Case of the Month

Answer to April’s Case of the Month...

B. Respiratory bronchiolitis (RB)

Respiratory bronchiolitis (RB) occurs exclusively in smokers and is almost always detected as an incidental finding in lung carcinoma resection specimens. Clinical manifestations are either absent or very mild (1) (2) (3). Aggregates of smokers’ macrophages occupy the lumens of respiratory and terminal bronchioles, and their walls may exhibit fibrosis, mild inflammation, or both (Figure 1, 2). The accumulation of smokers’ macrophages in alveolar ducts and airspaces immediately adjacent to bronchioles that exhibit RB is a common finding, but interstitial inflammation and fibrosis are not. The condition known as RB-ILD (respiratory bronchiolitis-interstitial lung disease) is currently classified as one of the idiopathic interstitial pneumonias. The diagnosis of RB-ILD is made in smokers who have clinical and radiological evidence of respiratory bronchiolitis and interstitial lung disease. However, microscopic examination of lung tissue from such individuals has not clearly defined or identified the nature of the alleged interstitial lesion. When smokers’ macrophages diffusely fill the airspaces of secondary lobules, the diagnostic term desquamative interstitial pneumonia (DIP) is often applied. DIP is a misnomer because it is primarily an alveolar filling process and the cells occupying the airspaces are macrophages, not desquamated alveolar epithelial cells as originally believed. DIP is currently a controversial term that many believe should be eliminated. The extent to which smokers’ macrophages fill the airspace in cases of RB is quite variable and represents a spectrum of severity rather than separate entities. An acceptable diagnostic term for this spectrum has not yet been suggested.

RB is invariably accompanied by various combinations of other smoking-related lesions, including emphysema (Figure 3), smoking-related interstitial fibrosis (Figure 4), smokers’ macrophages occupying a variable number of airspaces (Figure 5), and Langerhans cell histiocytosis (Figure 6).

References

1. DE Niewoehner et al., “Pathologic changes in the peripheral airways of young cigarette smokers”, N Engl J Med, 291, 755–758 (1974). PMID: 4414996.
2. “American Thoracic Society/European Respiratory International Multidisciplinary Consensus Classification of the Idiopathic Interstitial Pneumonias”, Am J Respir Crit Care Med, 165, 277–304 (2002). PMID: 11790668.
3. WD Travis et al., “An Official American Thoracic Society/European Respiratory Statement: Update of the International Multidisciplinary Consensus Classification of the Idiopathic Interstitial Pneumonias”, Am J Respir Crit Care Med, 188, 733–748 (2013). PMID: 24032382.

Submitted by Yale Rosen, MD, SUNY Downstate Medical Center, Brooklyn, NY, USA.