Skin fold versus pneumothorax

Abstract

Frequently, a fold of skin on the chest wall will create a curvilinear shadow which is cast over the lung parenchyma. This is usually recognized easily, although it can simulate a visceral pleural edge representing a pneumothorax. Luckily, these curvilinear shadows cross the chest in a location which obviously could not represent a slightly collapsed lung. Furthermore, lung markings can usually be seen to extend beyond these shadows, a finding which would usually exclude a pneumothorax. However, a fold of skin may be located peripherally, such as at the apex. In that instance the distal lung markings cannot be readily identified, making the differentiation from pneumothorax difficult. The visceral pleural edge and skin folds have identifiable characteristics that can be used for differentiation. When a pneumothorax occurs and the visceral pleura retracts from the chest wall, a thin distinct radiopaque curvilinear line can be seen. This line is the pleural edge (Fig. 1A). On the hilar aspect of this line is the relatively normal-appearing lung parenchyma. On the opposite side is a homogenous radiolucency produced by air in the pleural space. When viewing a skin fold projected over the periphery of the lung, a broad radiopaque curvilinear line is seen (fig. 1B). As in the pneumothorax, a homogenous density similar to normal-appearing lung parenchyma is present on the hilan aspect of this line. No sharp, thin opaque line is identified. On the chest wall side of this diffuse white line there is a 1-2 mm radiolucent band which then diffuses into normal lung density. This radiolucent band apparently represents a Mach band effect [1] and is also suggested adjacent to the pleural edge in the pneumo-