What’s Your Diagnosis? Identifying Pleural Effusion

Identifying Pleural Effusion

Signalment:10-year-old female spayed Labrador retriever

History: 24 hours of progressive respiratory distress

1. What are the primary findings?
2. What do you think is the primary reason for the clinical signs?

Radiographic findings: Increased soft tissue opacity is noted and is causing the pleural fissures and margins of the lung to be identified. The heart and ventral diaphragm are silhouetted by this opacity. The pleural fissures that are seen are wider at the periphery than centrally. Follow-up radiographs taken after thoracocentesis which revealed a hemorrhagic fluid show an obvious mass effect in the right cranial ventral thorax adjacent to the right side of the heart.

Radiographic interpretation: Pleural effusion. This is causing a silhouette sign with the intrathoracic structures and removal of the fluid reveals a large mass effect in the cranial thorax. Neoplasia is the primary rule-out in this situation.

Discussion: Knowledge of the pertinent anatomy of the thoracic structures facilitates understanding the abnormal. This is especially true in radiographic evaluation of thoracic lesions.

The pleura is a mesothelial lined connective tissue membrane covering the internal thoracic wall, the mediastinum, heart and lungs. The pleural space is a potential space containing a small amount of lymph for lubrication. The pleural lined lung lobes have a fairly constant anatomical arrangement. Knowledge of this arrangement or the lung fissures is important in recognition of pleural effusion or thickened pleura.

The natural fissures are formed where the individual lung lobes meet. Radiographic visualization of these fissures (air or soft tissue opacity) is usually due to disease of the pleural space. Pleural fissure lines may occasionally be seen in older dogs or dogs recovered from previous thoracic disease. These lines represent thickened pleural membranes that have been penetrated tangentially (end-on) by the X-ray beam but are thin and uniform in thickness.

Pleural effusion is the presence of any type of fluid within the pleural space (transudate, modified transudate and exudate). The radiological differentiation of thickened pleural membranes and slight pleural effusion may be difficult or impossible. The divergence and widening of the fissure as it progresses toward the periphery of the lung field is an indication of pleural effusion.

The amount of pleural fluid affects the ease with which it is recognized. Close to 100 ml of free fluid may be unrecognized in a medium-sized dog. Theoretically, the earliest area to detect pleural fluid is at the costodiaphragmatic angle. Fluid may collect here and cause blunting of the caudal lung lobe tips. 

Also, minimal amounts of fluid may be seen collecting in the lung fissure between the accessory and left caudal lung lobe. These changes are best seen on the recumbent VD view. It is important to remember that there is normally fat accumulation in the mediastinum between the accessory and left caudal lung lobe. Pleural fissures also may become visible on the lateral view prior to visualization on the VD/DV view. 

In the recumbent DV view, fluid collects along the sternum, obscuring the cardiac apex and widening the cranial mediastinum. With massive pleural fluid, lobes retract toward the hilus. The fluid obscures pulmonary detail. The lungs may undergo partial or complete atelectasis. 

In partial atelectasis the lung tissue retains its normal shape due to its natural “form elasticity.” Its volume diminishes but it still can be recognized as lung by its shape and air opacity. When complete atelectasis occurs, the cranial and middle lung lobes are most frequently involved. As the lungs retract, the lung/fluid border takes on a scalloped appearance. The fluid may be so extensive as to silhouette (absence of radiographic contrast between two objects) with the caudal heart and the cranial diaphragm. This may present confusion in distinguishing between a diaphragmatic hernia and pleural effusion alone. 

Chronic effusions, such as commonly seen in chylothorax in cats, can cause radiographically visible changes including rounding of the lung margins and possible diminished size of the lung lobe due to an inability to expand the lung due to pleural fibrosis.

Summary: Pleural effusion can often be easily recognized radiographically through knowledge of the normal pleural anatomy. However, the etiology is usually not visible radiographically when the effusion is present and thoracocentesis, including cytological analysis and additional imaging, is usually necessary to make a definitive diagnosis.


Anne Bahr, DVM, MS, Dipl. ACVR, is a consulting radiologist with PetRays Veterinary Radiology Consultants in Spring, Texas.

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