Pleural effusion

The pleural space normally holds only a thin film of lubricating fluid. An effusion develops whenever fluid production outpaces lymphatic drainage — from raised hydrostatic pressure, reduced oncotic pressure, increased capillary permeability, or blocked lymphatics. The first branch point in evaluation is whether the fluid is a transudate or an exudate, distinguished by Light's criteria comparing protein and lactate dehydrogenase levels in the fluid against serum.

Transudates result from systemic pressure imbalances with intact capillaries; the leading causes are heart failure, cirrhosis, and nephrotic syndrome. Exudates result from local pleural disease that makes capillaries leaky — pneumonia (parapneumonic effusion), malignancy, pulmonary embolism, tuberculosis, and connective tissue disease. Special categories include empyema (frank pus), hemothorax (blood), and chylothorax (lymphatic fluid).

Clinically, patients report dyspnea and sometimes pleuritic chest pain. Examination of the affected side shows dullness to percussion, decreased or absent breath sounds, and reduced tactile fremitus, with the trachea deviating away from the effusion when it is large. Upright chest radiography shows blunting of the costophrenic angle and a meniscus, and lateral decubitus films or ultrasound help determine whether the fluid layers freely. Thoracentesis both samples the fluid for analysis and relieves symptoms; definitive management targets the underlying cause.

Pleural effusion appears across several licensure exams. USMLE Step 1 tests it in respiratory pathology alongside conditions such as silicosis, emphasizing Light's criteria and the transudate versus exudate distinction, while the NPTE for physical therapists and physical therapist assistants covers the auscultation findings, differential diagnosis, and the pulmonary interventions appropriate for a patient with an effusion.