Bilateral skin folds mimicking pneumothoraces
Mild head injury. Septic screen.
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AP erect film. Heart is not enlarged. Lungs and pleural spaces clear.
On the left are two vertical lines paralleling the lateral chest wall and on the right a single line. These appearances are consistent with prominent bilateral skin folds which may mimic lung edges as seen with pneumothoraces. Lung markings are clearly visible peripheral to the skin folds. No subcutaneous emphysema. No rib fractures. No evidence of hemothorax.
Bilateral degenerative disease of the ACJs and glenohumeral joints.
Skin folds projected over the lungs on a frontal chest radiograph is a common finding. Usually differentiation from a pneumothorax is straightforward:
- line appearance
- lung edge is formed by a very narrow sharp line
- skin fold is formed by a thicker radiopaque line and the radiopacity fades in the direction of the hilum - this is an example of the Mach effect 1
- visibility of peripheral lung markings
- no lung markings peripheral to a lung edge in pneumothorax
- lung markings peripheral to a skin fold
- caveat: in the apex it may not be possible to see lung markings even in a normal patient
If there is any doubt then a film performed in expiration may help as a pneumothorax tends to look larger on an expiratory CXR. Ultrasound can be used to diagnose pneumothorax but it is rarely performed as few operators have the requisite experience. Rarely CT may be required.
This is important as I have personally seen thoracotomy tubes placed for non-existent pneumothoraces when skin folds were erroneously felt to represent lung edges.
Skin folds are commoner in elderly patients and on AP projections.