In usual interstitial pneumonia (UIP), the histological pattern is the substrate for idiopathic pulmonary fibrosis (IPF). The hallmark of UIP that distinguishes it from other fibrotic ILDs is the 'temporal heterogeneity' or 'temporal and spatial heterogeneity.' This means:

• A Fibrosis is uniform throughout both lungs without normal tissue
• B Old dense subpleural/paraseptal fibrosis (honeycombing) coexists with adjacent normal lung and areas of active fibroblastic foci — representing different stages of injury simultaneously ✓
• C Fibrosis alternates between upper and lower lobes on sequential biopsies
• D Both intra-alveolar and interstitial fibrosis occur in the same patient simultaneously

Explanation

The defining histological feature of UIP/IPF is spatial AND temporal heterogeneity: in the same biopsy, one sees old end-stage honeycomb fibrosis (dilated, mucus-filled spaces lined by bronchiolar epithelium), adjacent areas of normal or near-normal lung, and active zones of injury called 'fibroblastic foci' (spindle cells in myxoid stroma). This coexistence of old scar and active new injury at different times ('temporal') and in adjacent areas ('spatial') is unique to UIP. NSIP (nonspecific interstitial pneumonia) by contrast shows uniform fibrosis at the same stage throughout ('temporally uniform').

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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Written and medically reviewed by the StethoPrep medical team.