Thoracic Imaging 2003 - Society of Thoracic Radiology

SUNDAY
74
Small Airway Diseases
Michael B. Gotway, M.D.
The term small airway refers to airways 3mm or less in
diameter (most of which represent bronchioles). Small airways
are quite numerous, and thus contribute little to overall airway
resistance; this implies that considerable destruction of the small
airways must be present before the patient will become symptomatic.
Small Airway Diseases and Pulmonary Pathophysiology
In patients with small airways diseases, obstruction of the
bronchiolar lumen results in hypoxia of the underventilated lung
and air trapping. This hypoxia results in reflex vasoconstriction,
causing blood to be shunted airway from the underventilated
segments of lung to normal lung areas. Because much of pulmonary
parenchymal attenuation is the result of the lung’s blood
volume, redistribution of blood flow, due to small airway diseases,
appears on HRCT as inhomogeneous lung opacity. This
inhomogeneous opacity is appreciated as areas of relatively
increased and decreased lung parenchymal attenuation, which is
accentuated on expiratory imaging. In this circumstance, the
vessels within the areas of relatively diminished lung parenchymal
attenuation appear abnormally small, representing mosaic
perfusion (1).
Approach to Small Airway Diseases
Small airway diseases may be approached in several different
ways. The two major approaches to small airway diseases
are:
Classification based on pathologic descriptions
Classification based on HRCT appearances
Pathologic Classification of Small Airway Diseases
The major small airway disease pathologic categories
include cellular bronchiolitis (both acute and chronic bronchiolitis),
panbronchiolitis, follicular bronchiolitis, respiratory bronchiolitis,
constrictive bronchiolitis (bronchiolitis obliterans), and
proliferative bronchiolitis (aka bronchiolitis obliterans organizing
pneumonia) (2).
Cellular Bronchiolitis
Cellular bronchiolitis involves inflammation within the bronchiolar
lumen and bronchial walls. Diseases that fall within this
pathologic category include infections (virus, Mycoplasma, airway
invasive Aspergillosis), hypersensitivity pneumonitis, asthma,
chronic bronchitis, and bronchiectasis. The typical appearance
of cellular bronchiolitis is small ground-glass opacity,
commonly presenting as centrilobular nodules (3), sometimes
with coexistent air trapping. More diffuse ground-glass opacity
may reflect coexistent alveolitis (2).
Panbronchiolitis
Panbronchiolitis is a rare disease in the United States,
although it is common in Asia (especially Japan and Korea).
Pathologically it is characterized by macrophage and mononuclear
cell infiltration within the respiratory bronchioles and
adjacent alveoli. The HRCT correlate of these pathologic findings
is centrilobular nodules, with and without tree-in-bud, air
trapping, bronchiectasis, and bronchiolectasis (2, 3).
Follicular Bronchiolitis
Follicular bronchiolitis (FB) represents hyperplasia of the
bronchus-associated lymphoid tissue (BALT), characterized
histopathologically by hyperplastic lymphoid follicles with reactive
germinal centers distributed along bronchioles and, to a
lesser extent, bronchi. FB is associated with collagen vascular
diseases (especially rheumatoid arthritis and Sjögren’s syndrome),
immunodeficiencies, chronic inflammatory diseases
(such as bronchiectasis), and hypersensitivity reactions. FB may
be thought of as a less extensive “version” of lymphocytic interstitial
pneumonitis (LIP); both are associated with similar clinical
conditions (collagen vascular disease, immunodeficiency).
On HRCT, FB usually appears as lower lobe predominant, small
(3 mm or less, rarely up to 12 mm) nodules distributed along
the bronchioles and subpleural interstitium. These small nodules
along the bronchioles are often seen as centrilobular on HRCT.
Foci of ground-glass opacity may be present. Associated findings
include bronchiolectasis and bronchial wall thickening,
mild interlobular septal thickening, and air trapping. In immunocompromised
patients, LIP has a similar appearance, but is usually
more extensive than FB. In immunocompetent patients, LIP
often appears as small, lower lobe centrilobular ground glass
nodules (which histologically represent peribronchiolar cellular
infiltration), perhaps associated with cysts. Bronchial wall thickening,
adenopathy, and thickening of the peribronchovascular
interstitium may be associated (4).
Respiratory Bronchiolitis
Respiratory bronchiolitis is due to irritation from cigarette
smoke, and results in inflammation associated with alveolar
macrophage accumulation. The typical CT appearance is airway
thickening associated with centrilobular ground-glass nodules
predominantly in the upper lobes. When extensive, the condition
is referred to as respiratory bronchiolitis-interstitial lung disease
(2, 5). On the most extensive end of the spectrum, when alveolar
macrophage accumulation is very extensive, the condition is
referred to as desquamative interstitial pneumonia (DIP), or
alveolar macrophage pneumonia.
Constrictive Bronchiolitis (Bronchiolitis Obliterans)
Constrictive bronchiolitis (CB) is characterized pathologically
by submucosal and peribronchiolar fibrosis causing extrinsic
airway narrowing. There is little bronchiolar inflammation. The
major pathophysiologic consequence of this condition is airflow
obstruction, manifest as air trapping on HRCT. Associated conditions
include: prior childhood infection (Swyer-James syndrome,
Mycoplasma), toxic fume inhalation, connective tissue
diseases, rheumatoid arthritis, certain drugs (especially penicillamine),
in association with inflammatory bowel disease, and in
association with neuroendocrine cell hyperplasia. BO is the
major manifestation of chronic rejection following lung transplantation,
and also may represent chronic graft-versus-host disease
following bone marrow transplantation (2, 6).
Proliferative Bronchiolitis (Bronchiolitis Obliterans
Organizing Pneumonia)
Proliferative bronchiolitis, or bronchiolitis obliterans organizing
pneumonia (BOOP), is characterized histologically by
granulation tissue polyps within the lumina of bronchioles and
alveolar ducts, often associated with foci of organizing pneumonia.
BOOP, like BO, may be seen idiopathically (idiopathic
BOOP), or it may be seen in association with certain conditions
(BOOP reactions), such as infections (viral, bacterial, and fungal
pneumonia), chronic eosinophilic pneumonia, collagen vas-