Bronchiolitis, General Concepts
Small airway disease refers to a number of lesions that mainly involve the bronchioles, described as segments of <2 mm in diameter (
Fig. 35.1), a definition mainly derived from the radiology literature. The term “bronchiolitis” may sometimes be confusing to clinicians because it may be applied both as a descriptive term indicating inflammation of distal bronchioles and as a final pathologic diagnosis. Rarely, a surgical pathologist will encounter a biopsy or surgical specimen with no histologic alteration of small airways, in patients with clinically documented small airway disease. Therefore, a pathologic diagnosis of bronchiolitis only has a meaning when linked to the clinical and radiologic context.
1Bronchiolitis was first described more than 100 years ago with the term “bronchiolitis obliterans,” which has evolved to a varied terminology that sometimes differs in the clinical, radiology, and pathology literature.
2,3 Bronchiolitis may be an isolated pathologic finding but often is secondary to diseases affecting other parts of the airways or lung parenchyma. Inflammation of bronchioles is a finding present in a large variety of lung lesions, and this may add to the confusion of the terminology. The term “small airway disease” is a generic descriptive term (often used by radiologists) that encompasses several histologic appearances. A classification based on main histologic appearance, inflammatory cell type, and extent of disease is summarized in
Table 35.1.
The histologic findings of the main types of bronchiolitis overlap and the causes and clinical manifestations are extremely varied. In children younger than 2 years of age, bronchiolitis is usually acute, is associated with several viruses (respiratory syncytial virus, enterovirus, and rhinovirus), and is rarely biopsied.
3,4,5,6,7Chronic “cellular” bronchiolitis in adults is usually diagnosed on high-resolution computed tomography and is associated mainly with collagen vascular disease, infection, and allograft rejection. Follicular bronchiolitis, or chronic peribronchial inflammation with lymphoid follicles, is especially frequent in adults with collagen vascular disease (classically Sjögren syndrome), and immunocompromised states, and less often hypersensitivity pneumonia and as a reaction to neoplasia. Respiratory bronchiolitis is directly related to tobacco exposure. Constrictive bronchiolitis is a late phase of diffuse bronchiolitis and is associated with collagen vascular disease, fume exposure, or drug reaction or is idiopathic.
8 Table 35.2 shows the main causes of histologic types of chronic bronchiolitis. Small airway disease in hypersensitivity pneumonitis and asthma is discussed in Chapters 23 and 32. Follicular bronchiolitis is discussed more comprehensively in Chapter 36.
Radiologic Findings
Most patients with small airway disease are diagnosed clinically with high-resolution computed tomography imaging. While the findings are nonspecific regarding either etiology or the histologic appearance of bronchiolitis, it provides clues to the presence of an underlying bronchiolar disorder.
3 Bronchioles are not seen on imaging in healthy state, but air trapping, distal insufflation, or thickening of the bronchiolar wall on HRCT yields indirect evidence of small airway disease.
9Direct signs of bronchiolitis include centrilobular thickening, peripheral nodules, so-called tree-in-bud opacities, and bronchiolectasis. Some authors argue that the presence of
tree-in-bud opacities favors an infectious etiology to the bronchiolitis.
10 Tree-in-bud opacities may be caused by vascular thickening as well, for example, by microembolization and peribronchiolar fibrosis. Indirect signs of small airway disease are caused by obstruction of the bronchioles and subsequent underperfused lung from compensatory vasoconstriction, the appearance known as
mosaic attenuation pattern on HRCT (see
Chapter 10).
10,11 The second indirect sign is air trapping that can be accentuated on expiratory CT.
