Pathology of Respiratory Bronchiolitis - Interstitial Lung Disease (RB-ILD)

Respiratory bronchiolitis, a common finding in cigarette smokers (smoker’s bronchiolitis), may on rare occasions present as interstitial lung disease (RB-ILD).

Respiratory bronchiolitis associated with interstitial lung disease (RB-ILD), first was described by Niewoehner et al in an autopsy study of cigarette smokers who died from non pulmonary causes in 1974.

The clinical presentation resembles those of patients with other interstitial lung diseases - cough and dyspnea, with coarse rales on physical examination.

Diffuse fine reticulonodular interstitial opacities are found on chest radiograph, usually with normal-appearing lung volumes.

Bronchial wall thickening, prominence of peribronchovascular interstitium, small regular and irregular opacities, and small peripheral ring shadows are distinctive features.

Pulmonary function testing may be normal but usually demonstrates mild to moderate restriction and normal or slightly reduced diffusing capacity. A mixed obstructive-restrictive pattern is common.

Histologically it is characterized by patchy and non-uniform accumulation of finely pigmented tan brown macrophages within membranous, terminal and respiratory bronchioles and also within immediately adjacent alveoli, and sometimes in the interstitial tissues, associated with mild interstitial fibrosis, which is limited to the peribronchiolar tissues.

Unlike typical cases of DIP, the disease process is centri-lobular in distribution, sparing the periphery of the secondary lobules.

Iron stain shows very fine cytoplasmic positivity within the pigmented macrophages, unlike the coarse positivity of hemosiderin granules.

The differential diagnosis of RB-ILD includes small airways disease that may occur in asbestosis , Langerhans cell histiocytosis (LCH), and DIP. Unlike asbestosis, no asbestos bodies are seen and the macrophages are finely pigmented in RB-ILD.

In LCH the stellate bronchiolocentric fibrosis and the presence of S-100 positive cells are diagnostic.

UIP can be ruled out by the lack of the characteristic temporal and spatial heterogeneity.

The distinction from DIP may be difficult because of overlapping histological changes in some cases.

However, such distinction may be of little clinical significance as both diseases more or less share similar presentations and courses.

In differential diagnosis smoking related interstitial lung diseases (desquamative interstitial pneumonia , Langerhans cell histiocytosis , idiopathic pulmonary fibrosis) and other interstitial lung diseases have to be excluded). It has been suggested that RD-ILD and DIP may represent different stages of the same disease.

Long-term prognosis is good with cessation of smoking, in combination or not with corticosteroid therapy.