Concept and general imaging findings of SR-ILD
The term SR-ILD was proposed by radiologists and pulmonologists because classical "smoker's lung diseases" exhibit massive overlap in clinical, radiological, and histological features. This group traditionally includes respiratory bronchiolitis-associated interstitial lung disease (RB-ILD) and desquamative interstitial pneumonia (DIP). In addition, they share significant morphological similarities with idiopathic pulmonary fibrosis and pulmonary Langerhans cell histiocytosis (LCH).
The clinical course of these diseases remains a subject of debate. It has been proven that in a certain proportion of patients, structural changes in lung parenchyma visible on high-resolution computed tomography (HRCT) do not resolve. They persist even with adequate corticosteroid therapy and complete smoking cessation.
The overall SR-ILD tomographic pattern consists of a combination of the following elements:
- Fine reticular pattern (indicating the presence of a fibrotic process).
- Areas of decreased lung attenuation resembling "ground-glass opacities".
- Background manifestations of centrilobular or panlobular emphysema.
Differential CT semiotics of nosologies
Despite being grouped together, RB-ILD and DIP have their own classic patterns that a radiologist must be able to recognize.
Respiratory bronchiolitis-associated interstitial lung disease (RB-ILD) Characterized by subtle, poorly defined centrilobular nodules. The decreased attenuation pattern presents as ground-glass opacification (ground-glass opacification). These changes are detected in patients with a history of active smoking and clinical symptoms.
Desquamative interstitial pneumonia (DIP) Typically characterized by more extensive but nonspecific ground-glass opacities. The predilection site is the lower lung zones. The process may occur with or without concurrent mild reticulation.
Disease progression Using DIP as an example, the evolution of the process can be tracked: initially nonspecific reticulation and ground-glass opacity zones in the lower lobes can transform over time into diffuse involvement. The end result is established pulmonary fibrosis characterized by coarse reticulation and mild traction bronchiectasis.
Acute interstitial pneumonia (AIP) and DAD
Acute interstitial pneumonia (AIP) stands apart and is considered by the global community as an idiopathic form of acute respiratory distress syndrome (ARDS). Clinically and histologically, it is radically different from other interstitial pneumonias.
The fundamental pathomorphological pattern in AIP is diffuse alveolar damage (DAD). It is important to remember that DAD is not entirely specific to idiopathic pneumonia alone; the same pattern occurs in severe infections, systemic connective tissue diseases (CTD), toxic fume inhalation, and drug toxicity.
Morphologically, the process goes through three distinct phases:
- Acute exudative phase.
- Proliferative (organizing) phase.
- Fibrotic phase.
A key marker on lung biopsy is temporal homogeneity (temporal homogeneity). This means that all affected areas are at the same stage of development, which unmistakably indicates the consequences of a single, powerful damaging event.
Radiological findings in AIP/DAD
Imaging diagnosis of acute interstitial pneumonia requires a comprehensive approach, starting with basic modalities and ending with detailed HRCT analysis.
Chest radiography Standard radiographs visualize nonspecific bilateral airspace opacities.
HRCT findings in acute and subacute stages Tomography demonstrates a heterogeneous appearance:
- Focal or diffuse ground-glass opacities (present in all three phases of the disease).
- Areas of consolidation and lung collapse that are gravitationally dependent (located predominantly in the posterior-inferior regions in supine patients).
- Bronchial dilation and marked architectural distortion (architectural distortion).
- Traction bronchiectasis and reticulation can manifest extremely early—within just a few days of symptom onset—reflecting the rapid development of established fibrosis.
Follow-up evaluation Follow-up CT scans in surviving patients typically demonstrate reticular opacities, which are the substrate of residual fibrosis. An interesting feature is that such patients often exhibit fibrotic damage in the anterior (gravity-independent) lung regions. This is believed to be secondary changes caused by barotrauma during mechanical ventilation.
To memorize the phases of diffuse alveolar damage (DAD), use the acronym EOF: Exudative → Organizing → Fibrotic. Ground-glass opacity is visible on HRCT in any of these stages.
Class and exam questions
Why are RB-ILD and DIP grouped together as SR-ILD?
Due to the significant overlap of their clinical presentation, HRCT patterns, and histological features, as well as similarities with pulmonary Langerhans cell histiocytosis and fibrosis.
Do CT abnormalities always regress after smoking cessation?
No. In a subset of patients, characteristic HRCT patterns persist even after complete smoking cessation and corticosteroid therapy.
What does "temporal homogeneity" in a biopsy specimen indicate in acute interstitial pneumonia?
This histological feature indicates that the entire diffuse alveolar damage occurred simultaneously as a result of a single powerful injurious agent.
Where are consolidation zones most commonly localized in AIP/DAD?
On HRCT, areas of consolidation and collapse in acute interstitial pneumonia are predominantly located in gravitationally dependent lung regions (dorsally in supine patients).
What else to study on this topic
- CT findings in pulmonary Langerhans cell histiocytosis (LCH) and its differential diagnosis
- Lymphoid interstitial pneumonia (LIP): diagnostic criteria
- Pulmonary barotrauma: radiological signs of ventilator-induced lung injury
- Systemic connective tissue diseases (CTD) as a cause of the DAD pattern
- Traction bronchiectasis: formation mechanism and prognostic significance
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