ILD Diagnostic Approach Study Guide
ILD Diagnostic Framework: The 4-Step Radiologist's Guide
Systematically navigating Interstitial Lung Diseases, focusing on Fibrosis and Key Patterns (UIP, NSIP, FHP).
The Four-Step Diagnostic Approach
1
Confirm Fibrosis
Identify chronic scarring (not acute inflammation).
- Subpleural Reticulation (Irregular linear opacities)
- Traction Bronchiectasis (Dilated/distorted airways)
- Volume Loss / Architectural Distortion
- Irregular Septal Thickening
2
Evaluate Distribution
The first clue to the underlying pattern.
- UIP: Basal, Peripheral, Heterogeneous
- NSIP: Basal, Diffuse, Homogeneous, Subpleural Sparing
- FHP: Mid/Lower Lobes, Central/Perihilar Predominance
3
Identify Predominant Pattern
Zoom in on key radiological hallmarks.
- UIP: Honeycombing (Hallmark)
- NSIP: Ground Glass Opacities (Dominant)
- FHP: Mosaic Attenuation / Air Trapping (Predominant)
4
Integrate History & Ancillary Findings
Differentiate Idiopathic from Secondary Causes.
- Age, Gender, Smoking Status
- Occupational/Environmental Exposures (FHP)
- Extrapulmonary Clues (CT-ILD, Asbestosis)
- Drug History
Key Radiological Pattern Differentiation
| Feature | Usual Interstitial Pneumonia (UIP) | Nonspecific Interstitial Pneumonia (NSIP) | Fibrotic Hypersensitivity Pneumonitis (FHP) |
|---|---|---|---|
| Distribution | Heterogeneous, Apicobasal Gradient (lower lobes > upper), Strong Peripheral/Subpleural predominance, sparing central zones. | Homogeneous, Basal predominance, Diffuse involvement, Key: Subpleural Sparing. Symmetrical look. | Heterogeneity, Mid and Lower lobes biased, Key: Central/Perihilar predominance, often airway-centered. |
| Hallmarks | Honeycombing (clustered, multilayered, sub-5mm cysts in basal/subpleural), Reticulation, Peripheral Traction Bronchiectasis. | Ground Glass Opacities (often dominant), Reticulation, Central Traction Bronchiectasis. Can evolve to mimic UIP over time. | Mosaic Attenuation due to Air Trapping, Three-Density Sign (low air, intermediate normal, high GGO), Irregular opacities. |
| Atypical / Exclusions | Absence of extensive ground glass, air trapping, micronodules, or mosaic attenuation strengthens diagnosis. | Subpleural sparing is the key differentiator from UIP. | Honeycombing must not dominate the findings (typically absent or minimal). |
| Etiology Link | Idiopathic Pulmonary Fibrosis (IPF) if no secondary cause found (older, male, smoker). Also seen in secondary causes (e.g., RA-ILD). | Most Connective Tissue Diseases (CT-ILD): Systemic Sclerosis (SSc), Polymyositis/Dermatomyositis (PM/DM), Sjögren's Syndrome. | Environmental/Occupational Exposures (e.g., bird antigens, plastics). Antigen avoidance is key to management. |
Ancillary Clues: Differentiating Secondary ILDs
Connective Tissue Disease (CT-ILD)
- Anterior Upper Lobe Sign: Fibrosis concentrated in anterior upper lobes.
- Four Corner Sign: Bilateral anterolateral upper & posterosuperior lower lobes.
- Straight-Edge Sign: Sharply isolated basal fibrosis (no lateral extension).
- Exuberant honeycombing (>70% of fibrotic area).
Specific CT-ILD & Manifestations
- Systemic Sclerosis (SSc): Esophageal Dilatation (up to 97%), PH.
- Rheumatoid Arthritis (RA): Bony Erosions, Necrobiotic Nodules (often UIP pattern).
- PM/DM: Atoll Sign (crescent consolidations around GGO), migratory patches (OP).
- SLE: Pleural Effusions (50%), Pericardial involvement.
Other Causes & Associated Clues
- Asbestosis: Calcified Pleural Plaques, Calcified Lymph Nodes.
- Drug-Induced: Liver Hyperdensity (Amiodarone), Methotrexate, Nitrofurantoin.
- CPFE Pitfall: Honeycombing (basal/subpleural) vs. Paraseptal Emphysema (upper lobe, larger cysts).
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