A pneumothorax does not display classical signs when a patient is positioned supine for a chest radiograph as commonly occurs in acute trauma or the critical care setting. Of course, pneumothoraces are common in both these scenarios and even relatively small pneumothoraces may be significant due to the inherent risks of worsening from starting mechanical ventilation.
An erect chest radiograph has a sensitivity as high as 92% for detection of a pneumothorax, whilst a supine projection may only detect 50% .
Instead, the pneumothorax may be demonstrated by looking for the following signs:
- relative lucency of the involved hemithorax
- deep, sometimes tongue-like, costophrenic sulcus: deep sulcus sign
- increased sharpness of the adjacent mediastinal margin and diaphragm
- increased sharpness of the cardiac borders or diaphragm
- visualization of the anterior costophrenic sulcus: double diaphragm sign
- visualization of the inferior edge of the collapsed lung above the diaphragm
- depression of the ipsilateral hemidiaphragm
- lumpy appearance of the cardiac contour representing change in shape of the pericardial fat: pericardial fat tag sign
The subtle signs of a supine pneumothorax may be simulated by other pathologies :
- deep lateral costophrenic angle mimics deep sulcus sign e.g. in COPD
- unilateral radiolucent hemithorax due to air trapping or technique
- a subpulmonic pneumothorax may be mimicked by several entities
Usually chest CT or POCUS is done to clarify if there is any diagnostic doubt, although historically additional radiographic projections, e.g. lateral decubitus or apicolordotic, were often performed.