[Bronchiolitis. Part 1--anatomic features, classification, clinical presentation and imaging]

The term "bronchiolitis" refers to a broad spectrum of common conditions related to the small airways associated with a miscellaneous aetiology, histology, clinical features and course. Due to their variability, bronchiolar disorders are generally difficult to diagnose. History (smoking, collagen vascular disease, inhalational injury, medication usage, and organ transplant) may point towards a bronchiolar process. In addition, signs of systemic and pulmonary infection and evidence of air trapping may provide diagnostic hints. Although clinical presentation, physical examination, pulmonary function tests (obstructive ventilatory defect), and plain chest radiographs may demonstrate abnormalities suggesting small airways involvement, they are often non-specific and rarely diagnostic. In contrast, the high-resolution CT (HR-CT) scanning of the chest provides three distinct HR-CT patterns that assist in the diagnosis and differential diagnosis of bronchiolar conditions: (i) a tree-in-bud pattern, (ii) ill-defined centrilobular ground-glass nodules, and (iii) a mosaic attenuation pattern (best visible on expiratory images). The present paper summarises the current knowledge, the classification, imaging, and the clinical presentation of bronchiolar disorders.