Pleural Effusion in Cats: Radiographic Diagnosis, Differential Diagnosis by Signalment, and Triage Decisions

Pleural effusion is a frequent cause of respiratory distress in feline patients presenting to general practice and emergency settings. Feline pleural effusion refers to abnormal accumulation of fluid within the pleural space, resulting from disturbances in hydrostatic pressure, oncotic pressure, lymphatic drainage, or vascular permeability. Regardless of the underlying etiology, increasing intrapleural fluid volume compresses the lungs, decreases functional residual capacity, and ultimately compromises ventilation.

In cats, pleural effusion may develop insidiously and remain clinically unapparent until respiratory reserve becomes exhausted. Consequently, rapid recognition and appropriate triage are critical. In many practices, the first important clue comes from thoracic radiographs, where characteristic radiographic signs support the presence of pleural fluid and help guide immediate next steps.

Given the limited access to ultrasound and CT, thoracic radiographs remain the primary diagnostic modality for assessing respiratory disease. While thoracic radiographs are useful in diagnosing pleural effusion and assessing severity the underlying etiology usually requires integration of imaging findings with signalment, physical examination, and, when indicated, fluid cytology.  

This article reviews the radiographic hallmarks of pleural effusion in cats, discusses differential diagnoses organized by signalment, and outlines practical triage decisions regarding imaging versus drainage.

Recognizing Pleural Effusion on Radiograph

In healthy cats, the pleural space contains only a minimal amount of fluid and is radiographically invisible. When excess fluid accumulates, it separates the visceral pleura from the parietal pleura and creates several characteristic radiologic (roentgen) signs. Recognizing these signs on thoracic radiographs is essential for identifying feline pleural effusion and distinguishing it from other causes of feline dyspnea.

Interlobar Fissure Lines

Pleural fluid commonly collects within interlobar fissures, producing linear soft tissue opacities visible on thoracic radiographs. These fissure lines typically appear widest laterally and taper medially toward the pulmonary hilus.

When of sufficient volume, accumulation of fluid between lung lobes can cause multiple fissure lines as an early indicator of disease.

Interlobar fissure lines on a feline thoracic radiograph are one of the most consistent signs of pleural effusion.

Retraction of Lung Lobes

Pleural fluid separates the lungs from the thoracic wall and sternum. Radiographically, this produces dorsal or medial displacement of lung margins, with soft tissue opacity filling the pleural space.

In moderate to severe cases, the lung lobes become compressed and reduced in volume.This often causes increased opacity to the pulmonary parenchyma, due to compressive atelectasis. The retraction of the lung lobes is a helpful distinguisher of pleural space disease from pulmonary disease. 

The cardiac silhouette displays border effacement with the ventral pleural fluid. While its margins are not fully delineated, the heart is dorsally displaced and all lung lobes are retracted from the thoracic wall. 

Scalloped Lung Margins

A classic feature of feline pleural effusion is scalloping of the lung margins along the ventral thorax, particularly adjacent to the sternum. This appearance results from fluid outlining the compressed pulmonary lobes and can also be associated with chronic pleuritis

Scalloped margins are especially helpful when assessing thoracic radiographs with pleural effusion, as the rounding can indicate chronicity. 

Scalloped ventral lung margins are a useful visual clue when evaluating pleural effusion on feline chest radiographs.

Border Effacement, Silhouette Sign

Border effacement occurs when two similar radiographic opacities are in contact with each other, causing their margins to disappear. As fluid accumulates in the pleural space, the cardiac silhouette and diaphragm may become indistinct or completely obscured.. This loss of normal anatomical borders can also be referred to as the silhouette sign.

Border effacement is an important feature of feline pleural effusion, but it should be interpreted alongside other findings such as lung lobe retraction and fissure lines rather than in isolation.

With small volumes of effusion, one of the first radiographic signs may be border effacement of the cardiac margin on a dorsoventral projection due to fluid pooling along the cardiac apex. 

Pleural fluid often causes border effacement of the cardiac silhouette and diaphragm, creating the classic silhouette sign.

Costophrenic Angle Rounding

Early pleural effusion may produce blunting or rounding of the costophrenic angles, particularly on DV or VD views. This may be one of the earliest detectable radiographic signs in mild cases, although it is not specific on its own.

Tracheal Deviation

Large-volume effusions can cause dorsal displacement of the trachea due to compression of the lung lobes and mediastinal structures. This finding generally reflects substantial pleural fluid accumulation and should prompt careful assessment of the patient’s respiratory stability, although it is not present in every severe case.

Radiographic Severity

Radiographic appearance often correlates approximately with effusion volume:

• Mild effusion: costophrenic angle rounding and subtle fissure lines
  
• Moderate effusion: fissure line widening and partial lung retraction
  
• Severe effusion: marked border effacement, pronounced lung compression, and sometimes tracheal displacement
  

Recognizing these stages is useful for estimating severity and determining the urgency of thoracocentesis, although clinical decision-making should always be guided by respiratory status rather than imaging appearance alone.

Bilateral vs Unilateral Pleural Effusion

The distribution of pleural fluid can provide useful diagnostic clues, but distribution alone is not diagnostic. Interpretation is most helpful when combined with signalment, clinical findings, and follow-up diagnostics.

Bilateral Pleural Effusion

Pleural effusion in cats is often bilateral or appears relatively symmetric, in part because mediastinal fenestrations may permit communication between hemithoraces, although this is variable. Bilateral pleural effusion may be seen with:

• congestive heart failure
  
• neoplasia, especially lymphoma
  
• feline infectious peritonitis
  
• hypoproteinemia or modified transudate states
  
• inflammatory or infectious pleural disease
  

In older cats, bilateral pleural fluid with post-drainage evidence of cardiomegaly, left atrial enlargement, or pulmonary venous congestion should increase suspicion for cardiogenic effusion. In other patients, bilateral fluid may also occur with neoplastic, inflammatory, or infectious disease.

Unilateral Pleural Effusion

Unilateral or markedly asymmetric pleural effusion may increase suspicion for loculated fluid due to an exudate or fibrin accumulation although unilateral distribution is not specific to any single diagnosis. Conditions that may present this way include:

• pyothorax
  
• chylothorax
  
• lung lobe torsion
  
• pleural neoplasia
  
• focal pulmonary disease with secondary pleural involvement
  

In particular, unilateral fluid on a cat pleural effusion xray should prompt careful consideration of localized disease processes, but this finding should always be interpreted in context.

Dorsal tracheal displacement may be seen in severe feline pleural effusion and usually reflects substantial thoracic volume loss from lung compression.

While not definitive, unilateral feline pleural effusion may suggest more localized disease, while bilateral fluid is commonly seen with cardiac, inflammatory, infectious, or neoplastic causes.

Differential Diagnosis by Signalment

Signalment remains a useful tool in prioritizing differential diagnoses in feline pleural effusion. While radiographic findings are often not specific for a single etiology, combining imaging findings with age and clinical history can substantially narrow diagnostic possibilities.

Young Cats to Middle Age Cats

Feline Infectious Peritonitis, FIP

Effusive FIP can involve the pleural space, either alone or together with abdominal effusion, in the appropriate clinical setting. Radiographic features may include:

• bilateral pleural effusion
  
• concurrent ascites
  
• absence of obvious cardiomegaly
  

Pleural fluid is often straw-colored and protein-rich, but diagnosis depends on the broader clinical picture and additional testing.

Chylothorax

Chylothorax results from accumulation of lymphatic fluid within the pleural cavity due to disruption, obstruction, or impaired drainage of the thoracic duct or associated lymphatic pathways. Chylothorax may be idiopathic or secondary to cardiac disease, mediastinal disease, or other causes, and age alone does not determine etiology. However, it remains an important consideration in younger cats presenting with pleural effusion.

Radiographic findings may include:

• bilateral pleural effusion
  
• rounded or retracted lung margins
  
• possible pleural thickening in chronic cases
  

Fluid obtained via thoracocentesis is often milky white, and diagnosis is supported by triglyceride concentration in pleural fluid relative to serum. Chronic chylothorax cat cases may lead to fibrosing pleuritis and restrictive pulmonary expansion.

Pyothorax

Pyothorax is an important cause of pleural effusion in cats and should be considered especially when fluid is unilateral or loculated, pleural thickening is present, or the patient has fever, lethargy, pain, or evidence of systemic inflammation. Possible sources include:

• bite wounds
  
• migrating foreign bodies
  
• penetrating trauma
  
• hematogenous infection
  

Radiographic findings may include:

• unilateral or asymmetric effusion, especially early
  
• pleural thickening
  
• occasional gas pockets within the pleural space
  
• more diffuse fluid accumulation later in disease
  

Pleural fluid is typically turbid or purulent, and cytology may reveal degenerate neutrophils and bacteria. Immediate stabilization, drainage, and appropriate antimicrobial management are usually required.

Mediastinal Lymphoma

Lymphoma remains an important differential diagnosis in  cats with pleural effusion, particularly when a cranial mediastinal mass is present or they are FeLV positive. 

Radiographic findings may include:

• cranial mediastinal widening
  
• mediastinal mass effect
  
• bilateral pleural effusion
  

Pleural fluid cytology may reveal neoplastic lymphocytes, although not all cases are diagnosed from fluid evaluation alone.

Pleural effusion with cranial mediastinal widening and tracheal displacement should prompt consideration of feline lymphoma.

Older Cats

Congestive Heart Failure

In older feline patients with acquired heart disease, cardiogenic pleural effusion is an important differential diagnosis, especially when post-drainage imaging reveals cardiomegaly, left atrial enlargement, or pulmonary venous congestion. Cardiomyopathies are a common underlying cause of cardiac disease in cats though congenital cardiac disease is also possible and signalment becomes an important factor when cardiomegaly is present

Radiographic features may include:

• bilateral pleural effusion
  
• cardiomegaly
  
• pulmonary venous distension
  
• pulmonary edema
  

Unlike dogs, cats with CHF may present with pleural effusion, pulmonary edema, or both.

Thoracic Neoplasia

Thoracic neoplasia is another important cause of pleural effusion in older cats. Possible tumors include:

• pulmonary carcinoma
  
• mesothelioma
  
• metastatic disease
  
• pleural or mediastinal neoplasia
  

Radiographs may reveal pulmonary nodules, mediastinal masses, pleural thickening, or more diffuse pleural fluid accumulation, although some lesions only become apparent after drainage.

What Fluid Character Tells You, If Thoracocentesis Has Already Been Performed

The gross appearance of pleural fluid can provide immediate diagnostic clues, although fluid appearance alone is not definitive and should be interpreted alongside cytology, biochemical analysis, and culture when indicated.

Transudate

• clear, low protein
  
• may be associated with congestive heart failure or hypoproteinemia
  

Modified Transudate

• slightly turbid
  
• may be seen with chronic cardiac disease, neoplasia, or longer-standing pleural processes
  

Exudate

• turbid or purulent
  
• supports consideration of pyothorax or severe inflammation
  

Chylous Effusion

• milky white
  
• supports chylothorax when triglyceride concentration exceeds serum
  

Hemorrhagic Effusion

• red to dark red
  
• may occur with trauma, neoplasia, or coagulopathy
  

Cytology, culture, and biochemical analysis remain essential for definitive diagnosis.

When to Drain First vs Image First

Management of feline pleural effusion must prioritize respiratory stabilization. The decision to image before drainage depends on respiratory stability, handling tolerance, imaging availability, and clinician judgment. In unstable patients, stabilization and therapeutic drainage generally take priority over full radiographic positioning.

Indications for Immediate Thoracocentesis

Thoracocentesis should generally be performed before radiography when the patient exhibits:

• open-mouth breathing
  
• cyanosis
  
• severe tachypnea
  
• marked orthopnea
  
• inability to remain in sternal recumbency or tolerate restraint
  

Radiographic positioning and restraint may exacerbate respiratory compromise in these patients.

When Imaging Can Precede Drainage

Thoracic radiographs may be obtained before thoracocentesis when:

• respiratory distress is mild to moderate and the patient remains stable
  
• the patient tolerates minimal restraint
  
• imaging findings may influence procedural planning
  
• point-of-care assessment suggests that a brief study can be obtained safely
  

Even small volumes of removed fluid may markedly improve ventilation and allow safer imaging afterward.

Follow-Up Imaging After Drainage

Follow-up thoracic radiographs are often highly valuable after thoracocentesis because pleural fluid may obscure underlying pathology before drainage, although timing should depend on patient stability and clinical priorities.

Key features to evaluate include:

Pulmonary Parenchymal Disease

After fluid removal, radiographs may reveal:

• pulmonary masses or nodules
  
• consolidation
  
• atelectasis
  
• lung lobe torsion or focal collapse
• cardiomegaly
  

These findings may alter the differential list

Mediastinal Abnormalities

Cranial mediastinal masses consistent with lymphoma, thymoma, or other space-occupying lesions may become more apparent once fluid is removed.

Cardiac Silhouette Evaluation

Cardiac size and shape can be assessed more accurately after at least partial pleural fluid drainage.