Causes and mechanisms of development
Globally, lobar collapse is divided into two groups: that occurring due to endobronchial obstruction (internal or external) and that developing without obstruction.
- In adults, the leading causes are tumors and mucous plugs.
- In children, foreign body aspiration and mucous plugs come to the forefront, whereas neoplasms are extremely rare.
Collapse due to endobronchial obstruction:
- Internal causes: bronchogenic carcinoma, bronchial carcinoid, metastases (breast cancer, melanoma), lymphoma, benign tumors (lipoma, hamartoma), granulomatoses (tuberculosis, sarcoidosis).
- External causes: hilar or mediastinal lymphadenopathy, mediastinal masses, aortic aneurysms, and marked cardiomegaly.
Collapse without endobronchial obstruction: Occurs as a passive process against the background of pleural effusion or pneumothorax, as well as a result of radiation therapy or replacement of pulmonary parenchyma with tumor tissue (e.g., in adenocarcinoma). A distinct type is rounded atelectasis (rounded atelectasis) — a specific sublobar collapse associated with prior exudative pleurisy and visceral pleural thickening.
Direct signs of volume loss
The cardinal signs are increased density of the affected lobe and loss of its volume. The effect of collapse is most pronounced on immediately adjacent structures.
- Displacement of interlobar fissures (displacement of fissures). This is the most reliable sign.
- Crowding of pulmonary vessels and bronchi. One of the earliest markers, which is easily detected when compared with the patient's previous radiographs.
- Hilar alterations (hilar alterations). In upper lobe collapse, classical elevation of the hilum is observed. In lower lobe collapse, the hilum is described as a «small hilum» (small hilum): the airless lobe obscures the lower lobe pulmonary artery, and the margin of the interlobar artery ceases to form the border on the frontal projection.
- Change in the position of large bronchi. In upper lobe collapse, the ipsilateral main bronchus assumes a more horizontal position, and the bronchus intermedius deviates laterally. When the lower lobe is affected, the bronchi assume a vertical position.
Indirect signs
Indirect symptoms occur due to compensatory displacement and overexpansion of adjacent anatomical structures.
- Compensatory hyperinflation. Adjacent lobes overinflate, leading to a paucity of the pulmonary pattern (vascular attenuation). With large volume losses, the contralateral lung may cross the midline, forming a so-called mediastinal hernia (displacement of the anterior junctional or azygoesophageal line).
- Mediastinal shift. Assessed by the position of the trachea and the cardiac contour. The greatest shift is observed in lower lobe collapse, and the least in right middle lobe involvement.
- Luftsichel sign (Luftsichel sign). Characteristic of left upper lobe collapse. It appears as a paramediastinal lucency caused by the overinflated segment of the lower lobe filling the space between the aortic arch and the collapsed tissue.
- Juxtaphrenic peak (juxtaphrenic peak). A small triangular opacity on the diaphragmatic dome. It occurs due to volume loss in the anterior portions of the upper lobe and traction on the accessory minor fissure.
Role of CT in the diagnosis of lobar atelectasis
CT is the gold standard of diagnostic imaging, allowing the elimination of structural superimposition. The main indication in adults is the search for an endobronchial or compressive mass.
Differentiation between tumor and collapsed lung: On non-contrast scans, it is difficult to distinguish between a tumor and atelectasis. Following contrast administration, the collapsed lung tissue enhances more intensely than the tumor nodule. The optimal scanning window when the density difference is maximized ranges from 40 seconds to 2 minutes.
CT Semiologic Features:
- Air bronchogram. While the presence of air bronchograms on a radiograph helps rule out central cancer (favoring pneumonia), on CT this sign is not a reliable criterion for the absence of an obstructing tumor.
- CT Equivalent of Golden's Sign. Golden's S sign (Golden’s S sign) is a combination of a concave contour of the collapsed lobe and a central convexity created by the tumor itself. On CT, the combination of a retracted interlobar fissure and local bulging strictly indicates bronchogenic carcinoma.
- Limitations of the Method. False-positive diagnoses of obstruction can be caused by strictures, mucous plugs, or marked compression due to pleural effusion.
Briefly: answers to topical questions
What types of atelectasis are distinguished according to the pathogenetic mechanism of development?
According to the etiological mechanism of development, two main groups of atelectasis are distinguished: those occurring due to endobronchial obstruction and those without it.
- Obstructive or post-stenotic — develops due to internal (tumors, mucous plugs, foreign bodies) or external (lymphadenopathy, mediastinal masses) bronchial obstruction.
- Compressive or passive — occurs without obstruction as a result of external compression of the lung tissue (e.g., due to pleural effusion, pneumothorax, or a bulla).
What specific radiological signs are characteristic of rounded atelectasis?
Rounded atelectasis (Rounded atelectasis or Round Atelectasis) is a specific type of sublobar parenchymal collapse associated with thickening of the adjacent visceral pleura. This condition is characterized by the following features:
- Pleural thickening — thickening of the adjacent visceral pleura; this condition may be associated with prior exudative pleurisy.
- Peripheral localization — changes are most often detected in the lung periphery, predominantly in the dorsal regions of the lower lobes.
- «Folded lung» sign (folded lung) — a synonym for rounded atelectasis.
- Characteristic CT semiotics.
Golden's S sign is easy to visualize: the central part of the letter «S» (the convexity) represents the obstructing tumor itself, while the peripheral part (the concavity) represents the retracted, collapsed lung lobe.
Class and exam questions
Why does a lung lobe appear dense in atelectasis?
The collapsed lobe becomes opaque not only due to a decrease in aeration. The dominant process is the retention of secretions and fluid within the lobe, which causes an increase in density even with partial collapse.
Can an air bronchogram rule out cancer?
On a plain chest radiograph, the presence of an air bronchogram often allows central obstruction to be excluded. However, on CT, the presence of peripheral air lucencies within the bronchi does not rule out main bronchus cancer.
How to reliably differentiate a tumor from atelectasis on CT?
Using bolus contrast enhancement. Collapsed lung tissue enhances more actively and rapidly compared to the tumor. Scanning should be performed at 40–120 seconds.
What else to study on this topic
- Rounded atelectasis and its specific CT semiotics
- Shifting granuloma sign
- Mediastinal hernia: radiological signs of lung displacement across the midline
- Anatomic variants of pulmonary hilum displacement in lesions of different lobes
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