Chest X-ray in Lung Collapse

Collapse:

Partial or complete loss of volume of a lung is referred to as collapse or atelectasis. This contrasts with consolidation in which a diminished volume of air in the lung is associated with normal lung volume.

Mechanisms of collapse:

1. Relaxation or passive collapse:

This is the mechanism whereby the lung tends to retract toward its hilum when air or increased fluid collects in the pleural space.

2. Cicatrisation collapse:

Normal lung expansion depends upon a balance between outward forces in the chest wall and opposite elastic forces in the lung. When the lung is abnormal stiff, this balance is disturbed, lung compliance is decreased and the volume of the affected lung is reduced. This occurs with pulmonary fibrosis.

3. Adhesive collapse:

The surface tension of the alveoli is decreased by surfactant. If this mechanism is disturbed as in the respiratory distress syndrome, collapse of alveoli occurs, although the central airways remain patent.

4. Resorption collapse:

In acute bronchial obstruction, the gases in the alveoli are steadily taken up by the blood in the pulmonary capillaries and are not replenished, causing alveolar collapse. The degree of collapse may be modified by collateral air drift if the obstruction is distal to the main bronchus, and also by infection and accumulation of secretions. If the obstruction becomes chronic, subsequent resorption of intra-alveolar secretions, and exudate may result in complete collapse. This is the ususal mechanism of collapse seen in carcinoma of the bronchus.

Radiological signs of collapse:

The radiographic appearance in pulmonary collapse depends upon the mechanism of collapse, the degree of collapse, the presence or absence of consolidation, and the pre-existing state of the pleura. Signs of collapse may be considered as direct or indirect. Indirect signs are the results of compensatory changes which occur in response to the volume loss.

Direct signs of  collapse:

1. Displacement of interlobar fissures:

This is the most reliable sign and the degree of displacement will depend on the extent of the collapse.

2. Loss of aeration:

Increased density of a collapsed area of lung may not become apparent until collapse is almost complete. However, if the collapsed lung is adjacent to the mediastinum or diaphragm, obscuration of the adjacent structures may indicate loss of aeration.

3. Vascular and bronchial signs:

If a lobe is partially collapsed, crowding of its vessels may be visible, if an air bronchogram is visible, the bronchi may be crowded.

Indirect signs of collapse:

1. Elevation of the hemidiaphragm:

This sign may be seen in lower lobe collapse, but is rare in collapse of the other lobes.

2. Mediastinal displacement:

In upper lobe collapse, the trachea is often displaced toward the affected side, and in lower lobe collapse, the heart may be displaced.

3. Hilar displacement:

The hilum may be elevated in upper lobe collapse and depressed in lower lobe collapse.

4. Compensatory hyperinflation:

The normal part of the lung may become hyperinflated, and it may appear hypertransradiant with its vessels more widely spaced than in the corresponding area of the contralateral lung. If there is considerable collapse of a lung, compensatory hyperinflation of the contralateral lung may occur, with herniation across the midline.

Patterns of collapse:

An air bronchogram is almost never seen in resorption collapse, but is usual in passive and adhesive collapse, and may be seen in cicatrisation collapse if fibrosis is particularly dense. Pre-existing lung disease such as fibrosis and pleural adhesions may alter the expected displacement of anatomic landmarks in lung collapse. There also tends to be a reciprocal relationship between the compensatory signs, e.g. in lower lobe collapse, if diaphramatic elevation is marked, hilar depression will be diminished.

Complete collapse of a lung:

Complete collapse of a lung in the absence of pneumothorax or large pleural effusion or extensive consolidation, causes opacification of the hemithorax, displacement of the mediastinum to the affected side and elevation of the diaphragm. compensatory hyperinflation of the contralateral lung occurs, often with herniation across the midline. Herniation most often occurs in the retrosternal space, anterior to the ascending aorta, but may occur posterior to the heart or under the aortic arch.

Lobar collapse:

a. Right upper lobe collapse:

The normal horizontal fissure is usually at the level of the right fourth rib anteriorly. As the right upper lobe collapses, the horizontal fissure pivots about the hilum, its lateral end moving upward and medially toward the superior mediastinum, and its anterior end moving upward toward the apex. The upper half of the oblique fissure moves anteriorly. The two fissures become concave superiorly. In severe collapse, the lobe may be flattened against the superior mediastinum, and may obscure the upper pole of the hilum. The hilum is elevated, and its lower pole may be prominent. Deviation of the trachea to the right is usual, and compensatory hyperinflation of the right middle and lower lobes may be apparent.

b. Right middle lobe collapse:

In right middle lobe collapse, the horizontal fissure and lower half of the oblique fissure move toward one another. This can best be seen in the lateral projection. The horizontal fissure tends to be more mobile, and therefore usually shows greater displacement. Signs of right middle lobe collapse are often subtle on the frontal projection since the horizontal fissure may not be visible, and increased opacity does not become apparent until collapse is almost complete. However, obscuration of the right heart border is often present, and may be the only clue in this projection. The lordotic AP projection brings the displaced fissure into the line of the X-ray beam, and may elegantly demonstrate right middle lobe collapse. Since the volume of this lobe is relatively small, indirect signs of volume loss are rarely present.

c. Lower lobe collapse:

The normal oblique fissures extend from the level of the fourth thoracic vertebra posteriorly to the diaphragm close to the sternum anteriorly. The position of these fissures on the lateral projection is the best index of lower lobe volumes. When a lower lobe collapses, its oblique fissure moves posteriorly but maintains its normal slope. In addition to posterior movement, the collapsing lower lobe causes medial displacement of the oblique fissure, which may then become visible in places on the frontal projection.

d. Right lower lobe collapse:

This causes depression and medial rotation of the hilum, elevation of the right hemidiaphragm and hyperinflation of the right upper lobe. A completely collapsed lower lobe may be so small that it flattens and merges with the mediastinum, producing a thin, wedge-shaped shadow. On the left, this shadow may be obscured by the heart and a penetrated view with a grid may be required for its visualization.

e. Lingula collpase:

The lingula is often involved in collapse of the left upper lobe, but it may collapse individually, when the radiological features are similar to right middle lobe collapse. However, the absence of a horizontal fissure on the left makes anterior displacement of the lower half of the oblique fissure and increased opacity anterior to it important signs. On the frontal projection the left heart border becomes obscured.

f. Left upper lobe collapse:

The pattern of upper lobe collapse is different in the two lungs. Left upper lobe collapse is apparent on the lateral  projection as anterior displacement of the entire oblique fissure, which becomes oriented almost parallel to the anterior chest wall. With increasing collapse the upper lobe retracts posteriorly and loses contact with the anterior chest wall. The space between the collapsed lobe and the sternum becomes occupied by either hyperinflated left lower lobe or herniated right upper lobe. With complete collapse, the left upper lobe may lose contact with the chest wall and diaphragm and retract medially against the mediastinum. On a lateral film, therefore, left upper lobe collapse appears as an elongated opacity extending front the apex and reaching, or almost reaching, the diaphragm: it is anterior to the hilum and is bounded by displaced oblique fissure posteriorly, and by hyperinflated lower lobe anteriorly.

A collapsed left upper lobe does not produce a sharp outline on the frontal view. An ill-defined hazy opacity is present in the upper, mid and sometimes lower zones, the opacity being densest near the hilum. Pulmonary vessels in the hyperinflated lower lobe are usually visible through the haze. The aortic knuckle is usually obscured, unless the upper lobe has collapsed anterior to it, allowing it to be outlined by lower lobe. If the lingula is involved, the left heart border is obscured. The hilum is often elevated, and the trachea is often deviated to the left.

Rounded atelectasis:

This is an unusual form of pulmonary collapse which may be misdiagnosed as a pulmonary mass. It appears as a homogeneous mass upto 5 cm in diameter, with ill-defined edges. It is always pleural based and associated with pleural thickening. Vascular shadows may be seen to radiate from part of the opacity, resembling a comet's tail. The appearance is caused by peripheral lung tissue folding in on itself. It is often related to asbestos exposure, but may occur secondary to any exudative pleural effusion.