Pathomorphology and Etiology
The basis of the disease is irreversible circular submucosal fibrosis. The proliferation of fibrous tissue proceeds along the long axis of the airways, predominantly between the epithelium and the lamina muscularis mucosae. Unlike some other pathologies, intraluminal granulation polyps are absent here, and the epithelium remains intact (although it may flatten or undergo metaplasia). In a number of cases, the fibrotic process also involves the accompanying artery.
There are numerous causes of constrictive bronchiolitis. Most commonly, the process is triggered by:
- Infections: childhood viral infections (adenovirus, RSV), Mycoplasma pneumoniae, endobronchial tuberculosis.
- Toxic gases: nitrogen dioxide, ammonia, chlorine, phosgene.
- Transplantation: bone marrow or heart-lung transplantation.
- Autoimmune diseases: rheumatoid arthritis, Sjögren's syndrome.
- Medications: penicillamine, lomustine.
- Chronic aspiration: occult aspiration of gastric contents in elderly patients or individuals with dysphagia.
A special clinical variant is Swyer-James (MacLeod) syndrome. This is a post-infectious condition in which areas of decreased attenuation and impaired perfusion are limited predominantly to one lung.
CT Diagnostics: Inspiratory and Expiratory Phases
Radiography is of little diagnostic value in this pathology: images often appear normal or demonstrate only mild hyperinflation with a subtle decrease in the peripheral vascular pattern. The gold standard of diagnosis is high-resolution CT (HRCT), mandatory evaluated at both inspiration and expiration.
- Inspiration: Areas of decreased X-ray attenuation with a reduced vessel caliber are identified. Bronchi may be slightly dilated and their walls thickened.
- Expiration: «Air trapping» (air trapping) is revealed. Normally, tissue density increases during expiration, and the cross-sectional area of the lungs decreases. In obliterative bronchiolitis, the affected areas remain radiolucent (decreased density) due to trapped air, and lung volume remains almost unchanged.
Practice example: in a patient after bone marrow transplantation, low-dose CT at the end of forced expiration shows no physiological increase in lung tissue density. At the same time, complete collapse of the lower lobe bronchial lumens confirms that the expiration maneuver was performed correctly.
Mosaic Perfusion Pattern
Since bronchioles are affected heterogeneously, blood flow is reflexively redistributed to healthy areas of the lung with normal ventilation. On CT, this creates a «geographic» pattern, or mosaic perfusion:
- Areas of decreased attenuation (affected tissue) have sharply narrowed blood vessels due to hypoperfusion. Important: vessels are narrowed, but not deformed.
- Areas of normal attenuation (healthy tissue) contain dilated vessels that accommodate the redistributed blood flow.
The difference in vessel caliber is a key criterion that allows distinguishing mosaic perfusion from mosaic attenuation in infiltrative diseases (where vessels are uniform across all areas).
During expiratory CT, this contrast becomes much more pronounced because healthy areas increase in attenuation during expiration. However, if bronchiolar involvement is total, the regional heterogeneity disappears. In such cases, the lungs appear homogeneous on inspiration, and mosaicism can be detected only on expiratory scans.
Differential diagnosis with emphysema
The CT picture of bronchiolitis obliterans can closely resemble panlobular emphysema. Both conditions present with generalized decreased attenuation of the lung parenchyma, bronchial dilation, and bronchial wall thickening.
To avoid misdiagnosis, one must look for signs characteristic of panlobular emphysema:
- Pronounced and frequently encountered destruction of the lung parenchyma.
- Presence of long lines representing locally thickened interlobular septa.
To differentiate mosaic perfusion from infiltration, look at the vessels. If vessels are large in the "bright" zone and small in the "dark" zone, it is blood redistribution (mosaic perfusion). If the caliber is uniform everywhere, it is infiltration.
Class and exam questions
Why is a standard chest X-ray poorly suited for diagnosing bronchiolitis obliterans?
Chest radiographs in such patients often remain within normal limits. Findings such as mild hyperinflation or vascular attenuation are too subtle for a confident diagnosis.
Why is an expiratory scan mandatory?
On inspiration, areas of decreased attenuation may be missed (especially in diffuse involvement). Only the expiratory phase allows the detection of air trapping, as healthy tissue increases in attenuation on expiration while affected tissue remains aerated.
What is maximum intensity projection (MinIP)?
This is a CT image processing mode that highlights pixels with the lowest attenuation. It is ideal for visualizing air trapping on expiratory scans.
What else to study on this topic
- Technique and evaluation of thin-section post-expiratory CT
- Application of MIP and MinIP projections in thoracic radiology
- Allergic bronchopulmonary aspergillosis (ABPA): acute and chronic features
- Tree-in-bud sign and mucoid impaction
- Ciliary dyskinesia syndrome and Kartagener syndrome
Master the entire topic in the "Diagnostic Radiology" course
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Other topics in the «Chest Radiology» section
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