An occult pneumothorax is one that is seen only on CT scan, but not on conventional chest x-ray. They are noted in somewhere between 2% and 10% of major blunt trauma patients. Although management is usually conservative, this has not been well studied.

A paper was presented at the AAST earlier this year based on a prospective, multicenter trial. The authors attempted to determine what factors were predictive of failure of observation. They were able to quantify the size of the pneumothorax by measuring a line from the largest collection to the chest wall.

Sixteen centers participated and enrolled 569 patients, who had 588 occult pneumothoraces. Of those, 21% had immediate chest tube drainage (no reasons were given). The remaining 448 patients were observed, and 27 of those patients failed. Failure was determined if they had progression of the pneumothorax, developed respiratory distress, or developed a hemothorax. 

Risk factors were found to be: positive pressure ventilation (14% of observed group failed), size > 7mm, respiratory distress. 

The authors recommend that patients with respiratory distress and those placed on positive pressure ventilation have a drainage system inserted. Those with pneumothoraces greater than 7mm bear close watching.

Our practice is to monitor any patient with an occult pneumothorax with a followup chest x-ray (one view only) performed after six hours. If the pneumo is still not visible, no further observation is done. If it becomes visible, serial 6 hour x-rays are obtained until it is stable or requires a chest tube.

Reference: Management of blunt traumatic occult pneumothorax: is observation harmful? Results of a prospective multicenter study. Forrest O Moore, et al. Paper #5 presented at 69th Annual AAST Meeting, September 22, 2010.