Small Airway Disease Definition and Characteristics

Anatomical Definition

Small airways are defined as those with an internal diameter ≤2 mm without cartilage in their walls, including the membranous and respiratory bronchioles, which are distal to the terminal bronchioles 1, 2, 3, 4

The disease manifests through three key pathological processes: cellular inflammation (lymphocytic, neutrophilic, eosinophilic, or granulomatous), fibrosis, and architectural distortion, leading to airway narrowing, obliteration, and eventual complete airway loss 1, 2, 4, 5
In asbestos-related small airway disease, inflammation and fibrosis occur in the walls of membranous bronchioles with smooth muscle hyperplasia, while respiratory bronchioles show fibrosis extending into alveolated portions 4

Small airway disease can cause cough with or without mucous hypersecretion and bronchial hyperresponsiveness, and may result in incomplete or irreversible airflow limitation 1, 3, 6

HRCT resolution is limited to airways >2 mm in diameter, so normal bronchioles cannot be visualized, and chest radiographs are often normal despite clinically significant bronchiolar disease 2, 6
Direct HRCT signs include airway dilation, wall thickening, nodular branching (2-4 mm), and tree-in-bud abnormalities, while indirect HRCT signs include air-trapping (mosaic attenuation on expiratory scans) and subsegmental atelectasis 2, 6, 7

Small airway disease must be distinguished from conditions affecting larger airways, such as asthma, COPD/emphysema, and bronchiectasis, and excludes bronchiolar abnormalities associated with these conditions when they are the primary diagnoses 1, 9
In COPD, small airway disease represents the airway component (inflammation, increased wall muscle mass, fibrosis, narrowing) as distinct from emphysema (parenchymal destruction) 5

Small airway disease occurs in multiple clinical contexts, including nonbronchiectatic suppurative airway disease (bronchiolitis), inflammatory bowel disease-related lung involvement, diffuse panbronchiolitis, asbestos exposure, connective tissue diseases, and post-infectious states 1, 4, 10, 11, 12