Detection of Small Slit-Like Anterior Basilar Pneumothorax on Standard CXR
A small slit-like left anterior basilar pneumothorax may be missed on a standard upright posteroanterior (PA) chest radiograph, but can often be detected with additional imaging techniques.
Why Standard CXR May Miss This Finding
The location you describe—anterior and basilar—is particularly challenging for standard upright PA chest radiographs. Air in a pneumothorax rises to the highest point in the pleural space relative to patient position. In an upright patient, this typically means apical air collection. However, when air collects anteriorly and basally, standard PA radiographs have limited sensitivity 1.
The BTS guidelines explicitly acknowledge that plain PA radiographs are poor methods of quantifying pneumothorax size and typically underestimate it 1. This is especially problematic for small, anteriorly located pneumothoraces.
Detection Strategies
For Supine or Semi-Recumbent Patients
In supine positioning, anterior basilar pneumothorax becomes more detectable because air redistributes anteriorly. Look for:
- Deep sulcus sign: An abnormally deep, lucent costophrenic angle
- Double diaphragm sign: Two diaphragm-lung interfaces visible
- Increased lucency over the upper quadrant with an abrupt curvilinear density change 2
Research demonstrates that occult pneumothoraces account for 52-63% of all traumatic pneumothoraces on supine radiographs 3, highlighting the significant limitation of standard technique.
Recommended Additional Views
When clinical suspicion is high but the PA radiograph appears normal, obtain lateral or lateral decubitus views 1. The evidence shows:
- Lateral radiographs provide additional diagnostic information in up to 14% of cases where PA films are equivocal 1
- Lateral decubitus radiographs are superior to erect or supine films and approach CT sensitivity for pneumothorax detection 1
- Oblique chest radiographs can reveal occult pneumothoraces in trauma patients 3
The BTS guidelines specifically state: "The upright lateral or lateral decubitus radiograph is clinically helpful where findings on the upright PA radiograph are unclear" 1.
When to Escalate to CT
CT scanning is the most robust approach for detecting small pneumothoraces 1. CT is specifically recommended when 1:
- Differentiating pneumothorax from complex bullous lung disease
- The plain radiograph is obscured by surgical emphysema
- Clinical suspicion remains high despite negative standard imaging
Research confirms that CT detects 40% of pneumothoraces versus only 25% detected on standard CXR in post-procedural settings 4.
Clinical Implications
Important Caveats
- Even small pneumothoraces have significant clinical implications in patients with underlying lung disease 1
- The anterior basilar location you describe is precisely where air may collect in supine patients, making it easily missed on standard upright films
- Clinical symptoms are not reliable indicators of pneumothorax size 1
Practical Approach
If clinical suspicion exists (chest pain, dyspnea, mechanism of injury) but PA CXR is negative:
- Obtain lateral or lateral decubitus views immediately 1
- Consider patient positioning during the radiograph (supine films show different signs)
For secondary pneumothorax concerns (underlying lung disease):
- Lower threshold for additional imaging, as even small pneumothoraces require intervention 1
If additional views remain equivocal but suspicion persists:
- Proceed to CT scanning 1
Do not rely on expiratory films—they add little diagnostic value 1.
The bottom line: A small anterior basilar pneumothorax will frequently be occult on standard PA CXR and requires either supplementary radiographic views or CT for reliable detection.