Tag Archives: occult

Using Chest CT To Detect Occult Injuries

There are major belief systems when it comes to the use of trauma CT: selective scan vs pan scan. The selective scanners believe that too much radiation can be bad, and that the risk of excess exposure outweighs the value of scanning everything. The pan scanners believe that valuable information might be missed unless they routinely image everything.

Who is right? There’s probably value in each side of the argument. But do we have data? Good data? Two emergency medicine groups from UC-Irvine and UC-San Diego tried to answer this question via a prospective study involving 10 Level I trauma center EDs in California.  They tagged onto data collection underway for the NEXUS chest and chest CT studies from 2009-2012.

Patients with fresh (< 24 hours) blunt trauma who underwent chest imaging in the ED were included. Patients needed to have both CT scan and chest x-ray within 24 hours, at the discretion of the emergency physician. Weirdly, they skewed their sample by enrolling patients from 7am to 11pm daily due to availability of research personnel.

The researchers were looking for minor and major interventions necessitated by data discovered on the CT scan. Occult injuries were defined as clinically important if an intervention occurred because of it. Major interventions included surgery, mechanical ventilation for pulmonary contusion, or chest tube for hemo- and pneumothoraces.

Here are the factoids:

  • Nearly six thousand patients were enrolled, and 2,048 had at least one injury identified on either study
  • A total of 1,454 of these injuries (71% of injuries, but only 25% of patients) were occult, only being seen on the CT scan
  • Chest x-ray found all injuries in only 29% of patients (not surprising)
  • When pulmonary contusion was seen by CT only, 6% were placed on ventilators; when hemo- or pneumothorax were seen, 41% and 29% respectively had chest tubes inserted (wow!)
  • The authors tallied 241 major interventions for occult injury in 202 patients, 154 chest tubes for hemo/pneumothorax and/or mechanical ventilation, 9 operations for diaphragm or aortic injury, and the remainder appear to be for other chest wall fractures

The authors concluded that occult injuries were found in 71% of their patients, with the majority of those “requiring” chest tubes. They recognized some of the shortcomings in their study and stopped short of recommending a pan-scan type approach to major chest trauma.

Bottom line: This argument always boils down to diagnostic yield vs money vs radiation. Radiologists like to find as many things as they can, so CT is great. For me, it always comes back to that old saying: “if a tree falls in the woods when no one is around, does it make a sound?” 

The corollary is “if a diagnosis is found on CT that is not clinically relevant, do we care?” But wait, you say, they did have to intervene. Or did they?

Have you ever scanned a chest and seen something that makes you intubate the patient immediately and put them on a ventilator? Probably not. It’s a clinical judgement. The scan may make you a bit more wary, but you will still wait for some clinical signs that the patient needs that extra help. 

And what about chest tube insertion? I’m sure most of you have seen a modest pneumothorax on chest x-ray (1 cm away from the chest wall, extending to the 6th intercostal space, say). Ho hum. And then you get a CT scan and your eyes widen. It always looks much larger on the scan. It always does. Yet the patient is still lying there comfortably, with normal oxygen saturations. Do you really need to put a tube in? For decades, we used only the x-ray, and patients did fine.

So I don’t buy that the CT result made them do the interventions. It was the clinician’s choice based on how they interpreted the scan, not the patient’s clinical condition. Without specific guidelines that determine when interventions are indicated, it just boils down to “I do an intervention when I think the patient needs it.” And every clinician will have their own criteria and thresholds. It’s tough to learn from things done this way.

So I stick by my guns. We know that chest x-ray is flawed. But it does provide good clinical data even without a bunch of diagnostic minutiae. A good practice guideline that helps select the patients most likely to benefit from a CT scan is paramount.

As you can probably tell, I’m a selective scan kind of guy and still have not run across a study that is clean and compelling enough to make me change. And I think I’ll be waiting for a while for one of those to pop up!

Reference: Prevalence and clinical import of thoracic injury identified by chest computed tomography but not chest radiography in blunt trauma: multicenter prospective cohort study. Ann Emerg Med 66(6):589-600, 2015.

Is It Safe to Watch Occult Pneumothorax in Ventilated Patients?

An occult pneumothorax is one that is visible on chest CT but not conventional chest xray. The pneumo can be a single bubble, or it can be a larger one that layers out over the lung but cannot be seen on plain xray. This air is generally watched for a period of time, typically 6 hours, then a repeat plain radiograph is obtained to see if it has become visible. 

The pneumothorax literature cautions us about watching visible pneumothoraces in patients who are placed on positive pressure ventilation. The rationale is that this may force more air out of an acutely injured lung, resulting in an enlarging pneumothorax. Many have recommended that a chest tube be placed in any patient with a visible pneumothorax on positive pressure ventilation to avoid the possibility of developing a tension pneumothorax.

But what about the occult pneumothorax? Since they are generally very small, do they pose the same risk? A paper from 2008 retrospectively reviewed 79 patients with occult pneumothorax , 20 of whom were placed on ventilators. 51 of 59 of the non-ventilated patients had no change in their occult pneumo (86%), while 16 of 20 of the ventilated patients had no progression (80%).

The study numbers are small, but suggest that occult pneumothoraces can be safely watched. The real question is, how long do you have to watch it? Typically, ventilated patients get regular chest xrays, so monitoring for progression of the pneumo should be easy.

Reference: American Surgeon 74(10):958, 2008.

Factors Predicting Failure of Observation of Occult Pneumothorax

An occult pneumothorax is defined as one that is seen on CT scan, but not on plain chest x-ray. It is a common finding in blunt trauma that is evaluated using CT 2-12% of scans), but there is no consensus on management. It is recognized that some of these progress and require insertion of a chest tube, while many can be observed safely. The authors try to define what factors predict the need for chest tube management.

The authors reviewed their experience over a 3 year period, and identified 642 patients (10% of their registry entries) with a pneumothorax. 283 were occult, and 98 ultimately received a chest tube.

They found that age>35, ISS>24, more than 4 rib fractures, and need for positive pressure ventilation increased the risk for chest tube insertion. These seem to make sense, but there was one significant limitation in this study: there were no standard indications for a chest tube insertion among the surgeons involved with these patients. There was significant variability, so the actual need for tube insertion was probably less than reported.

An audience member related one anecdotal factor for chest tube as well: a heavy smoking history. This makes intuitive sense, but not everything that makes sense is borne out by research.

At Regions, we define an occult pneumothorax the same way these authors did. We routinely get a delayed chest xray 6 hours later. If there is still no visible pneumothorax, we stop looking. If it is visible, we will obtain periodic (q12-24 hrs) xrays until it stabilizes or grows to a size that demands tube or pigtail insertion.

Given the data conveyed in this paper, we will consider watching a bit longer than 6 hours in patients at higher risk.

Reference: Factors Predicting Failed Observation of Occult Pneumothorax in Blunt Trauma. Selander, Minshall, couillard, Leon. Medical University of South Carolina.

Presented at the 23rd Annual Scientific Assembly of the Eastern Association for the Surgery of Trauma