Exercise: Pleural Abnormalities

EXERCISE 4-12. PLEURAL ABNORMALITIES

4-20. The most likely diagnosis in Case 4-20 (Figure 4-59 A,B) is

A.   pulmonary embolism.

B.   overinflation associated with asthma.

C.   pneumothorax.

D.   normal chest, with a skin fold projected over the right hemithorax.

E.   left lower lobe atelectasis.

Radiologic Findings

4-10. In Figure 4-59, there is increased radiolucency in the periphery of the right hemithorax. On the close-up of the right lung (Figure 4-60 A), there is a thin white line (arrows) paralleling, but displaced from, the right lateral chest wall. The thin line represents the visceral pleura. There is air-filled lung medial to this thin white line, and there is air within the pleural space lateral to this line. Note the absence of pul-monary vessels lateral to the pleural line (C is the cor-rect answer to Question 4-20).

Discussion

Pneumothorax is the presence of air in the pleural space. The lung collapses away from the chest wall because of its normalelastic recoil. In some instances, a ball valve mechanism is present, and air continues to enter the pleural space and further collapses the lung and displaces the mediastinum away from the side of the pneumothorax. The relationship of the air in the pleural space to the lung and chest wall can be clearly seen on the CT scan of a patient with a right pneumothorax (Figure 4-60 B). Note that air rises to the highest point in the thorax, the anterior thorax in a supine patient and the lung apex in an upright patient. The visceral pleura covering the lung is visible as a thin white line on both chest radiographs and CT scans. No pulmonary vessels may be seen extending beyond the pleural line, and the air in the pleural space appears more radiolucent than the ad-jacent lung.

The most common mimic of a pneumothorax, particu-larly in a supine patient, is a skin fold. The image receptor for portable AP chest radiographs is placed behind the pa-tient’s back. Skin folds may be pressed between the pa-tient’s back and the receptor. Radiographically, a skin fold produces an interface, or an edge of thick tissue outlined by the greater radiolucency of the superimposed lung (Figure 4-61). If you can distinguish an edge from a line, then you can distinguish a skin fold from a pneumothorax. The ab-sence of pulmonary markings beyond the pleural line is supporting evidence for a pneumothorax. Because the vessels taper as they approach the lung periphery, the ves-sels in the extreme periphery of the lung may be too tiny to see.

Pneumothorax is considered spontaneous if it occurs in the absence of trauma (including barotrauma). Sponta-neous pneumothorax may be primary and occur in the ab-sence of significant other lung disease, or it may occur secondarily because of lung disease. Apical blebs are present in a high percentage of patients with primary sponta-neous pneumothorax, and their rupture is thought to be the most frequent cause of spontaneous pneumothorax. For unknown reasons, it occurs most frequently in tall young men. Secondary spontaneous pneumothorax may occur in association with any cavitary lesion that lies in the periphery of the lung, as well as in emphysema, in bullous disease, and in pulmonary fibrosis of a variety of etiologies.