In extrinsic allergic alveolitis (hypersensitivity pneumonitis, HP), chronic fibrotic HP resembles UIP/IPF on HRCT. Which histological finding on lung biopsy would strongly favour HP over IPF/UIP?
- A Poorly formed non-necrotizing granulomas in peribronchiolar distribution, lymphocytic bronchiolitis, and multinucleate giant cells — occurring in a setting of predominantly upper/mid-lobe or peribronchiolar-centric fibrosis rather than exclusively subpleural basal fibrosis ✓
- B Fibroblastic foci in subpleural zones with honeycombing and absence of inflammation
- C Diffuse alveolar damage pattern with hyaline membranes and type II cell hyperplasia in a background of fibrosis
- D Extensive organizing pneumonia (Masson bodies) filling alveolar spaces diffusely in a subpleural distribution
Explanation
Chronic fibrotic hypersensitivity pneumonitis (HP) is a major diagnostic pitfall vs. IPF/UIP because both can show fibrosis, honeycomb change, and traction bronchiectasis on HRCT. The histological hallmarks favouring HP over UIP/IPF are: (1) poorly formed peribronchiolar non-necrotizing granulomas and multinucleate giant cells (even when sparse), (2) predominant peribronchiolar/centrilobular distribution of fibrosis (rather than pure subpleural basal), (3) lymphoplasmacytic interstitial infiltrate that is disproportionately prominent for the degree of fibrosis, and (4) bridging fibrosis between bronchioles. Finding even scattered granulomas or giant cells in a UIP-like setting should prompt HP consideration and detailed antigen exposure history. DAD pattern and OP bodies are not the discriminating features.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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