Pleural Effusions for USMLE Step 1
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Pleural Effusions for the USMLE Step 1 Exam
Pathophysiology
- Normal Physiology:
- The pleural space normally contains a small amount of fluid (approximately 5-15 mL) that allows smooth lung movement.
- Fluid balance is maintained by hydrostatic and oncotic pressures across the pleural membranes, along with lymphatic drainage.
- Mechanisms of Fluid Accumulation:
- Increased hydrostatic pressure: Commonly due to left-sided heart failure, which raises pulmonary capillary pressures.
- Decreased oncotic pressure: Seen in nephrotic syndrome or cirrhosis, reducing albumin and lowering plasma oncotic pressure.
- Increased vascular permeability: Inflammation from infections or malignancies increases capillary leak.
- Impaired lymphatic drainage: Obstruction of pleural lymphatics, often due to malignancies, causes fluid retention.
Classification of Pleural Effusions
Transudative vs. Exudative Effusions
The differentiation between transudative and exudative effusions is vital, and this is commonly achieved using Light’s criteria.
- Transudative Effusions: Result from systemic factors affecting the balance of hydrostatic and oncotic pressures.
- Common causes:
- Heart failure: Increased hydrostatic pressure.
- Cirrhosis: Decreased oncotic pressure due to hypoalbuminemia.
- Nephrotic syndrome: Massive proteinuria leading to hypoalbuminemia.
- Exudative Effusions: Result from local factors like inflammation or neoplasia, which increase capillary permeability.
- Common causes:
- Pneumonia (parapneumonic effusion): Inflammatory response increases vascular permeability.
- Malignancy: Lymphatic obstruction by tumor cells.
- Tuberculosis: Granulomatous inflammation increases pleural fluid production.
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 Presentation
The symptoms depend on the size and underlying cause of the effusion.
- Symptoms:
- Dyspnea: Most common symptom, especially with larger effusions.
- Pleuritic chest pain: Sharp, localized pain, worsened by breathing or coughing, typically seen in inflammatory causes.
- Cough: Often dry and non-productive.
- Physical Exam Findings:
- Dullness to percussion: Indicates fluid accumulation.
- Decreased breath sounds: Over the area of the effusion.
- Decreased tactile fremitus: Less vibration transmission due to fluid.
Diagnosis
Imaging
- Chest X-ray (CXR): Initial imaging study.
- Findings: Blunting of the costophrenic angles. Lateral decubitus X-ray helps differentiate free-flowing from loculated effusions.
- Ultrasound: Sensitive and can guide thoracentesis.
- CT scan: Detailed imaging, useful in identifying underlying causes such as malignancies.
Thoracentesis
Thoracentesis is the diagnostic gold standard for pleural effusions. Fluid is aspirated for analysis.
- Pleural Fluid Analysis:
- Protein and LDH: To differentiate between transudative and exudative effusions.
- Cell count and differential:
- Neutrophils: Suggest infection (e.g., parapneumonic effusion).
- Lymphocytes: Suggest tuberculosis or malignancy.
- Glucose: Low in infections or malignancy.
- pH: Low (<7.2) in complicated parapneumonic effusions or malignancy.
Key Points
- Pleural effusions occur due to abnormal fluid accumulation in the pleural space and are classified as transudative or exudative.
- Transudative effusions are caused by systemic factors (e.g., heart failure, cirrhosis), while exudative effusions result from local inflammation or malignancy.
- Diagnosis is made using Light’s criteria, with thoracentesis serving as the primary diagnostic procedure.
- Common symptoms include dyspnea and pleuritic chest pain, with imaging and fluid analysis guiding further management.