Causes of Unilateral Atelectasis on Chest X-ray
The most common causes of unilateral atelectasis identified on chest X-ray include airway obstruction (mucus plugging, foreign body, tumor), compression from pleural effusion or masses, and post-surgical changes. 1, 2
Primary Mechanisms and Classification
- Atelectasis can occur through three main mechanisms: airway obstruction, compression of lung parenchyma, and increased surface tension in alveoli 2
- Obstructive atelectasis results from blockage of airways by mucus plugs, foreign bodies, tumors, or broncholithiasis (calcified lymph nodes eroding into airways) 1, 3
- Compressive atelectasis occurs due to pleural effusion, pneumothorax, or space-occupying lesions in the chest 1, 3
- Adhesive atelectasis stems from surfactant deficiency, commonly seen in neonatal respiratory distress syndrome 1
- Passive atelectasis can result from diaphragmatic dysfunction or hypoventilation 3
- Cicatrization atelectasis develops from pulmonary fibrosis 3
Common Specific Causes
- Mucus impaction is a frequent cause, particularly in patients with limited mobility or underlying lung conditions 4, 2
- Malignancy (especially lung cancer) can cause atelectasis through direct airway obstruction or associated pleural effusion 5, 1
- Pleural effusion is a significant cause of compressive atelectasis 1, 3
- Foreign body aspiration should be considered, especially with persistent cough and localized atelectasis 1
- Post-surgical atelectasis is common, particularly after thoracic or upper abdominal procedures 5
- Rounded atelectasis (folded lung) can develop from infolding of thickened visceral pleura, most commonly associated with asbestos exposure 1
Radiographic Features
- Direct signs of atelectasis include crowded pulmonary vessels, crowded air bronchograms, and displacement of interlobar fissures 3
- Indirect signs include pulmonary opacification, elevation of the diaphragm, shift of mediastinal structures, displacement of the hilum, and compensatory hyperexpansion of surrounding lung 3
- Rounded atelectasis presents with a characteristic "comet sign" that is pathognomonic, more visible on HRCT than plain films 1
- Complete "white-out" of the affected lung may be seen in severe cases 4
Diagnostic Considerations
- Chest X-rays in both anterior-posterior and lateral projections are essential to document atelectasis 2
- Differentiation from lobar consolidation (pneumonia) can be challenging and may require clinical correlation 2, 3
- When atelectasis is associated with pleural effusion, consider tuberculosis and malignancy as potential underlying causes 5, 6
- In HIV patients with atelectasis and pleural effusion, consider Kaposi's sarcoma, parapneumonic effusions, and tuberculosis 6
- The diagnosis of atelectatic pneumonia should be based on clinical signs and symptoms of infection plus identification of pathogenic bacteria, not just radiographic findings 3
Management Approaches
- Treatment depends on the underlying cause, duration, and severity of atelectasis 2
- Conservative measures include chest physiotherapy, postural drainage, and bronchodilator therapy 2
- Persistent mucus plugs should be removed by bronchoscopy 4, 2
- For refractory cases with hypoxemia, more aggressive interventions may be needed, such as differential lung ventilation in critical care settings 7
- Recruitment maneuvers may temporarily reverse atelectasis, though long-term benefits vary 1
- When atelectasis is associated with pleural effusion, diagnostic thoracentesis should be performed to determine the underlying cause 8
- In cases of persistent undiagnosed pleural effusion with atelectasis, always reconsider tuberculosis and pulmonary embolism as they are treatable disorders 8, 6