A 47 year old man falls 4m onto a wall, hitting his left chest wall. He is complaining of chest pain and you wonder whether there is a pneumothorax.
View 2: Longitudinal 3rd interspace
View 3: Longitudinal left chest 4th interspace anterior axillary line
View 4: Longitudinal right chest 2nd interspace mid axillary line
Describe and interpret these scans
Image 1: Longitudinal left chest 2nd interspace mid axillary line. Air has spread along the interpectoral fascial plane. It obscures the view of deeper structures including the pleura.
Image 2: Longitudinal 3rd interspace further laterally. At this point the air not a continuous sheet but small foci. The pleural line with lack of sliding can be see through it in places.
Image 3: Longitudinal left chest 4th interspace anterior axillary line. At this point there is no subcutaneous emphysema and the ribs and pleural line can be seen. Although the chest wall (ribs and intercostals) moves with each respiration there is no lung sliding seen at the pleural line confirming pneumothorax.
Image 4: Longitudinal right chest 2nd interspace mid axillary line. Normal chest wall anatomy and normal lung sliding is seen on the right. This means at the point of examination there is no air between the parietal and visceral pleural surfaces.
Subcutaneous emphysema and pneumothorax.
Subcutaneous or surgical emphysema is commonly seen in trauma and usually associated with rib fractures or penetrating trauma and underlying pneumothorax. It can also occur spontaneously or after barotrauma with pneumomediastinum and can be seen with or without pneumothorax.
Air within soft tissues has a diverse appearance and this depends on the size and surface of the air pocket being observed.
In Image 1 air has tracked along the fascial plane between the pectoral muscles, creating a flat sheet of air through which ultrasound energy does not pass. As the layer of air is flat and not moving it can mimic the appearance of pneumothorax, complete with lack of sliding and horizontal reverberation artefactual lines similar to A-lines.
Air will obscure structures lying deep to it and in this case the ribs, deeper muscles of the chest wall, and the pleural surface are obscured. As we explore the chest wall further laterally the sheet of air becomes small foci and glimpses of the parietal pleura are seen lying below. Further laterally still there is no subcutaneous air and the parietal pleural surface is well seen without any sliding we are able to confirm the presence of pneumothorax.
The take home message is recognize air lying in the soft tissues and understand you cannot see what is lying deep to an air interface. Gentle firm pressure sometimes moves small amounts of subcutaneous emphysema allowing a glimpse of what lies below. Alternatively use other windows to approach deeper structures, or alternative imaging modalities.