Can pneumonia cause pleural effusions?

Can Pneumonia Cause Pleural Effusions?

Yes, pneumonia commonly causes pleural effusions, with parapneumonic effusions developing in up to 40-60% of hospitalized pneumonia patients, making it one of the most frequent complications of bacterial pneumonia. 1, 2, 3

Incidence and Clinical Significance

• Parapneumonic effusions occur in 40-60% of all hospitalized pneumonia cases, though many small effusions remain undetected and underreported in clinical series 2, 4, 3
• The actual prevalence may be even higher than traditionally reported, as small parapneumonic effusions (<10mm) are frequently missed on standard chest radiography 1
• Approximately 5-10% of patients with parapneumonic effusions will progress to empyema, requiring more aggressive intervention 2, 3
• Patients with parapneumonic effusions have significantly higher morbidity and mortality compared to those with pneumonia alone 5, 2

Pathophysiology

• Pleural effusions develop when there is an imbalance between pleural fluid formation and drainage, with the lymphatic system becoming overwhelmed 6
• In pneumonia, local inflammatory changes increase capillary permeability, leading to fluid accumulation in the pleural space 6
• As infection progresses, fibrin deposition occurs, leading to septation and loculation of the fluid 6
• The pleural fluid pH and glucose levels fall while lactate dehydrogenase (LDH) levels increase, indicating progression from simple to complicated parapneumonic effusion 5, 4

Clinical Presentation

Patients with parapneumonic effusions are typically more unwell than those with simple pneumonia, presenting with: 1, 7

• Persistent high fever despite appropriate antibiotic therapy (most common indicator) 1, 7
• Pleuritic chest pain, with patients often lying on the affected side to splint the hemithorax 1
• Unilateral decreased chest expansion 1, 7
• Dullness to percussion over the affected area 1, 7
• Reduced or absent breath sounds 1, 7
• Possible scoliosis on examination 1, 7

Diagnostic Approach

If a patient remains febrile or unwell 48 hours after admission for pneumonia, parapneumonic effusion must be actively excluded through careful clinical examination and repeat chest radiography. 1

Imaging Strategy:

• Chest radiography should be obtained first, looking for obliteration of the costophrenic angle or meniscus sign of fluid ascending the lateral chest wall 1
• Ultrasound is the gold standard for confirming pleural fluid presence, quantifying size, and identifying internal characteristics (septations, loculations, complex fluid) 6, 1
• Ultrasound has 92% sensitivity and 93% specificity for detecting effusions, superior to CT for characterization 6
• CT chest with IV contrast is indicated when evaluating for complications such as necrotizing pneumonia, lung abscess, or empyema 6

Pleural Fluid Analysis:

All parapneumonic effusions should be aspirated for diagnostic purposes, which may require ultrasound guidance if the effusion is small or loculated. 5

Critical pleural fluid parameters include: 5, 4

• pH < 7.20: Indicates need for immediate drainage
• Glucose < 40-60 mg/dL: Suggests complicated effusion requiring drainage
• LDH > 1,000 IU/L: Consider drainage, individualize based on clinical course
• Positive Gram stain or culture: Immediate drainage required
• Purulent appearance: Immediate drainage required

Management Algorithm

Drain any parapneumonic effusion meeting at least one of the following criteria: 1, 5

1. Size ≥ 1/2 of the hemithorax
2. Loculations present on imaging
3. Pleural fluid pH < 7.20
4. Pleural fluid glucose < 60 mg/dL
5. Positive pleural fluid Gram stain or culture
6. Purulent appearance

Specific Management by Severity:

• Small effusions (<10mm rim): Treat with antibiotics alone without drainage 1
• pH 7.00-7.20 or LDH 1,000-3,000: Consider tube thoracostomy; serial pleural fluid studies may guide decision 4
• pH < 7.00 or glucose < 40 mg/dL: Immediate tube thoracostomy required 4
• Organized empyema with persistent sepsis: Early surgical consultation for possible thoracoscopy or decortication 6, 5

Common Pitfalls

• Underestimating effusion size on standard chest radiography—always use ultrasound for accurate assessment 6, 1
• Delaying thoracentesis in febrile patients—waiting beyond 48 hours of persistent fever increases risk of progression to empyema 1
• Relying solely on radiographic appearance—pleural fluid analysis is essential to distinguish simple from complicated effusions 5, 4
• Missing bilateral effusions—while uncommon in bacterial pneumonia, bilateral effusions should prompt consideration of tuberculosis or other systemic causes 6

Prognosis

• The prognosis in children with parapneumonic effusions is usually excellent, with the majority making complete recovery and lung function returning to normal 1
• Adults with appropriately managed parapneumonic effusions generally have good outcomes, though delayed drainage results in prolonged hospitalizations and increased mortality 8
• Residual pleural fibrosis may cause mechanical pleuritic pain for weeks to months but does not indicate active infection 9