Typical UIP Pattern: Radiologic and Histologic Features
Radiologic Features on High-Resolution CT
The typical UIP pattern on HRCT is characterized by basal-predominant subpleural reticulation, traction bronchiectasis, and honeycombing, which is associated with histologic UIP in >95% of patients. 1
Definite UIP Pattern
- Honeycombing: Defined by as few as two adjacent honeycomb cysts along the pleural surface or stacked on one another 1
- Basal and subpleural reticulation with traction bronchiectasis 1
- Distribution: Characteristically basal and subpleural predominant, though occasionally may be nearly uniform from apex to base 1
Probable UIP Pattern
- Shares the same features as definite UIP (basal and subpleural reticulation with traction bronchiectasis) but without honeycombing 1
- Also highly likely to be associated with UIP on histologic analysis in patients with high clinical likelihood of IPF 1
- Both definite and probable UIP patterns are sufficient for a presumptive diagnosis of IPF without pathologic confirmation 1
Features That Argue Against UIP
- Peribronchovascular disease predominance with subpleural sparing 1
- Complete sparing of the costophrenic angles (though relative sparing is more challenging to interpret) 1
- Extensive ground-glass abnormalities when extent exceeds reticular abnormality 2
Histologic Features
The histologic UIP pattern demonstrates temporospatial heterogeneity with patchy subpleural and paraseptal dense fibrosis, fibroblastic foci, and honeycomb change adjacent to preserved lung. 1, 3
Key Histologic Characteristics
- Fibroblastic foci: Present in 82-100% of cases, representing active fibrosis 3
- Dense scarring fibrosis with architectural distortion 1
- Honeycombing: Present in 64-95% of cases depending on disease stage 3
- Patchy distribution: Temporospatial heterogeneity with alternating areas of normal lung and fibrosis 1, 3
- Subpleural and paraseptal predominance 1, 3
Histologic Pitfalls
- NSIP areas may be present in approximately 60% of UIP cases, particularly in explants 3
- Peribronchiolar fibrosis occurs in 41% of cases 3
- DIP-like areas are uncommon (18-27%) but can occur 3
- Inflammation, bronchiolar metaplasia, and vascular changes are more frequent in advanced disease 3
Typical Patient Demographics
The classic IPF clinical profile includes male sex, age older than 60 years, and a history of cigarette smoking, with likelihood increasing when features are seen in combination. 1
Demographic Features Favoring IPF
- Male predominance: 59% male in research cohorts 4
- Age: Mean age approximately 67-68 years, with age >60 years favoring IPF 1, 4
- Smoking history: 61% have smoking history, though this alone is not diagnostic 1, 4
Common Clinical Presentations
- Dyspnea and cough are universal features 1
- Inspiratory crackles on examination 1
- Gastroesophageal reflux disease is commonly associated 1
- Family history of ILD may be present 1
Critical Diagnostic Considerations
Multidisciplinary Approach Required
The diagnosis requires close communication between clinician, radiologist, and when appropriate, pathologist, as the process is dynamic and patterns must be contextualized. 1
Common Pitfalls to Avoid
- UIP pattern is not synonymous with IPF: The UIP pattern can occur in connective tissue disease-associated ILD, fibrotic hypersensitivity pneumonitis, drug toxicity, and asbestos exposure 1, 5, 6
- Systematic exclusion of secondary causes is mandatory before diagnosing IPF, including comprehensive autoimmune workup, detailed exposure history (occupational and environmental antigens), and medication review 7, 5
- CT signs suggesting CTD-ILD in UIP pattern: The "straight-edge" sign (94% specificity), "exuberant honeycombing," and "anterior upper lobe" signs should raise suspicion for connective tissue disease rather than IPF 6
- Indeterminate patterns on HRCT warrant surgical lung biopsy when treatment strategies would differ markedly, as histologic confirmation can change diagnosis and prognosis 7, 8