What is Subsegmental Atelectasis
Subsegmental atelectasis is a partial collapse of lung tissue affecting areas smaller than a complete bronchopulmonary segment, appearing as linear or platelike opacities on chest imaging and representing one of the most common radiographic findings in clinical practice. 1
Definition and Anatomic Scope
Subsegmental atelectasis refers to collapsed and non-aerated regions of lung parenchyma that are smaller than a complete anatomic segment. 2 This type of atelectasis is also described as "platelike," "linear," or "discoid" atelectasis in radiologic terminology. 1
- The collapsed areas typically manifest as horizontal linear densities on chest radiographs, most commonly in the lung bases. 1
- Unlike lobar or segmental atelectasis which involves larger anatomic units, subsegmental atelectasis affects smaller portions of lung tissue. 2
Clinical Context and Significance
Subsegmental atelectasis is particularly common in specific clinical scenarios and should not be automatically dismissed as clinically insignificant:
- In bronchiolitis, chest radiographs frequently show evidence of subsegmental atelectasis due to airway edema, sloughing of respiratory epithelium, and generalized hyperinflation coupled with poorly developed collateral ventilation. 3
- In chronic lung disease of infancy, recurrent subsegmental atelectasis may indicate central airway obstruction and can be associated with failure to wean from mechanical ventilation. 3
- In bronchiolar diseases, subsegmental atelectasis may be an indirect HRCT finding suggesting small airways disease, particularly when larger airway abnormalities are absent. 3
Mechanisms of Formation
The development of subsegmental atelectasis occurs through several pathophysiologic mechanisms:
- Absorption atelectasis occurs when alveolar air is resorbed distal to obstructing lesions in small airways. 1
- Compression from pleural effusions or other space-occupying processes can cause collapse of subsegmental lung units. 4
- Adhesive atelectasis results from surfactant deficiency or dysfunction affecting smaller lung units. 1
- In perioperative settings, up to 15-20% of lung bases regularly collapse during anesthesia, often manifesting as subsegmental atelectasis. 5
Radiographic Identification
Direct and indirect signs help distinguish subsegmental atelectasis from other pathology:
- Direct signs include crowded pulmonary vessels and minimal displacement of interlobar fissures (less pronounced than in larger atelectasis). 1
- Subsegmental atelectasis appears as linear or platelike opacities, typically horizontal and measuring 1-3 mm in thickness. 1
- On HRCT, subsegmental atelectasis may be an indirect feature of bronchiolar disease, particularly when seen with mosaic attenuation on expiratory scans. 3
Common Clinical Pitfalls
A critical caveat is that subsegmental atelectasis should not be automatically equated with pneumonia:
- The diagnosis of atelectatic pneumonia requires clinical signs and symptoms of infection plus identification of pathogenic bacteria in respiratory specimens, not just radiographic atelectasis alone. 1
- Subsegmental atelectasis can be a non-specific finding in pulmonary embolism and should not distract from this diagnosis. 4
- In bronchiolitis specifically, the presence of subsegmental atelectasis does not justify chest physiotherapy, as multiple trials have shown no clinical benefit from vibration, percussion, or passive expiratory techniques. 3