Recognizing Pneumothorax, Pneumomediastinum, Pneumopericardium, and Subcutaneous Emphysema

Chapter 8 Recognizing Pneumothorax, Pneumomediastinum, Pneumopericardium, and Subcutaneous Emphysema

Recognizing A Pneumothorax

A pneumothorax occurs when air enters the pleural space.

• When this occurs, the negative pressure normally present in the pleural space rises higher than the intralveolar pressure and the lung collapses.

• The parietal pleura remains in contact with the inner surface of the chest wall, but the visceral pleura retracts toward the hilum with the collapsing lung.

• The visceral pleura becomes visible as a thin, white line outlined by air on both sides, marking the outer border of the lung and indicating the presence of the pneumothorax. The visible visceral pleura is called the visceral pleural white line or simply the visceral pleural line.

You must be able to identify the visceral pleural line (Fig. 8-1) to make the definitive diagnosis of a pneumothorax!

Even as the lung collapses, it tends to maintain its usual lunglike shape so that the curvature of the visceral pleural line parallels the curvature of the chest wall; that is, the visceral pleural line is convex outward toward the chest wall (Fig. 8-2).

• Most other linear densities that mimic a pneumothorax do not demonstrate this spatial relationship with the chest wall.

There is usually, but not always, an absence of lung markings peripheral to the visceral pleural line.

Figure 8-1 Visceral pleural line in a pneumothorax.

You must see the visceral pleural line to make the definitive diagnosis of a pneumothorax (solid white arrows). The visceral and parietal pleurae are normally not visible, both normally lying adjacent to the lateral chest wall. When air enters the pleural space, the visceral pleura retracts toward the hilum along with the collapsing lung and becomes visible as a very thin, white line with air outlining it on either side. Notice how the contour of the pneumothorax parallels the curvature of the adjacent chest wall.

Figure 8-2 Pneumothorax seen on CT.

As the lung collapses, it tends to maintain its usual shape so that the curve of the visceral pleural line (solid white arrows) parallels the curve of the chest wall (dotted white arrows). This is important in differentiating a pneumothorax from artifacts or other diseases that can mimic a pneumothorax. As it collapses, the lung on the side of the pneumothorax also tends to remain lucent until the lung loses almost all of its normal volume, at which point it appears opaque. This patient also has subcutaneous emphysema—air in the soft tissues—of the left lateral chest wall (white stars). The patient had suffered a stab wound.

Pitfall: Pleural adhesions may keep part, but not all, of the visceral pleura adherent to the parietal pleura, even in the presence of a pneumothorax (Fig. 8-3).

Figure 8-3 Pneumothorax with pleural adhesions.

Lung markings may be visible on a conventional radiograph of the chest, distal to the visceral pleural line if pleural adhesions are present. In (A), a pneumothorax (solid white arrows) is prevented from collapsing the lung by pleural adhesions (solid black arrows). On a CT scan (B), the pleural adhesions (solid black arrows) are seen tethering the lung (solid white arrows) to the parietal pleura. Adhesions most frequently result from prior infection or blood in the pleural space.

• On conventional radiographs, it may be possible to visualize lung markings in front or in back of the pneumothorax and to overlook the presence of a pneumothorax because lung markings appear to extend to the chest wall.

• Absence of lung markings alone is not sufficient for the diagnosis of pneumothorax nor is the presence of lung markings distal to the visceral pleural line sufficient to eliminate the possibility of a pneumothorax.

The presence of an air-fluid interface in the pleural space is, by definition, an indication that a pneumothorax is present (see Fig. 6-14).

• For more about recognizing a hydropneumothorax, see Chapter 6, Recognizing a Pleural Effusion.

In the supine position, air in a relatively large pneumothorax may collect anteriorly and inferiorly in the thorax and manifest itself by displacing the costophrenic sulcus inferiorly while, at the same time, producing increased lucency of that costophrenic sulcus.

• This is called the deep sulcus sign and it is presumptive evidence for the presence of a pneumothorax on a supine chest radiograph (Fig. 8-4).

The key signs for recognizing a pneumothorax are summarized in Box 8-1

Figure 8-4 Deep sulcus sign.

In the supine position, air in a relatively large pneumothorax may collect anteriorly and inferiorly in the thorax and manifest itself by displacing the costophrenic sulcus inferiorly while, at the same time, producing increased lucency of that sulcus (solid black arrow). This is called the deep sulcus sign and is an indication of a pneumothorax on a supine radiograph. Notice how much lower the left costophrenic sulcus appears than the right sulcus (solid white arrow).

Recognizing the Pitfalls in Overdiagnosing A Pneumothorax

Several pitfalls can lead to the mistaken diagnosis of a pneumothorax.

Pitfall 1: Absence of lung markings mistaken for a pneumothorax.

• The simple absence of lung markings is not sufficient to warrant the diagnosis of a pneumothorax as other diseases produce such a finding.

• These diseases include:

• Bullous disease of the lung (Fig. 8-5)

• Large cysts in the lung

• Pulmonary embolism, which can lead to a lack of perfusion and hence a decrease in the number of vessels visible in a particular part of the lung (Westermark sign of oligemia).

• In none of these diseases would the treatment ordinarily include the insertion of a chest tube. In fact, insertion of a chest tube into a bulla might actually produce an intractable pneumothorax.

• Solution: Look at the contour of the structure you believe is the visceral pleural line. Unlike the margin of a bulla, the visceral pleural line will be convex outward toward the chest wall and will parallel the curve of the chest wall (Fig. 8-6).

Figure 8-5 Bullous disease, right upper lobe.

A thin, white line is visible on this close-up of the right upper lobe (solid white arrows) and no lung markings are seen peripheral to it. Unlike the visceral pleural line of a pneumothorax, this white line is convex away from the chest wall and does not parallel the curve of the chest wall. This is the classical appearance of a bulla in a patient with emphysema. It is important to differentiate between a pneumothorax and a bulla because inadvertently placing a chest tube into a bulla will almost always produce a pneumothorax which may be difficult to re-expand. The walls of several bullae are visible in this patient (dotted white arrows). On rare occasions, the bullae can grow so large as to render the hemithorax seemingly devoid of visible lung tissue (vanishing lung syndrome).

Figure 8-6 Bullous disease on right; pneumothorax on left.

This axial section from a chest CT demonstrates the different appearances of bullous disease, seen on the right with its border convex away from the chest wall (dotted white arrow), and a pneumothorax seen here on the left with its border convex toward and paralleling the chest wall (solid white arrow). This patient also has subcutaneous emphysema on the left (solid black arrow).

Pitfall 2: Mistaking a skin fold for a pneumothorax.

• When the patient lies directly on the radiographic cassette (as for a portable supine radiograph), a fold of the patient’s skin may become trapped between the patient’s back and the surface of the cassette.

• This can produce an edge in the expected position of the visceral pleural line which may, in fact, parallel the chest wall just as you would expect the visceral pleural line in a pneumothorax (Fig. 8-7).

• Solution: Unlike the thin, white line of the visceral pleura, skin folds produce a relatively thick, white band of density.

Figure 8-7 Skin fold mimicking a pneumothorax.

When patients lie directly on the radiographic cassette as they might for a portable, supine radiograph, a fold of the patient’s skin may become trapped between the patient’s back and the surface of the cassette. This can produce an edge (dotted white arrow) in the expected position of a pneumothorax, and that edge may parallel the chest wall just as you would expect a pneumothorax to do (A). Unlike the thin, white line of the visceral pleura in a different patient with a pneumothorax (solid white arrow—in B), skin folds produce relatively thick, white bands of density. A skin fold is an edge; the visceral pleura produces a line.

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Tags: Learning Radiology Recognizing the Basics

Mar 2, 2016 | Posted by admin in GENERAL RADIOLOGY | Comments Off

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