What is a Pleural Effusion?
A pleural effusion is an abnormal accumulation of fluid in the pleural space—the potential space between the visceral and parietal pleura—exceeding the normal 15-20 mL present in healthy individuals. 1, 2
Pathophysiology
The pleural space normally contains a small amount of fluid that lubricates the pleural surfaces during respiration. Pleural effusions develop when this balance is disrupted through several mechanisms: 3
- Increased permeability of the pleural membrane (allowing protein and fluid to leak into the pleural space) 3
- Increased pulmonary capillary pressure (as seen in heart failure) 3
- Decreased negative intrapleural pressure (from airway obstruction or lung entrapment) 3
- Decreased oncotic pressure (from hypoalbuminemia) 3
- Obstructed lymphatic drainage (from malignancy or inflammation) 3
Clinical Presentation
Dyspnea is the most common presenting symptom, occurring in more than half of cases, initially manifesting on exertion and progressing as the effusion enlarges. 3, 2 Additional symptoms include: 2, 4
- Predominantly dry cough 2
- Pleuritic chest pain (sharp, localized pain worsening with breathing) 2
- Dull, aching chest pain (particularly in mesothelioma) 3
- Weight loss, anorexia, and malaise (in malignant effusions) 3
The dyspnea results from decreased chest wall compliance, contralateral mediastinal shift, reduced ipsilateral lung volume, and reflex stimulation from the lungs and chest wall. 3
Physical Examination Findings
On examination, large effusions produce decreased or absent breath sounds, dullness to percussion, decreased tactile fremitus, and reduced chest expansion on the affected side. 5, 6 These findings are typically present when the effusion volume exceeds 500 mL. 3
Classification
Pleural effusions are fundamentally classified into two categories: 3
Transudates
Transudates occur when hydrostatic forces favoring pleural fluid accumulation are altered, but capillary permeability to proteins remains normal. 3 Common causes include: 3
- Congestive heart failure
- Cirrhosis with ascites
- Hypoalbuminemia
- Nephrotic syndrome
- Dialysis patients
Exudates
Exudates develop when the pleural surface and/or local capillary permeability are altered, allowing protein-rich fluid to accumulate. 3 Common causes include: 3
- Pneumonia (parapneumonic effusions/empyema)
- Malignancy (lung cancer, breast cancer, lymphoma, mesothelioma)
- Pulmonary embolism
- Tuberculosis
- Rheumatoid arthritis and systemic lupus erythematosus
- Drug-induced effusions
Diagnostic Approach
Clinical assessment alone can often identify transudative effusions in appropriate settings such as left ventricular failure, and these do not require sampling unless atypical features are present or they fail to respond to treatment. 3 However, unilateral effusions and those with atypical features require thoracentesis. 3
Diagnostic thoracentesis should be performed using a 21G needle to obtain a 50 mL sample, which is analyzed for protein, LDH, pH, glucose, cell count with differential, Gram stain, acid-fast bacilli stain, cytology, and microbiological culture in blood culture bottles. 3, 7, 6
Clinical Significance
Pleural effusions indicate the presence of underlying disease, which may be pulmonary, pleural, or extrapulmonary in origin. 3 Approximately 5-12% of patients presenting for emergency medical treatment are diagnosed with pleural effusion, making it a common clinical finding. 1 The etiology remains unclear in nearly 20% of cases despite thorough investigation. 2
In the context of trauma or neurological issues, hemothorax (blood in the pleural space) should be considered, which presents as a pleural effusion on imaging but requires specific management including drainage. 3