Distinguishing Infrahilar Atelectasis from Fibrotic Scarring
The critical distinction between infrahilar atelectasis and fibrotic scarring is best made using high-resolution CT (HRCT) with prone imaging—prone views will show resolution or redistribution of dependent atelectasis while fibrotic scarring remains fixed with architectural distortion including traction bronchiectasis and/or honeycombing. 1, 2
Imaging Approach to Differentiation
Essential Technical Requirements
- HRCT is the diagnostic procedure of choice, using 1-2mm slice thickness with high spatial frequency reconstruction algorithm to maximize spatial resolution 2, 3
- Prone views are mandatory to distinguish dependent atelectasis from true parenchymal fibrosis in posterior lung fields 1, 2
- Obtain images at 2-cm intervals for accurate assessment of all abnormalities 1
- No intravenous contrast is needed for this evaluation 2
Key Distinguishing Features
Atelectasis characteristics:
- Disappears or redistributes on prone imaging 1, 2
- Shows crowded pulmonary vessels and air bronchograms without architectural distortion 4
- Can migrate with positional changes—infrahilar atelectasis may shift superiorly when supine 5
- No traction bronchiectasis or honeycombing present 3
Fibrotic scarring characteristics:
- Remains fixed on prone imaging with persistent architectural distortion 1
- Traction bronchiectasis and/or honeycombing are the definitive signs of fibrosis 1, 3
- Reticular opacities with distortion of normal lung architecture 1, 3
- Typically bilateral and predominantly subpleural/basal in distribution 1
- May show parenchymal bands and subpleural lines 1
Common Pitfalls to Avoid
- Failing to obtain prone images is the most critical error—dependent atelectasis can perfectly mimic fibrosis on supine-only imaging 1, 2
- Interpreting isolated reticulation without traction bronchiectasis as definite fibrosis—this is insufficiently specific unless progression is documented 1
- Less experienced observers have substantially lower accuracy; consider expert thoracic radiology consultation when findings are equivocal 1, 2
Management Based on Diagnosis
If Atelectasis is Confirmed
- Identify and address the underlying cause (obstruction, hypoventilation, pleural disease) 4
- Consider bronchoscopy if central airway obstruction is suspected 4
- Serial imaging only if clinical deterioration occurs 1
If Fibrotic Scarring is Confirmed
When fibrosis involves >5% of total lung volume by visual estimate, this represents interstitial lung disease (ILD) rather than just interstitial lung abnormalities (ILA): 1
- Pursue definitive ILD diagnosis through multidisciplinary discussion (MDD) integrating clinical context, exposure history, and imaging patterns 1
- Evaluate for specific fibrotic patterns (UIP, fibrotic hypersensitivity pneumonitis, fibrotic NSIP) which have treatment implications 1, 6
- Obtain baseline pulmonary function tests including DLCO, as this is often the earliest physiologic abnormality 1
- Consider antifibrotic therapy if progressive pulmonary fibrosis is identified 6, 7
- Follow-up HRCT within 12 months is advisable for patients with progressive pulmonary fibrosis 2
For fibrotic abnormalities involving ≤5% of lung volume (fibrotic ILA):
- Active monitoring with serial HRCT and pulmonary function tests 1
- Progression of reticulation or development of symptoms/physiologic decline warrants reclassification as ILD 1
Specific Clinical Contexts
High-risk populations requiring lower threshold for concern:
- First-degree relatives of patients with familial pulmonary fibrosis or idiopathic pulmonary fibrosis 1
- Patients with connective tissue disease 1
- Occupational exposures (particularly asbestos) 1
- Even unilateral findings may be significant in these populations 1
Differential diagnosis considerations for basal/infrahilar fibrotic patterns: