ABSTRACT
Bronchiolitis is an inflammatory and fibrosing disorder that primarily affects the
small conducting airways, often sparing a considerable portion of the lung
parenchyma and usually with a mild involvement of the larger airways. Damage
to the bronchiolar epithelium is usually considered the first step of the process.
Repair leads to influx of immune and inflammatory cells, proliferation of
granulation tissue in the airway walls, the lumen, or both. Epithelium atrophy or
hyperplasia may be part of the tissue reaction to damage. Recovery or, vice
versa, scarring with collagen deposition and architectural derangement are the
two extremes of the repairing processes. As bronchioles are in between bronchi
at one side and alveoli to the other side, involvement of these structures may
be also evident with bronchiectasis or centrilobular inflammation and fibrosis as
accompanying findings. Most cases of bronchiolitis are infectious in nature or
related to inhalation of toxins, dusts, or gases. Other causes of bronchiolitis include
drugs, collagen vascular disease, graft versus host disease, lung transplantation,
chronic occult aspiration, and inflammatory bowel disease (IBD). Idiopathic
forms are increasingly recognized and, finally, peculiar forms such as diffuse
panbronchiolitis (DPB) or cicatricial bronchiolitis associated with diffuse neuro-
endocrine peribronchiolar cell hyperplasia are part of the clinicoradiologic and
anatomic spectrum of these disorders.
