Pneumothorax Detection on Plain Chest X-Ray
When pneumothorax is suspected but not confirmed on standard postero-anterior (PA) chest radiographs, lateral radiographs provide added diagnostic information in up to 14% of cases 1. This means approximately 14% of pneumothoraces are missed on initial PA chest X-ray alone.
Key Diagnostic Limitations
The sensitivity of plain chest radiography for pneumothorax detection is notably limited:
In trauma settings, the miss rate is substantial: Studies show that 67% of pneumothoraces in blunt trauma patients are visible only on CT scan and not on chest X-ray 2. In another trauma cohort, chest X-ray sensitivity for pneumothorax was only 39.1% 3.
"Occult pneumothoraces" (those not visible on supine chest X-ray but detected on CT) account for 52-63% of all traumatic pneumothoraces 4, with one study finding 69% of diagnosed pneumothoraces were occult 5.
Retrospective review by radiologists identified only 12-24% of occult pneumothoraces when re-examining chest X-rays, even when specifically looking for subtle signs 6.
Clinical Approach When Suspicion Remains High
If clinical suspicion for pneumothorax is high despite a normal PA chest radiograph, obtain a lateral or lateral decubitus radiograph 1. The BTS guidelines specifically state this approach is clinically helpful when PA findings are unclear 1.
Additional imaging considerations:
Lateral decubitus radiographs are superior to erect or supine chest radiographs and are considered as sensitive as CT scanning for pneumothorax detection 1
Expiratory radiographs add little value and are not recommended as routine investigation, even for suspected small apical pneumothorax 1
CT scanning is indicated when:
- Differentiating pneumothorax from bullous lung disease
- Lungs are obscured by surgical emphysema
- Standard radiographs remain unclear despite high clinical suspicion 1
Common Pitfalls
The most commonly missed radiographic sign is the deep sulcus sign (75-90% of identified occult pneumothoraces) 6. Other subtle findings include the double diaphragm sign 4. These signs require careful scrutiny and are frequently overlooked even by experienced radiologists.
Clinical symptoms are unreliable indicators of pneumothorax size 1, so symptomatic patients warrant intervention regardless of radiographic appearance. In secondary pneumothorax patients, even small pneumothoraces have significant clinical implications and warrant additional imaging if not clearly visible 1.
The inter-observer agreement for detecting occult pneumothoraces on chest X-ray is poor (kappa 0.55-0.56) 6, emphasizing the need for additional imaging modalities when clinical suspicion persists despite negative initial radiographs.