Interstitial Lung Diseases
Overview
Interstitial Lung Diseases (ILDs) are a heterogeneous group of diffuse parenchymal lung disorders characterised by progressive inflammation and fibrosis of the alveolar walls and interstitium. All produce a restrictive ventilatory defect and are best characterised on HRCT (1 mm slices, inspiratory + expiratory series).
Key HRCT Patterns and Their Diagnoses
- UIP (Usual Interstitial Pneumonia) = IPF: Subpleural, basal-predominant bilateral reticular opacities + traction bronchiectasis + honeycombing (stacked subpleural cysts). Temporally heterogeneous — key feature.
- Sarcoidosis: Perilymphatic nodule distribution — along bronchovascular bundles, interlobular septa, and subpleural regions. Bilateral symmetric hilar and mediastinal lymphadenopathy.
- Hypersensitivity Pneumonitis: Centrilobular ground-glass nodules + mosaic attenuation/air trapping (expiratory CT). Upper and mid-lung predominance. History of antigen exposure.
- Silicosis: Upper lobe calcified nodules, progressive massive fibrosis (PMF). Eggshell (peripheral) calcification of lymph nodes — pathognomonic.
- Asbestosis: Lower lobe-predominant fibrosis. Bilateral pleural plaques (calcified). High mesothelioma risk.
WarningHoneycombing = End-Stage Fibrosis
Honeycombing on HRCT (clustered subpleural air cysts 3-10 mm with shared walls) is the hallmark of end-stage pulmonary fibrosis. Combined with temporal heterogeneity, it gives a confident UIP/IPF diagnosis without surgical biopsy, per 2022 ATS/ERS/JRS/ALAT guidelines.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
The distribution of CT abnormality (peribronchovascular vs. subpleural peripheral vs. centrilobular) is the most diagnostically discriminating radiological feature. Add expiratory scans to detect air trapping in HP and smoking-related ILD. Prone images eliminate gravity-dependent atelectasis mimicking fibrosis.