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  • Insidious onset of cough and dyspnea.

  • Irreversible airflow obstruction and air trapping on pulmonary function testing.

  • Minimal findings on chest radiograph, heterogeneous airflow obstruction, and air trapping on chest CT scan.

  • Relevant exposure or risk factors: toxic fumes, viral infections, organ transplantation, connective tissue disease.


Bronchiolitis is a generic term applied to varied inflammatory processes that affect the bronchioles, which are small conducting airways less than 2 mm in diameter. In infants and children, bronchiolitis is common and usually caused by respiratory syncytial virus or adenovirus infection. In adults, bronchiolitis is less common but is encountered in multiple clinical settings. Disorders associated with bronchiolitis include organ transplantation, connective tissue diseases, and hypersensitivity pneumonitis. Inhalational injuries as well as postinfectious and drug-induced causes are identified by association with a known exposure or illness prior to the onset of symptoms. Idiopathic cases are characterized by the insidious onset of dyspnea or cough.

The clinical approach to bronchiolitis divides patients into groups based on etiology, but different clinical syndromes may have identical histopathologic findings. As a result, no single classification scheme has been widely accepted, and there is an overlapping array of terms to describe these disorders from the viewpoints of the clinician, the pathologist, and the radiologist.


Acute bronchiolitis has a neutrophilic and mononuclear infiltration in the absence of fibroblast proliferation or collagen deposition. Constrictive bronchiolitis (also referred to as obliterative bronchiolitis, or bronchiolitis obliterans) is characterized by patchy chronic inflammation, concentric submucosal and peribronchiolar fibrosis, and smooth muscle hypertrophy causing luminal obstruction (obliteration).

In contrast to the minimally cellular pattern of constrictive bronchiolitis, proliferative bronchiolitis occurs when there is an organizing intraluminal exudate, consisting of fibroblasts, lipid laden (“foamy”) macrophages, and other cells that obstruct the bronchiolar lumen. These findings are more common than the constrictive pattern. When the organizing intraluminal exudate extends through the bronchiole into the alveolar space, with prominent intraluminal buds of fibroblasts embedded in immature collagen, the proliferative pattern is referred to as cryptogenic organizing pneumonitis (COP) (see Table 9–17). Follicular bronchiolitis is characterized by chronic peribronchiolar inflammation and hyperplastic lymphoid follicles with reactive germinal centers arising from bronchus-associated lymphoid tissue (BALT). Respiratory bronchiolitis is a disorder of small airways in cigarette smokers characterized by accumulation of pigmented alveolar macrophages within respiratory bronchioles, associated with mild interstitial fibrosis and chronic inflammation. In some patients, however, respiratory bronchiolitis causes diffuse parenchymal infiltrates, a syndrome referred to as respiratory bronchiolitis–associated interstitial lung disease (RB-ILD) and considered with desquamative interstitial pneumonia (DIP) to represent part of the spectrum of the smoking-related interstitial lung diseases.

Diffuse panbronchiolitis is an idiopathic disorder of respiratory bronchioles, characterized by a peribronchiolar mixed inflammatory cell infiltrate and accumulation of lipid laden (“foamy”) macrophages within the interstitium and alveolar spaces.



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