Radiologic patterns of lobar atelectasis

INTRODUCTION — Atelectasis describes the loss of lung volume due to the collapse of lung tissue. Radiologic findings characteristic of atelectasis are reviewed here. The appearance of atelectasis on a chest radiograph is emphasized, but its appearance on computed tomography (CT) is also described. The types and pathogenesis of atelectasis are discussed separately. (See "Atelectasis: Types and pathogenesis in adults".)

LOBAR ATELECTASIS — Radiologic signs of lobar atelectasis can be categorized as direct or indirect [1-5]. Direct signs include increased opacification of the airless lobe and displacement of fissures. Indirect signs include displacement of hilar and cardiomediastinal structures toward the side of collapse, narrowing of the ipsilateral intercostal spaces, elevation of the ipsilateral hemidiaphragm, compensatory hyperexpansion and hyperlucency of the remaining aerated lung, and obscuration or desilhouetting of the structures adjacent to the collapsed lung (eg, diaphragm and heart borders). On CT scans with intravenous contrast material, atelectatic lung enhances markedly (image 1 and image 2) and much more than nonaerated lung due to pneumonia (mean values of 119 versus only 62 Hounsfield units [HU]) [6]. Additional radiologic features vary according to the site of atelectasis.

Right upper lobe — In right upper lobe atelectasis, the collapsed lobe tends to shift cephalad and medially, producing a triangular apical opacity on the frontal view of a chest radiograph (image 3 and image 4 and image 5). The opacity may be subtle when the right upper lobe is completely collapsed (especially if chronic), forming only a thin right apical cap. These findings are accompanied by elevation of the right hilum and the minor fissure, with the latter displaying a convex contour cranially [2]. A juxtaphrenic peak (ie, a steeple-shaped pleural retraction above the diaphragm) may also be present (image 6) [7]. This finding is due to tenting of the ipsilateral diaphragmatic pleura at either the base of the inferior accessory fissure or, less likely, at the insertion site of the inferior pulmonary ligament.

These radiologic findings are different depending upon the orientation and shift of the upper lobe collapse.

●Medial collapse of the right upper lobe may mimic a right paratracheal mass on the frontal view of a chest radiograph.

●In contrast, lateral collapse, also called peripheral atelectasis, may generate a peripheral mass-like opacity that mimics a loculated pleural effusion (image 7 and image 8) [8,9]. This is generally accompanied by superior and medial hyperexpansion of the right middle and right lower lobe.

When right upper lobe atelectasis is due to a central mass with or without an enlarged interlobar lymph node, the minor fissure may have a lateral upward convexity and a medial caudal convexity (similar to the inverted letter S) on the frontal view of a chest radiograph (image 3 and image 4 and image 5). This configuration of the minor fissure is called the inverted S-sign of Golden and is suggestive of a neoplastic etiology causing atelectasis of the right upper lobe.

On a CT scan, the atelectatic right upper lobe appears plastered medially against the mediastinum and the minor fissure has a shape that is concave laterally. This appearance is characteristic of a right upper lobe collapse, even though it may occasionally be difficult to distinguish from a right paratracheal mass [10].

Right middle lobe — Collapse of the right middle lobe tends to have little impact on surrounding structures because the right middle lobe accounts only for approximately 10 percent of total lung volume.

●On the frontal view of a chest radiograph, the usual finding is a small triangular opacity with its apex pointing laterally. In some patients, however, an absent contour of the right heart border is the only hint that right middle lobe atelectasis is present (image 9). A lordotic frontal view can enhance the visibility of the atelectatic middle lobe by lengthening the path of the x-ray beam through opaque lung tissue [2].

●On the lateral view of a chest radiograph, the typical finding is an obliquely oriented triangular opacity with its apex pointing toward the hilum (image 10). This appearance results from anterosuperior shift of the major fissure and posteroinferior displacement of the minor fissure [11].

The right middle lobe has a greater tendency to collapse than the other lobes because of three anatomic factors: it has less collateral ventilation (since it is surrounded by two fissures), a collar of lymph nodes surrounds the origin of the right middle lobe bronchus and can extrinsically compress the lumen if enlarged, and the right middle lobe bronchus is long, thin, and curved, resulting in increased ventilatory resistance. Lung cancer can occlude the right middle lobe bronchus and invade the actual collapsed lobe (image 11).

Acute right middle lobe collapse is common during the early postoperative period following right upper lobectomy (image 12 and image 13 and image 14). The collapsed right middle lobe appears radiographically as an irregular opacity in the right upper paramediastinal region because of the change in lobar configuration and postoperative lung distribution. It can also mimic right upper lobe atelectasis, if the history of right upper lobe lobectomy is not known (image 15) [12]. Chronic right middle lobe atelectasis (often called right middle lobe syndrome) is relatively common. When it occurs, it is frequently nonobstructive and accompanied by scarring and bronchiectasis (Lady Windermere Syndrome) [13]. Chronic right middle lobe collapse is particularly common among elderly women with atypical mycobacterial infection (eg, Mycobacterium avium complex, MAC) (image 16) [14].

On a CT scan, right middle lobe atelectasis appears as a triangular opacity with its apex pointing laterally and with its medial contour apposed against the right heart border (image 17 and image 18) [10]. This configuration has been likened to an ice cream cone.

Right lower lobe — The right lower lobe retracts posteromedially and inferiorly when it collapses because it is tethered to the mediastinum by the hilar structures and the pulmonary ligament. As a result, an early sign of right lower lobe collapse is increased visibility of the major fissure on the frontal chest radiograph. This occurs because the inferior displacement and slight rotation of the major fissure during the early collapse orients the major fissure tangentially to the x-ray beam.

As right lower lobe collapse progresses, it initially forms a triangular or wedge-shaped opacity that obscures the right lower lobe pulmonary artery (image 19) and eventually forms a right paraspinal mass that projects behind the right atrium. The superior mediastinal structures shift to the right and form a superior right paratracheal triangular opacity with an apex that points toward the right hilum (the superior triangle sign) (image 20) [11]. The dome of the right hemidiaphragm is often not obscured because the hyperexpanded right middle lobe is abutting the right hemidiaphragm, preserving its contour. On the lateral view of the chest radiograph, the posterior third of the right hemidiaphragm is obscured by the collapsed right lower lobe.

Right lower lobe collapse that occurs in the presence of a pleural effusion or pneumothorax can lead to an atypical collection of subpulmonic pleural effusion or a posteromedial pneumothorax, respectively. These abnormalities are a consequence of right lower lobe retraction causing an increase in the focal relative negative pressure in the adjacent pleural space, which causes a so-called suction cup effect [15].

On a CT scan, a completely atelectatic right lower lobe can mimic a paraspinal mass (image 21 and image 22 and image 23). Air bronchograms, if present, may reveal the true nature of the space occupying lesion [10].

Left upper lobe — An atelectatic left upper lobe tends to retract anteriorly, producing a faint, veil-like opacity in the left upper hemithorax on the frontal view of a chest radiograph (image 24). This opacity may be mistaken for pleural thickening seen en face. Additional findings include the following: the trachea is deviated to the left, the main pulmonary artery and the upper contour of the left pulmonary artery are obscured, the left hilar structures are retracted cephalad, and the pulmonary vessels of the hyperexpanded left lower lobe appear as an array of linear and tubular opacities that are distinctly different from the normal distribution of the left upper lobe vasculature. The left heart border may be obscured if the lingula is atelectatic. A large left pericardial fat pad can mimic isolated lingular atelectasis on chest radiography but CT scanning can display the fat content of the pseudoatelectasis (image 25). Similar to right upper lobe atelectasis, a left-sided juxtaphrenic peak can be observed, although less frequently than on the right side.

Whenever the left upper lobe is atelectatic, the left lower lobe expands and its superior segment occupies most of the left upper hemithorax, mimicking an aerated upper lobe. The left lower lobe basal pulmonary arteries become elevated and clearly visible in the retrocardiac space. In the roughly 50 percent of individuals who have a complete major fissure, the atelectatic left upper lobe moves medially towards the mediastinum. In the remaining individuals, a tongue of aerated lower lobe is pulled forward by the atelectatic upper lobe and becomes interposed between the atelectasis and the aortic arch (image 24). This creates a crescent of aerated lung (ie, Luftsichel), which is an indirect sign of left upper lobe atelectasis [16,17].

On the lateral view of a chest radiograph, the major fissure is markedly displaced anteriorly and the atelectatic left upper lobe forms a narrow crescent adjacent to the anterior chest wall. A hyperexpanded anterior segment of the right upper lobe can shift across the midline into the retrosternal clear space, sharply outlining the anterior contour of the ascending aorta (image 26).

On a CT scan, the collapsed left upper lobe is seen in apposition to the anterior chest wall and anterior mediastinum, with the aerated lung tissue of the right upper lobe interposed between the ascending aorta and the collapsed left upper lobe (image 27) [10]. Cicatrizing left upper lobe atelectasis with bronchiectasis after necrotizing pneumonia or granulomatous disease is illustrated in the figure (image 28).

Left lower lobe — The left lower lobe retracts posteromedially and inferiorly when it collapses. As a result, findings of left lower lobe atelectasis on the frontal view of a chest radiograph include increased retrocardiac opacity, obscuration of the left lower lobe vessels, of the lateral contour of the descending aorta, and left hemidiaphragm, caudal displacement of the left hilum, and levorotation (ie, leftward rotation) of the cardiac silhouette with flattening of the cardiac waist [18]. The latter observation is called the flat waist sign, which describes the loss of the normal concavity of the upper left heart border. Additional findings in left lower lobe collapse include the left major fissure being parallel to the left heart border and partial obliteration of the aortic arch (top of the "knob" sign) due to leftward superior mediastinal shift (image 29A-B). A completely atelectatic left lower lobe can mimic a left paraspinal mass.

Collapse of the left lower lobe is frequently seen after cardiac surgery. This is probably multifactorial, including cold saline cardioplegia with damage to the left phrenic nerve, compression of the left lower lobe by an enlarged heart, possible postoperative contusion, and retention of secretions in the supine position.

On a CT scan, the atelectatic lobe is displaced posteromedially and inferiorly (image 30). Plate-like atelectasis in the lingula is occasionally seen because compensatory hyperexpansion of the left upper lobe reorients the lingular bronchus (Nordenstrom sign) (image 31) [19,20]. Compressive atelectasis of the left lower lobe occurs with marked elevation of the left hemidiaphragm, either due to congenital eventration of the left hemidiaphragm or left phrenic nerve paralysis (image 32).

Passive, segmental atelectasis involving the basal segments of the left lower lobe can be seen with localized pleural effusions (image 33).

MULTILOBAR ATELECTASIS — Atelectasis of more than one lobe may have atypical radiologic features. This can lead to errors in interpretation, as well as misdiagnoses [4,7,21-23].

Right middle and lower lobes — The most common combined multilobar atelectasis involves the right middle and right lower lobe. On the frontal view of a chest radiograph, it is characterized by a caudally displaced right minor fissure forming a straight interface between the hyperexpanded right upper lobe and the atelectatic right middle lobe, which may be difficult to distinguish from an elevated right hemidiaphragm or a subpulmonic pleural effusion (image 34A-B). In addition, the major fissure shifts inferomedially with subsequent caudal displacement of the right hilar structures. Obscuration of the hilar structures, basal pulmonary vessels, heart border, and hemidiaphragm are observed in the right hemithorax (image 35A-B and image 36 and image 1). The right upper lobe demonstrates compensatory hyperexpansion with relative oligemia.

The relatively high frequency of combined right middle and right lower lobe atelectasis is explained by their common air supply via the bronchus intermedius: a solitary obstructing lesion will affect the aeration of both lobes and lead to their combined atelectasis. Common obstructing lesions include mucous plugs, bronchogenic carcinoma, foreign bodies, and carcinoid tumors. Rare obstructing lesions include endobronchial tuberculosis, histoplasmosis, broncholithiasis, hamartomas, and inflammatory pseudotumors.

Combined right middle and right lower lobe atelectasis has been described after lung transplantation in the setting of ischemia of the bronchus intermedius as failure of bronchial revascularization of the donor airway. This occurrence has been referred to as the vanishing bronchus intermedius syndrome [24].

Right upper and middle lobes — The combination of right upper and right middle lobe atelectasis is less common. On the frontal view of a chest radiograph, the right upper and middle lobes are shifted anteriorly against the chest wall and medially against the mediastinum. This creates a veil-like opacity, which effaces the ascending aorta and right heart border, and then fades laterally (image 37A-C and image 38). The opacity's appearance is reminiscent of an elephant’s ear. In addition, the right hilum is shifted cephalad and the mediastinum is displaced to the right. The right lower lobe is hyperexpanded, with its superior segment occupying the apex of the right hemithorax. This hyperexpansion displaces the right major fissure anteriorly and medially. A hyperexpanded left upper lobe may extend across the midline and a juxtaphrenic peak (ie, a steeple-shaped pleural retraction above the diaphragm) may be present. On the lateral view of a chest radiograph, the combination of right upper and right middle lobe atelectasis can occasionally mimic an anterior mediastinal mass.

Combined right upper and right middle lobe atelectasis occurs less frequently than combined right middle and right lower lobe atelectasis. This is because the former requires simultaneous occlusion of two lobar bronchi by either two lesions or a large lesion that encompasses the takeoff of the right upper and right middle lobe bronchi, while sparing the bronchus intermedius. Potential causes of the obstruction include bronchogenic carcinoma, metastatic tumors, granulomatous infections, and inflammatory myofibroblastic tumors (plasma cell granulomata). These lesions are usually more extensive and more infiltrative then the single lesions that can lead to combined right middle and right lower lobe atelectasis.

Right upper and lower lobes — Combined right upper and right lower lobe atelectasis is the rarest type of multilobar atelectasis. It requires two separate synchronous obstructing lesions since the lobar bronchi arise independently and are separated by both the bronchus intermedius and the right middle lobe bronchus. A large pleural effusion can lead to passive atelectasis of both the upper and lower lobes.

Radiographically, the findings are similar to those described separately for right upper lobe atelectasis and right lower lobe atelectasis, except that they occur simultaneously (image 39). The right middle lobe is hyperexpanded to fill the right hemithorax. Potential causes are those that can affect the right lower and right upper lobe bronchi simultaneously: mucous plugs, enlarged lymph nodes, and cicatrization from prior pneumonia as well as passive atelectasis caused by a large pleural effusion (image 2).

ATELECTASIS OF AN ENTIRE LUNG — Obstruction of the right or left mainstem bronchus can lead to atelectasis of the entire right or left lung, respectively. On the frontal view of a chest radiograph, a totally collapsed lung causes opacification of the hemithorax with ipsilateral cardiomediastinal shift (image 40A-C). The ipsilateral cardiomediastinal shift distinguishes the atelectasis from a massive pleural effusion with complete opacification of one hemithorax since the latter causes a contralateral cardiomediastinal shift. On the lateral view of a chest radiograph, the cardiac silhouette, one hemidiaphragm, and one hilum are obscured.

The complete opacification and loss of volume of the atelectatic lung, as well as the rotational shift of the cardiomediastinal structures toward the atelectatic lung, are better seen on a CT scan than on a chest radiograph. The cause of the obstruction can be elucidated on a CT scan as well (image 41 and image 42).

SEGMENTAL ATELECTASIS — Two major types of segmental or subsegmental atelectasis are distinguished: plate-like atelectasis and rounded atelectasis.

Plate-like atelectasis — Plate-like atelectasis (also called discoid, linear, or subsegmental atelectasis) is probably the most common, yet least understood, type of atelectasis. Its appearance is usually band-like and it is frequently perpendicular to a pleural surface. Plate-like atelectasis is frequently associated with scars, pleural infoldings or clefts, and accessory fissures, rather than bronchial obstruction [25].

Plate-like atelectasis is frequently seen in patients with low lung volumes due to hypoventilation, elevation of the diaphragm caused by eventration, phrenic nerve paralysis, or massive ascites; pulmonary embolism or early bronchopneumonia can lead to plate-like atelectasis as well.

Patients with plate-like atelectasis tend to have poorer respiratory function than would be expected based upon the size and volume of the plate-like atelectasis; this suggests that plate-like atelectasis is a consequence of persistent low lung volumes with ventilation-perfusion mismatch, rather than a cause of the poor respiratory function. Concurrent alveolitis (with occlusion of the pores of Kohn) and obstruction of a small bronchus may contribute to the formation of plate-like atelectasis. (See "Atelectasis: Types and pathogenesis in adults".)

Rounded atelectasis — Rounded atelectasis (historically called folded lung, atelectatic pseudotumor, or Blesovsky syndrome) presents as a subpleural mass, out of which emanates a swirl of vessels and bronchi that curve like a comet tail as they connect the atelectatic lung parenchyma to the hilum (image 43A-B and picture 1 and image 44 and image 45 and image 46) [26-28]. It is usually located in the lower lobes, lingula, or right middle lobe.

Rounded atelectasis can be diagnosed on the basis of radiologic findings alone. These include the characteristic subpleural mass with comet tail (also called the vacuum cleaner effect) associated with pleural plaques, diffuse pleural thickening, and/or an existing or resolving pleural effusion, as well as accompanying loss of volume in the adjacent lung and concurrent adjacent accumulation of extrapleural adipose tissue [29]. In rare cases with chronic inflammation of the adjacent pleura, a small systemic artery originating from the aorta, intercostal or bronchial arteries can supply the rounded atelectasis. If the characteristic comet tail is not identified, a biopsy becomes necessary to rule out a malignant tumor [28]. Characteristic rounded atelectasis with all of the above-mentioned findings does not have to be biopsied and does not need rigorous follow-up with CT scanning.

Rounded atelectasis can persist for years, can clear spontaneously, or, in rare cases, can grow. Approximately 70 percent of cases of rounded atelectasis are associated with previous asbestos exposure. Rounded atelectasis has also been reported in association with pleural tuberculosis [30]. (See "Asbestos-related pleuropulmonary disease".)

SUMMARY AND RECOMMENDATIONS

●Definition – Atelectasis describes the loss of lung volume due to the collapse of lung tissue. (See 'Introduction' above.)

●Lobar atelectasis – On contrast-enhanced CT scans, atelectatic lung can enhance markedly. The radiologic signs of lobar atelectasis can be categorized as direct and indirect. (See 'Lobar atelectasis' above.)

•Direct signs include the increased opacification of the airless lobe and the displacement of fissures.

•Indirect signs include the displacement of hilar and cardiomediastinal structures toward the side of collapse, narrowing of the ipsilateral intercostal spaces, elevation of the ipsilateral hemidiaphragm, compensatory hyperexpansion and hyperlucency of the remaining aerated lung, and obscuration of the structures adjacent to the collapsed lung (eg, diaphragm and heart borders).

Additional radiologic features vary according to the site of atelectasis.

●Multilobar atelectasis – Atelectasis may involve more than one lobe of the lung. The most common combination of multilobar atelectasis involves the right middle and right lower lobes. However, the combination of right upper and right middle lobe atelectasis, as well as the combination of right upper and right lower lobe atelectasis, can occur. Each combination of multilobar atelectasis has characteristic imaging findings. (See 'Multilobar atelectasis' above.)

●Whole-lung atelectasis – Obstruction of the right or left mainstem bronchus can lead to atelectasis of the entire right or left lung, respectively. This appears as opacification of the hemithorax with ipsilateral cardiomediastinal shift on a chest radiograph. (See 'Atelectasis of an entire lung' above.)

●Plate-like and rounded atelectasis – Two major types of segmental or subsegmental atelectasis are recognized: plate-like atelectasis and rounded atelectasis. (See 'Plate-like atelectasis' above and 'Rounded atelectasis' above.)