Small Airway Disease Introduction to Radiology

Small airway disease refers to a group of conditions that affect the
small bronchioles (airways less than 2 mm in diameter) within the
lungs, leading to inflammation, narrowing, and sometimes
obstruction of these airways. This can result from chronic exposure
to irritants like cigarette smoke, environmental pollutants, or
conditions such as chronic obstructive pulmonary disease (COPD),
asthma, bronchiolitis, and rheumatoid arthritis-associated lung
disease. The pathogenesis involves inflammation, mucus
production, and sometimes fibrosis, which narrows the airways and
restricts airflow, causing symptoms like chronic cough, wheezing,
and shortness of breath. Diagnosis is often made through
pulmonary function tests (PFTs) showing airflow obstruction, and
high-resolution CT (HRCT) scans may show features like air
trapping, mosaic attenuation, and bronchial wall thickening

Small Airways Definition – <2mms in diameter

Overview of the Anatomy of the Lungs Large Airways and Small Airways
This image shows the division of the airways in the lungs classified as large airways and small airways.
Image (a) shows the airways starting in the trachea and continuing to the mainstem bronchi, lobar bronchi, segmental bronchi, and subsegmental bronchi.
Image b shows the structures that make up the small airways starting with the terminal bronchiole (tb) followed by the respiratory bronchiole (rb) alveolar duct, (ad) and alveolar sacs (as)
Image (c) shows the histologic makeup of the large airways that include a pseudostratified ciliated columnar epithelium with mucus secreting goblet cells a muscular layer (red) and a prominent cartilage layer (white) In the larger bronchioles (d) the epithelium remains as a pseudostratified, ciliated, columnar epithelium with prominent muscular layer (red). The columnar epithelium transitions to a stratified ciliated cuboidal epithelium by the terminal bronchiole s (f) both still with a muscular layer. The respiratory epithelium transitions from a cuboidal epithelium to a squamous epithelium (f) with alveoli and type I and II pneumocytes starting to branch (g)
Ashley Davidoff MD TheCommonVein.net lungs-0740nL

Small Airways
The diagram allows us to understand the the components and the position of the small airways starting in (a) which is a secondary lobule that is fed by a lobular bronchiole(lb) which enters into the secondary lobule and divides into terminal bronchioles (tb) which is the distal part of the conducting airways, and at a diameter of 2mm or less . It divides into the respiratory bronchiole (rb) a transitional airway which then advances into the alveolar ducts(ad) and alveolar sacs (as) Diseases isolated to the small airways do not affect the alveoli and hence there is peripheral sparing Ashley Davidoff MD TheCommonVein.net

Small Airways
- - <2mm
  - membranous bronchioles
  - respiratory bronchiole
  - lack cartilage
  - lack submucosal glands
Acinus

Terminal bronchiole Conducting Zone

Respiratory Bronchiole – Transitional Zone

Alveolar Ducts

Alveolar Sacs

Small Airways
The diagram allows us yo understand the the components and the position of the small airways starting in (a) which is a secondary lobule that is fed by a

lobular bronchiole enters into the secondary lobule and
divides into 3-12 terminal bronchioles (tb) which is the distal part of the conducting airways, and at a diameter of <than 2mm are considered the beginning of small airways. The terminal bronchiole divides into 2 respiratory bronchioles

Overview of the Anatomy of the Lungs
Image a shows the airways starting in the trachea and continuing to the mainstem bronchi, lobar bronchi, segmental bronchi, and subsegmental bronchi,. The subsegmental bronchi have 3 subsequent generations until the bronchiole is reached. The terminal bronchiole is the last of the transporting airways and is considered the most proximal small airway with a diameter of 2mm or less, and it gives rise to the respiratory bronchiole which is the feeding airway for the acinus . The acinus is the functional unit of the lung.
Image b is a 3D reconstruction of a CT scan showing the proximal airways from the trachea to the segmental airways.
Image c shows the structures that make up the acinus and the other parts of the small airways, starting with the respiratory bronchiole (rb) . The diagram in d, shows the detail of the small airways that participate in gas exchange, including the respiratory bronchiole, (rb) alveolar duct, (ad) and alveolar sac (as)
Image e shows the secondary lobule made from about 20-30 acini, arising from a single lobular bronchiole accompanied by a single pulmonary arteriole (pa).. Structure that surround and enclose the secondary lobule include the pulmonary venule, (red) lymphatics,(yellow) and a fibrous septum (pink).
Ashley Davidoff MD TheCommonVein.net
lungs-0739

Small Airways Terminal Bronchiole and Alveolar Duct
Cross section diagrams of the small airways. The top diagram shows a normal terminal bronchiole with columnar epithelium (pink), and muscularis (maroon). The respiratory bronchiole starts to have features of evolving respiratory airways, and he mucosa becomes cuboidal with persistence of the muscularis.
The alveolar duct has a squamous epithelium (pink), and is surrounded by a capillary network (blue – arteriolar component, and red venular component)
Ashley Davidoff MD thecommonvein.net lungs-0776b

Inflammatory response in the small airways attracts cellular interstitial infiltrates which surround the bronchiole. The diagram shows a bronchiole surrounded by an acute cellular inflammatory response
Ashley Davidoff MD TheCommonVein.net lungs-0722

Small Airway Surrounded by an Inflammatory process
Ashley Davidoff MD TheCommonVein.net wall 56 001

Small Airway Surrounded by an Inflammatory process
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Small Airway Surrounded by an Inflammatory process
Ashley Davidoff MD TheCommonVein.net wall 56 003

The collage provides a persepective of disease of the small airways and the alveoli that results in ground glass appearance on Xray. A process that incereases the densityof the lungs to a net “gray”regional density will result in a ground glass opacity whether it is inflammation of the walls ((second column) fluid within the lumen of the small aireways and alveoli (3rd column) or whether it is fibrosis in the walls of the small airways or alveolar septa (last column alveoli. The net result on CT is a ground glass opacity (bottom row). In fibrosis there are secondarychanges which include bronchiolectasis in this case, butother associated changes may include reticulations or centrilobuar nodules
Ashley Davidoff MD TheCommonVein.net lungs-0733

Disease
- COPD 75-85% have small airway disease
  - airway remodelling
  - mucus plugging
  - immune cell infiltration
- Inflammatory Disease
  - Acute
    - Hypersensitivity Pneumonitis
  - Chronic
    - RA
- Infections
  - Post Infectious
  - Swyer james
- Radiology
  - Bronchiolar Wall Thickening
  - Tree in Bud
  - Centrilobular Nodules and Air Trapping
  - Bronchiolectaisis
  - bronchiectasis
  - Low Caliber vessels
  - Accentuation of Mosaic pattern on expiration
- Common Causes
  - Constrictive bronchiolitis
    - post infectious
      - patchy bilateral
      - Swyer James/Macleod
        
        unilateral
      - RA and other collagen vascular diseases
      - Post lung transpalnt bronchiolitis obliterans syndromes
      - Chronic Graft vs host
  - Hypersensitivity Pneumonitis
    - Poorly defined centrilobular Nodules
  - Cystic Fibrosis
  - Pulmonary Hypertension

Small Airways Definition
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Bronchioles are rarely visible within 1cms of the pleural space

When they become visible we have
- Inflammatory bronchiolitis
- Cellular Bronchiolitis

Cellular Bronchiolitis
small airway disease ATS 003
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Constrictive Bronchiolitis or Fibrotic Disease
Mosaic Attenuation
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Classification
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Cellular Bronchiolitis
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Infectious Bronchiolitis
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Infectious Bronchiolitis
TB
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Adenovirus
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Aspiration
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Diffuse Panbronchiolitis
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Bronchiectasis – Cystic Fibrosis
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Bronchiectasis – Kartagener’s
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Society of Thoracic Radiology Santiago Rossi

Small Airways Definition Signet ring
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Note Sparing of the subpleural space
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Centrilobular Nodules
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Cavitation and Tree in Bud
TB
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ACUTE ALVEOLAR PNEUMONITIS
CT scan through the 4 chambers of the heart using lung windows is from a a 54 year old female with SLE. Recent CXR showed bibasilar ground glass infiltrates.
The scan shows basilar multicentric infiltrates with elements of ground glass change and small airway wall thickening (red circles in the right lower lobe middle lobe and lingula, as well as interlobular septal thickening (green circle) in the lateral basal segment of the left lower lobe. A small pericardial effusion is present (yellow arrowhead)
Ashley Davidoff MD
key words
SLE
acute pneumonitis
pericardial effusion

Representative photomicrographs of individual bronchiolar lesions observed in surgical lung biopsy in patients with small airways disease. a) Cellular bronchiolitis: a narrowed and contracted airway is infiltrated by numerous inflammatory cells without a specific pattern. b) Granulomatous bronchiolitis: the small airway is surrounded by an inflammatory infiltrate with a sarcoid granuloma (arrowheads), which increases the volume of the airway wall resulting in lumen narrowing. c) Follicular bronchiolitis: the small airway is surrounded by a large lymphoid follicule (arrowheads), which increases the volume of the airway wall resulting in lumen narrowing. d) Bronchiolitis obliterans is characterised by lumen obstruction with a fibro-inflammatory polyp. e) Obliterative (constrictive) bronchiolitis: the airways lumen is narrowed by subepithelial fibrosis. Although inflammatory cells and mucous exudates are present within the lumen, no fibro-inflammatory polyp is found. f) Mucous plugging: the airway lumen is obstructed by mucus exudates.
Burgel, P-R et al Small airways diseases, excluding asthma and COPD: an overview European Respiratory Review 2013 22: 131-147;

Mosaic Attenuation from Small Airways Disease

Mosaic attenuation
Small Airways are obstructed and air is trapped Sometimes small vessel disease, as in vasculltis, can lead to the air trapping
Ashley Davidoff MD TheCommonVein.net bronchioles 003

Mosaic attenuation bronchioles
Small Airways and Smaller Airways are Filled with Mucus in a patient with COPD – Note Centrilobular Impaction of Mucus
Ashley Davidoff TheCommonVein.net bronchioles 004

Representative images of computed tomography (CT) scans in patients with small airways disease. a) An inspiratory CT scan in a patient with hypersensitivity pneumonitis showing mosaic pattern of attenuation. b) Expiratory CT scan in the same patient showing air trapping that is characteristic of small airways disease. c) Ill-defined centrilobular nodules in a patient with farmer’s lung (personal communication; J.C. Dalphin). d) Localised micronodules branching with bronchovascular structures (tree-in-bud pattern) related to tuberculosis in a patient with rheumatoid arthritis receiving treatment with anti-tumour necrosis factor-α. Reproduced from [21] with permission from the publisher.
Burgel, P-R et al Small airways diseases, excluding asthma and COPD: an overview European Respiratory Review 2013 22: 131-147;

Broncholith

Small Airway Disease Bronchiole wall thickening
Ashley Davidoff MD TheCommonVein.net

ABPA

Bronchiolar Inflammation
- AEIOU
  - A
    - Asthma ABPA Asbestosis
  - E
    - Eosinophilic Pneumonia
  - Infections
    - Endobronchial
      - TB
        
        Mycobacterium
        
        Non TB Mycobacteria and
        
        Other Granulomatous Infections
      - ABPA
    - viruses such as
      - adenovirus,
      - influenza, and
      - respiratory syncytial virus (RSV),
  - Inflammatory
    - - Sarcoidosis
      - Inhalational
        
        Cigarette Smoke
        
        smokers bronchiolitis
        
        Langerhans Cell
        
        chemicals,
        
        fumes, or toxic gases
        
        occupational exposures,
        
        industrial chemicals
        
        diacetyl in the popcorn industry
  - Immune
    - - HP
        
        RA
        
        Follicular Bronchiolitis (MALT Lymphoid Hyperplasia in collagen vasc and immune deficiency)
        
        Graft vs Host
  - Inherited
    - Cystic Fibrosis
  - Idiopathic Bronchiolitis Obliterans

Links and References

Burgel, P-R et al Small airways diseases, excluding asthma and COPD: an overview European Respiratory Review 2013 22: 131-147;

Rossi Santiago Small Airway Disease

Rossi Santiago Case Based Hypersensitivity Pneumonitis You Tube