Lobar Collapse Tutorial

Introduction

Contents

• Introduction
• The silhouette sign
• The causes of collapse
• General features of lobar collapse
• The normal PA chest X-ray
• The normal lateral film
• Extent of lobes on normal films
• Right upper lobe collapse
• Right lower lobe collapse
• Right middle lobe collapse
• Left lower lobe collapse
• Left upper lobe collapse
• Total collapse of a lung
• Pleural effusions

The silhouette sign

Borders, outlines and edges seen on plain radiographs depend on the presence of two adjacent areas of different density. Roughly speaking, only four different densities are detectable on plain films; air, fat, soft tissue and calcium (five if you include contrast such as barium). If two soft tissue densities lie adjacent, then they will not be visible separately (eg the left and right ventricles, or any internal structures of the heart). If, however, they are separated by air, the boundaries of both will be seen. This has applications elsewhere too; the reason renal outlines are visible on plain films is the presence of perinephric fat between the kidneys and the surrounding soft tissues.

The silhouette sign has two uses:

• It can localise abnormalities on a PA film without a lateral view. If a mass lies overlaps the aortic knuckle and the outline of the knuckle is lost (air is no longer adjacent to it), then the mass lies posteriorly, against the knuckle (which represents the posteriorly placed arch and descending aorta). If the edges of the mass and the knuckle are both visible, then the mass lies anteriorly.

• The loss of clarity of a structure, such as the hemidiaphragm or heart border, suggests there is adjacent soft tissue shadowing, such as consolidated lung, even when the abnormality itself is not clearly visualised. This is particularly valuable in some cases of lobar collapse.

The causes of collapse

Although there are many causes for collapse, the most frequent are listed below:

• The commonest cause is a proximal stenosing bronchogenic carcinoma, which occludes a bronchus. Patients are middle aged or elderly, and almost always smokers.

• In a young adult or older child asthma is by far the commonest cause. Collapse occurs secondary to mucous plugging of the major airways, and responds to physiotherapy.
• In an infant consider an inhaled foreign body, such as a peanut.

• Retention of secretions is a frequent cause of post operative collapse.

• In ventilated patients, including neonates, collapse may occur if the endotracheal tube is inserted too far, entering one main bronchus and occluding the other.

General features of lobar collapse

• Tracheal displacement towards the side of the collapse.
• Mediastinal shift towards the side of the collapse.
• Elevation of the hemidiaphragm.
• Reduced vessel count on the side of the collapse.
• Herniation of the opposite lung across the midline.
• A hilar mass, which also suggests carcinoma as the cause.
• Other evidence of malignant disease (eg. rib metastases, lymphangitis, effusion)
• The presence of a foreign body; however these are rarely easy to see.
• The presence of an endotracheal tube; is it sited too low?

The normal PA Chest X-ray

Note the mediastinal contours (from upper left); the aortic arch (1), pulmonary trunk (2), left atrial appendage (3), left ventricle (4), right ventricle (5), superior vena cava (6). The hemidiaphragms (7 & 8) are also sharply outlined, though the medial part on the left is not always clear where it lies against the heart.

The horizontal fissure is visible on about 50% of films (9). The oblique fissures are not normally visible on the frontal film as they are not tangential to the X-ray beam.

The normal lateral film

This is less frequently obtained, and often harder to interpret. The diaphragms are again seen. The one furthest from the film cassette is magnified and so extends further back.

One or both oblique fissures (1) and the horizontal fissure (2) are usually visible. The thoracic spine should appear progressively darker as one looks more inferiorly, and the retrocardiac space (3) and retrosternal space (4) should be relatively dark, and of similar density.

Extent of lobes on normal films

PA view: extent of upper and lower lobes. Note the extensive overlap. The lower lobes extend considerably higher than many realise.

Lateral view: extent of lobes on the lateral view (note that the left and right sides are in practice superimposed upon one another).