Radiological presentation of middle lobe atelectasis
When assessing the condition of the right middle lobe, primary attention is paid to changes in the frontal projection.
In the case of collapse (atelectasis), a specific triangular opacity is visualized on the radiograph. The main topographic feature of this shadow is that it directly abuts the right heart border. Due to the identical radiological density of the collapsed airless lung tissue and the heart muscle, their borders optically merge. This leads to loss of clarity of the right heart border contour, which is a leading diagnostic landmark.
Differential diagnosis: lower lobe collapse
To make an accurate diagnosis, the presentation of middle lobe atelectasis must be differentiated from lower lobe collapse, which has its own strict patterns in both frontal and lateral projections.
Right lower lobe collapse:
- Frontal projection: a triangular opacity is determined. Unlike other conditions, it has a specific relation to the diaphragmatic and cardiac silhouettes.
- Lateral projection: the typical picture includes increased density (opacity) in the posterior costophrenic angle. The contour of the posterior parts of the right diaphragmatic dome is lost in this process.
Left lower lobe collapse:
- Frontal projection: a triangular opacity forms, located behind the heart shadow. Meanwhile, the contour of the medial part of the left diaphragmatic dome is lost (not traceable on the image).
- Lateral projection: an increase in density is noted in the posterior regions (similar to changes in right-sided lower lobe collapse). The silhouette of the posterior parts of the left diaphragmatic dome is also lost.
General signs of lower lobe collapse
In addition to local opacities, lower lobe collapse is accompanied by a number of general radiological phenomena associated with changes in the architectonics of the thoracic cavity.
- Invisibility of the lower lobe pulmonary artery. On the frontal radiograph, this vessel is usually not visualized. The reason is that the artery is no longer contrasted (surrounded) by the adjacent aerated lung tissue.
- Airway displacement. Major bronchi, including the right and left main bronchi, change their normal anatomical direction and shift to a more vertical position.
- Air bronchogram sign. In the zone of the triangular opacity of the collapsed lobe, the corresponding air-containing bronchus leading directly to the lesion zone can often be identified.
Additional signs (superior mediastinum)
In the diagnosis of lower lobe atelectasis, specific symptoms reflecting secondary changes in the superior mediastinum provide substantial assistance.
- «Superior triangle sign». This sign occurs exclusively in collapse of the right lower lobe. It represents a triangular opacity located to the right of the mediastinum. The mechanism of its formation is due to the displacement of anterior junctional structures. It is important to remember: this radiological picture must categorically not be confused with right upper lobe collapse.
- «Flat waist sign». This phenomenon is observed in extensive collapse of the left lower lobe. Its essence lies in the characteristic flattening of the contours of the aortic knuckle and the pulmonary trunk.
Briefly: answers to topical questions
What does atelectasis of the right middle lobe look like on a lateral chest radiograph?
On a lateral radiograph, right middle lobe atelectasis appears as a wedge-shaped consolidation or a triangular shadow. The apex of this opacity is directed toward the lung hilum, and the consolidation itself extends anteriorly from it. In mild collapse, the collapsed lobe is also clearly visualized on the image.
Which bronchopulmonary segments form the right middle lobe?
The right middle lobe is formed by two bronchopulmonary segments ventilated by their corresponding segmental bronchi. According to the nomenclature of the British Thoracic Society, these include:
- Lateral segment (Lateral bronchus) — designated as the 4th segment.
- Medial segment (Medial bronchus) — designated as the 5th segment.
These segments project overlapping one another on standard radiographs.
What is the physical mechanism of the air bronchogram sign in atelectasis?
The physical mechanism of air appearing in the bronchi during atelectasis is associated with gas penetration into the collapsed lobe. Air can enter the periphery of the bronchial tree via collateral air drift. In addition, the cause of gas inside the collapsed lobe may be tumor necrosis. As a result, an air-containing bronchus leading directly into the zone of triangular opacity of the collapsed lung tissue is identified on images.
Why is the contour of the medial part of the left diaphragmatic dome lost in left lower lobe collapse?
In left lower lobe collapse, a triangular opacity is formed behind the heart shadow, and the contour of the medial part of the left diaphragmatic dome is lost.
What pathological conditions, other than lower lobe collapse, should middle lobe atelectasis be differentiated from?
Right middle lobe atelectasis must be differentiated from the following pathological conditions and anatomical features:
- Lingular segment atelectasis (Lingular Collapse) — on the lateral radiograph, the picture resembles middle lobe collapse.
- Fat pads (Fat pads) — pronounced fat accumulations on the right can mimic middle lobe pathology.
- Pneumonia (Pneumonia) — unlike atelectasis, inflammatory consolidation does not show intense contrast enhancement on CT.
- Pleural effusion (Pleural effusion) — with massive fluid accumulation, the opacity is more homogeneous, and the mediastinum is shifted to the contralateral side.
Class and exam questions
Why is the lower lobe pulmonary artery not visible on a frontal radiograph during lower lobe collapse?
The artery becomes invisible because it is no longer contrasted against the background of the adjacent aerated lung tissue (the tissue has collapsed and become airless).
What is the «flat waist sign» and when does it occur?
This is the flattening of the contours of the aortic knuckle and pulmonary trunk. It is observed in extensive collapse of the left lower lobe.
What is the «superior triangle sign» associated with?
It occurs in right lower lobe collapse and is caused by the displacement of anterior junctional structures, resulting in a triangular opacity to the right of the mediastinum.
What else to study on this topic
- Radioanatomy of the lobes and segments of the right lung
- Air bronchogram sign: physical and radiological rationale
- Differential diagnosis of right upper and lower lobe collapse
- Biomechanics of main bronchi displacement in lobar atelectasis
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