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  • Insidious onset of cough and dyspnea.

  • Irreversible airflow obstruction and air trapping on PFTs.

  • 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 viruses (most commonly respiratory syncytial virus, but other viruses include rhinovirus, parainfluenza, coronavirus, and adenovirus). Bronchiolitis is less common in adults than in children, but it is encountered in multiple clinical settings, such as postinfectious, inhalational injury (such as vaping), organ transplantation, connective tissue diseases, and hypersensitivity pneumonitis.

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.


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).

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.

Follicular bronchiolitis is characterized by chronic peribronchiolar inflammation and hyperplastic lymphoid follicles with reactive germinal centers arising from bronchus-associated lymphoid tissue.

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 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.


Acute bronchiolitis can be seen following viral infections.

Constrictive bronchiolitis (also referred to as obliterative bronchiolitis or bronchiolitis obliterans) is relatively infrequent, although it is the most common finding following inhalation injury (ammonia, welding fumes, and heavy metals). It may also be seen in rheumatoid arthritis; medication reactions (busulfan, gold, and penicillamine); and chronic rejection following heart-lung, lung, or hematopoietic stem cell transplantation (bronchiolitis obliterans syndrome). Patients with constrictive bronchiolitis have airflow obstruction ...

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