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.