Pleural Effusion

A pleural effusion is an abnormal accumulation of fluid within the pleural space.

It is classified biochemically as transudate or exudate, a distinction that substantially narrows the differential diagnosis and guides further investigation.

Transudative effusions arise from an imbalance of hydrostatic and oncotic pressures, most commonly from heart failure, and also from cirrhosis with hypoalbuminemia, nephrotic syndrome, and hypothyroidism.

Exudative effusions arise from increased capillary permeability or impaired lymphatic drainage, most commonly from pneumonia (a parapneumonic effusion), malignancy, pulmonary embolism, and tuberculosis, with a longer list of less common causes including connective tissue disease and pancreatitis.

Progressive exertional dyspnea is the most common symptom, with the degree of breathlessness generally proportional to the volume of fluid and the rapidity of its accumulation.

Pleuritic chest pain suggests pleural inflammation and is more typical of exudative causes, particularly infection. A dry cough is common.

Examination findings include reduced chest expansion on the affected side, stony dull percussion note, reduced or absent breath sounds, and reduced vocal fremitus and resonance over the effusion.

Associated features point toward the underlying cause: signs of heart failure (raised jugular venous pressure, peripheral edema), signs of chronic liver disease, fever and features of pneumonia, or weight loss and other features suggesting malignancy.

CXR is usually the first investigation, showing blunting of the costophrenic angle (typically requiring at least 200 to 300 mL of fluid to be visible on an erect film) and, with larger effusions, a meniscus sign.

Ultrasound is more sensitive for detecting smaller effusions, helps distinguish fluid from pleural thickening, and is used to guide safe thoracocentesis.

CT chest with contrast provides further characterization and is particularly useful for identifying an underlying cause such as malignancy or loculated infection.

Diagnostic thoracocentesis should be performed for any new, unexplained pleural effusion where the cause is not already clear from the clinical context, such as a patient with obvious, symmetric bilateral effusions in the setting of clearly decompensated heart failure.

Pleural fluid analysis should include protein, LDH, cell count and differential, pH, glucose, cytology, and microbiology (Gram stain and culture, with tuberculosis testing where relevant).

Light's criteria are applied to distinguish exudate from transudate where serum protein is not straightforwardly high or low: an effusion is exudative if the pleural fluid to serum protein ratio exceeds 0.5, the pleural fluid to serum LDH ratio exceeds 0.6, or the pleural fluid LDH exceeds two thirds of the upper limit of normal serum LDH. A low pleural fluid pH (below 7.2) or low glucose in the context of a parapneumonic effusion suggests a complicated effusion or empyema, indicating a need for chest drainage rather than antibiotics alone.

Differentials for the underlying cause span the wide list noted above, alongside consideration of hemothorax (bloody effusion, typically traumatic) and chylothorax (milky effusion from lymphatic disruption, elevated triglycerides on fluid analysis, typically from malignancy or thoracic duct injury).

Management is directed primarily at the underlying cause: diuresis and heart failure optimization for a cardiac transudate, antibiotics for a simple parapneumonic effusion, and specific oncological management for a malignant effusion.

Therapeutic thoracocentesis provides symptomatic relief for a large, symptomatic effusion, with a general limit of around 1.5 liters removed in a single procedure to reduce the risk of re-expansion pulmonary edema.

Complicated parapneumonic effusion or empyema (frank pus, positive Gram stain or culture, or pleural fluid pH below 7.2) requires chest tube drainage in addition to antibiotics, since antibiotics alone are unlikely to adequately clear an infected, loculated collection; intrapleural fibrinolytics combined with DNase are used for loculated collections not draining adequately with a chest tube alone, and surgical decortication is considered for cases failing to resolve with these measures.

Recurrent malignant pleural effusion is managed with either an indwelling pleural catheter, allowing intermittent outpatient drainage, or pleurodesis (chemical, most commonly with talc, instilled via chest drain or at thoracoscopy) to obliterate the pleural space and prevent fluid reaccumulation, with the choice guided by patient prognosis, performance status, and whether the underlying lung is able to fully re-expand.

Referral: respiratory medicine for diagnostic uncertainty, any exudative effusion requiring further workup, or complicated parapneumonic effusion or empyema requiring drainage; oncology for confirmed malignant effusion; cardiothoracic surgery for cases requiring decortication.