axillary line above the diaphragm.

Key Ultrasound Findings

• Pleural effusion is visualized as an anechoic space between the parietal and visceral pleura 1
• The sinusoid sign, a dynamic sign showing the variation of the interpleural distance during respiratory cycles, can be used to diagnose pleural effusion 1
• Internal echoes or septa within the effusion are suggestive of exudate or hemothorax 1

Ultrasound Technique

• Use a microconvex transducer for optimal imaging 1
• Scan at the posterior axillary line above the diaphragm for best detection of non-loculated pleural effusions 1
• Real-time ultrasound guidance can be used to assist in drainage of pleural effusions, particularly small or loculated effusions 1

Comparison to Other Imaging Modalities

• Lung ultrasound is more accurate than supine radiography and as accurate as CT for detecting pleural effusions 1
• Thoracic ultrasound can detect smaller amounts of pleural fluid (>20 mL) compared to chest radiographs (>75 mL on lateral view, >175 mL on frontal view) 1

Clinical Considerations

• Ascites may not directly affect the ultrasound diagnosis of pleural effusion, but it may be a related condition that needs to be considered in the clinical context 1
• Physical examination findings have a lower positive likelihood ratio for detecting pleural effusions, making imaging studies like ultrasound more reliable 1

From the Research

Diagnosing Pleural Effusion by Ultrasound in a Patient with Ascites

To diagnose pleural effusion by ultrasound in a patient with ascites, several criteria and techniques can be utilized:

• The diaphragm sign, the displaced crus sign, and the bare area sign can be used to differentiate ascites from pleural effusion by ultrasound 2.
• Ultrasound can reliably detect small volumes of fluid and is a useful first-line imaging modality for clinical triage, including the detection of ascites and pleural effusion 3.
• The A-mode ultrasonic technique can detect pleural fluid in 93% of cases, and can differentiate between loculated pleural fluid and pleural thickenings 4.
• Ultrasound can quantify the volume of ascites and aid in the decision process for fluid drainage, and is superior to computed tomography in the qualitative assessment of fluid 3.

Ultrasound Techniques for Diagnosing Pleural Effusion

The following ultrasound techniques can be used to diagnose pleural effusion:

• A-mode ultrasound investigation can correctly identify fluid at a site different from the aspiration attempts and can successfully remove the fluid 5.
• Ultrasound can detect very small amounts (even 3 to 5 ml) of loculated pleural fluid, and can permit easy differentiation between loculated pleural fluid and pleural thickenings 4.
• Ultrasound can be used to guide needle aspiration of pleural fluid, and can make thoracocentesis possible in a high percentage of cases 4.

Differentiating Ascites from Pleural Effusion

To differentiate ascites from pleural effusion, the following can be considered:

• The simultaneous presence of both ascites and pleural effusion can be difficult to identify with plain radiographs, but can be differentiated using computed tomography (CT) or ultrasound 2.
• The four criteria (the diaphragm sign, the displaced crus sign, the interface sign, and the bare area sign) can be used to differentiate ascites from pleural effusion by CT, but may not be reliable when both fluid collections are present 2.
• Ultrasound can differentiate ascites from pleural effusion using three of the above-mentioned signs (the diaphragm sign, the displaced crus sign, and the bare area sign) 2.