Occult pneumothorax is a pleural air collection that is seen only on CT. It is not detected by standard chest xray either because of small size, location of the air, or position of the patient during xray (usually supine).
Approximately 15% of major trauma patients undergoing CT are diagnosed with an occult pneumothorax. The tough question is, what to do about it. Larger pneumothoraces are frequently treated with thoracostomy, but this procedure has its own list of associated complications. Patients undergoing positive pressure ventilation with a visible pneumothorax have an increased risk for progression to tension pneumothorax.
At our trauma center, we manage occult pneumothorax expectantly. If a pneumothorax is seen on the chest portion of a CT scan but not on the initial supine chest xray, a repeat conventional chest xray is scheduled for 6 hours later. Ideally, this xray is taken using the best technique (upright, PA, xray source 6ft from patient). However, this is not always practical for severely injured patients.
If the pneumothorax remains occult on the followup xray, no further monitoring is performed. If the pneumothorax becomes visible, repeat chest xrays are obtained every 6 hours until it is stable or it becomes large enough to warrant insertion of a chest tube.
How large is large enough for a chest tube? That’s the subject for another day.