Perspectives in veterinary medicine: Description and classification of bronchiolar disorders in cats

Reinero, Carol R.; Masseau, Isabelle; Grobman, Megan; Vientos-Plotts, Aida; Williams, Kurt

With more commonplace use of CT in cats, we are recognizing some features similar to those described in humans. Paired inspiratory and expiratory scans and thin section reconstruction (0.625-2 mm slice thickness) 27 provide optimal detail. Normal bronchioles in humans and cats are below the limits of resolution on HRCT, but with dilatation, mural thickening, and intraluminal plugging, small airways become visible. 8 Bronchiolar disorders have characteristic HRCT features, classified as direct or indirect signs. 8, 28 Direct signs imply a change in the walls or lumens of bronchioles resulting in an ability to visualize them. These signs include centrilobular nodules (humans only), tree-in-bud pattern (nodular opacities connected to branching linear structures originating from a single stalk reflective of impacted debris, cells, or fluid within bronchioles), peribronchiolar ground glass opacity or consolidation, and dilatation (bronchiolectasis). 5, 14, 27 Indirect signs reflect changes to the pulmonary parenchyma distal to the diseased bronchiole and include mosaic attenuation (because of air trapping) or, in the setting of bronchiolar disorders, rarely mosaic perfusion. 27 Because air trapping may be imperceptible on inspiratory images, expiratory scans are crucial to accentuate air trapping. 8 Direct findings reflect inflammation or proliferative changes within the bronchiolar lumens, whereas indirect findings reflect fibrosis within the bronchiolar wall (eg, constrictive bronchiolitis). 8 Definitive confirmation of bronchiolar involvement requires histopathology. The microscopic anatomy of bronchioles is similar between humans and cats. Distal bronchioles are lined by simple cuboidal epithelium supported on a thin lamina propria and surrounded by smooth muscle. Airways end as respiratory bronchioles before the alveolar parenchyma in the cat ( Figure 1 ) and in humans. Microscopic morphology has been used to categorize lesions as inflammatory or fibrotic, recognizing there are a limited number of ways airways respond to injury. 9 Inflammatory or cellular lesions often are subcategorized as acute, chronic, acute on chronic, granulomatous, or eosinophilic. Fibrotic lesions reflect the site affected (intraluminal or intramural); overlap in inflammatory and fibrotic changes can be noted within the same patient. 9 Multiple wedge biopsy specimens are recommended to capture patchy and sometimes subtle lesions. 29 A particular histologic pattern of disease may have a wide variety of causes. For example, inflammatory bronchiolitis can be caused by infection, aspiration, transplant rejection, and extension from large airway disease or systemic collagen vascular disease, among other causes. 9 Fibrosis may be a sequela to chronic inflammation so that in end-stage disease, the inflammatory etiology can be missed. Thus, classification schemes based solely on histopathologic features are likely to be less clinically useful, and multidisciplinary F I G U R E 1 Normal cat lung histology (hematoxylin and eosin stain). Terminal bronchioles (TB) are lined by low cuboidal epithelium and surrounded by thin smooth muscle. The airway terminates as respiratory bronchiole (RB) before entering the alveoli input should be sought to allow meaningful interpretation of histologic findings. This should not dampen enthusiasm for the central role of histopathology in an understanding of bronchiolar disorders. In fact, in humans, histopathology may be the only means to