INTRODUCTION

Bronchiolitis and bronchiolitis obliterans are general terms used to describe a nonspecific inflammatory injury that primarily affects the small airways (eg, less than 2 mm in diameter), often sparing a considerable portion of the interstitium [1-3]. The terms are often confusing because they describe both a clinical syndrome and a constellation of histopathologic abnormalities that may occur in a variety of disorders [4]. Unfortunately, much of the literature about bronchiolitis consists of isolated case reports or small case series. In addition, tissue confirmation of the diagnosis has not been described in many of these reports [5]. As a result, many uncertainties remain regarding the epidemiology, pathophysiology, long-term sequelae, and therapy of bronchiolitis. The most important clinical syndromes associated with bronchiolitis are listed in the table (table 1) [3,6].

An overview of bronchiolitis in adults is provided here. The acute infectious bronchiolitis that occurs predominantly in young children is described separately. (See "Bronchiolitis in infants and children: Clinical features and diagnosis" and "Bronchiolitis in infants and children: Treatment; outcome; and prevention".)

A discussion of the idiopathic form of organizing pneumonia also known as bronchiolitis obliterans organizing pneumonia (BOOP) is provided separately. (See "Cryptogenic organizing pneumonia".) "BOOP" refers to disorders characterized histologically by intraluminal polyps in the respiratory bronchioles, alveolar ducts, and alveolar spaces, accompanied by organizing pneumonia in the more distal parenchyma.

Another potentially confusing entity is bronchiolitis obliterans syndrome (BOS). BOS is a clinical term that refers to the progressive airflow limitation caused by small airway obstruction that may occur following lung transplantation. BOS is defined by lung function changes, not by histology. (See "Chronic lung transplant rejection: Bronchiolitis obliterans".)

PATHOGENESIS

In most instances, the pathogenesis of bronchiolitis remains poorly defined. Injury to the bronchiolar epithelium appears to initiate the process (figure 1). The alveoli immediately adjacent to the small airways are also frequently involved. The repair process may result in complete recovery or may be characterized by excessive proliferation of granulation tissue that causes narrowing or obliteration of the airway lumen. In some cases, fibrosis is primarily submucosal and peribronchiolar in distribution, resulting in extrinsic narrowing or obliteration of the bronchiolar lumen.