Pleural Effusion

A pleural effusion is a symptom of an underlying pathology rather than a primary diagnosis. In the UK, the commonest causes of transudates (protein <25g/L) are Heart Failure, Cirrhosis, and Nephrotic Syndrome. Commonest exudates (protein >35g/L) are Malignancy (lung, breast, lymphoma), Parapneumonic effusions (pneumonia), and Rheumatoid arthritis. Light's Criteria are used if the protein is between 25-35g/L: an exudate is likely if (1) Effusion/Serum Protein ratio >0.5, (2) Effusion/Serum LDH ratio >0.6, or (3) Effusion LDH > 2/3 the upper limit of normal serum LDH. Diagnosis starts with an erect CXR (showing blunting of costophrenic angles or a meniscus). Ultrasound-guided pleural aspiration (thoracocentesis) is mandatory for any new effusion (except obvious heart failure). Fluid should be sent for protein, LDH, glucose, pH, cytology, and microbiology. A pH <7.2 indicates an empyema or complicated parapneumonic effusion, requiring a chest drain. Malignant effusions may require talc pleurodesis or indwelling pleural catheters (IPCs) if they recur rapidly. Management must address the primary cause (e.g., diuretics for heart failure).

Pleural fluid is normally produced by the parietal pleura and absorbed by the visceral pleura and lymphatics; a small amount (10-20ml) acts as lubrication. Effusions occur when this balance is disrupted. Transudates are caused by changes in systemic factors: increased hydrostatic pressure (e.g., Heart Failure) or decreased oncotic pressure (e.g., Hypoalbuminaemia in cirrhosis/nephrosis). The pleura itself is healthy. Exudates are caused by local factors that increase capillary permeability or decrease lymphatic drainage (e.g., inflammation from infection, malignancy, or infarction). Parapneumonic effusions occur in 40% of pneumonias; 'complicated' effusions occur when bacteria invade the pleural space, leading to low glucose (metabolised by bacteria) and low pH (lactic acid production). If pus forms, it is called an empyema.

Symptoms: Often asymptomatic if small; large effusions cause progressive dyspnoea, pleuritic chest pain, and dry cough. Physical signs: Reduced expansion on the affected side, trachea shifted *away* from the side of a large effusion (>1000ml), stony dull percussion, diminished or absent breath sounds over the fluid, and reduced vocal resonance. 'Aegophony' (E-to-A change) may be heard at the upper border of the fluid. Massive effusions are usually due to Malignancy or TB. Bilateral effusions usually suggest a transudative cause (e.g. Heart Failure). Complications: Empyema, pleural thickening (trapped lung), and respiratory failure.

Imaging: Erect Chest X-ray (requires 200ml fluid to show blunting of costophrenic angle; 'meniscus' sign; opaque hemithorax with mediastinal shift if massive); Ultrasound (detects small effusions, identifies loculations, and guides aspiration). Pleural Fluid Analysis: Appearance (straw-coloured, bloody, or milky/chylous); Protein & LDH (for Light's); pH (pH <7.2 = drain); Glucose (low in RA/empyema); Cytology (for cancer); Gram stain/Microbiology. Bloods: Total protein, LDH, FBC, CRP, U&Es.

Conservative: Small, asymptomatic transudates (treat the underlying cause, e.g. Furosemide). Medical: Ultrasound-guided aspiration for diagnosis/relief. Empyema/Complicated Parapneumonic Effusion (pH <7.2): Urgent chest drain + Antibiotics. Malignant Effusion: Pleurodesis (using talc or doxycycline) to stick the pleural layers together; Indwelling Pleural Catheter (IPC) for home drainage; Palliative care. Surgical: VATS (Video-Assisted Thoracoscopic Surgery) for decortication if the effusion is loculated or the lung is 'trapped' by thick pleura.