Back
ditki Logo
medical & biological sciences

Pleural Effusions for USMLE Step 2

Start your One-Week Free Trial
Already subscribed? Log in »
Pleural Effusions for the USMLE Step 2 Exam
Pleural effusion is the abnormal accumulation of fluid in the pleural space, commonly resulting from systemic or local diseases.
Classification of Pleural Effusions
Transudative vs. Exudative Effusions
The initial approach to pleural effusion involves determining whether it is transudative or exudative using Light’s criteria. This distinction helps narrow down the underlying cause.
  • Transudative Effusion: Results from systemic factors like increased hydrostatic pressure or decreased oncotic pressure.
    • Common causes:
    • Heart failure: The most frequent cause, leading to increased pulmonary capillary pressures.
    • Cirrhosis: Hypoalbuminemia lowers plasma oncotic pressure.
    • Nephrotic syndrome: Severe protein loss in the urine decreases oncotic pressure.
  • Exudative Effusion: Arises from local inflammatory or neoplastic processes that increase vascular permeability or impair lymphatic drainage.
    • Common causes:
    • Parapneumonic effusion: Inflammatory response to bacterial pneumonia.
    • Malignancy: Tumor obstructing lymphatics.
    • Tuberculosis: Chronic infection causing granulomatous inflammation.
Transudate vs Exudate Pleural Effusion
Light’s Criteria
Exudative effusions meet at least one of the following criteria:
  • Pleural fluid protein/serum protein ratio > 0.5.
  • Pleural fluid LDH/serum LDH ratio > 0.6.
  • Pleural fluid LDH > two-thirds of the upper limit of normal for serum LDH.
Clinical Features
Symptoms of pleural effusion can vary based on the underlying cause and size of the effusion.
  • Symptoms:
    • Dyspnea: Common, especially in larger effusions.
    • Pleuritic chest pain: Seen in exudative effusions like parapneumonic effusion or pulmonary embolism.
    • Cough: Non-productive, often present with both types of effusions.
  • Physical Exam Findings:
    • Dullness to percussion: Indicates fluid accumulation.
    • Decreased breath sounds: Over the area of fluid.
    • Decreased tactile fremitus: Suggests fluid impeding sound transmission.
    • Egophony: Increased voice resonance just above the effusion.
Diagnostic Approach
Imaging
  • Chest X-ray (CXR): The first imaging study used.
    • Findings: Blunted costophrenic angles and a meniscus sign. A lateral decubitus X-ray helps assess if the fluid is free-flowing or loculated.
  • Ultrasound: More sensitive than CXR and helps guide thoracentesis.
  • CT scan: Useful in evaluating complex or recurrent effusions and identifying underlying causes, such as malignancy.
Thoracentesis
Thoracentesis is both diagnostic and therapeutic. It involves draining pleural fluid for analysis.
  • Pleural Fluid Analysis:
    • Protein and LDH: Used to classify the effusion as transudative or exudative (via Light's criteria).
    • Cell count and differential:
    • Neutrophils suggest bacterial infection.
    • Lymphocytes point to tuberculosis or malignancy.
    • Glucose: Low in infection, malignancy, or rheumatoid pleuritis.
    • pH: Low (<7.2) in complicated parapneumonic effusions or malignancies.
    • Cytology: Detects malignant cells in exudative effusions.
    • Culture and Gram stain: To detect bacteria in infectious effusions.
Treatment
Management depends on the underlying cause of the effusion.
General Approach
  • Therapeutic thoracentesis: Indicated for large, symptomatic effusions causing respiratory compromise.
  • Chest tube drainage: Necessary for empyema (infected pleural fluid), as simple drainage is insufficient.
Cause-Specific Treatment
  • Heart failure: Diuretics to reduce fluid overload.
  • Parapneumonic effusion/empyema: Antibiotics combined with chest tube drainage if the fluid is infected.
  • Malignant pleural effusion: Repeated thoracentesis or pleurodesis to prevent recurrence.
Key Points
  • Pleural effusions are categorized as transudative or exudative, with exudates meeting one of Light’s criteria.
  • Transudative effusions are caused by systemic conditions such as heart failure or cirrhosis, while exudative effusions are often due to local processes like infection, malignancy, or pulmonary embolism.
  • Key diagnostic tools include imaging (CXR, ultrasound) and thoracentesis with pleural fluid analysis.
  • Treatment focuses on managing the underlying cause, with therapeutic thoracentesis providing relief for symptomatic effusions.

Related Tutorials