Bronchioles are precisely defined as conducting airways that do not exhibit cartilage the walls and do not contain gas-exchanging structures (the alveoli).

The term “small airways” was used to describe both the smallest bronchi and the bronchioles and “peripheral airways” has a similar but even less precise connotation. Lesions in these airways play a significant role in chronic airflow obstruction ; the term “small airways” disease is used to describe them.

The term “bronchiolitis” is used because it involves specific structures and because the lesions are due to inflammation and its consequences.

Bronchiolitis is almost always related to cigarette smoking and occurs in patients with the usual features of chronic airflow obstruction.

 It is important to recognize that lesions in these airways have been studied chiefly at the two extremes of mild or severe chronic airflow obstruction.

Mild Chronic Airflow Obstruction:

The lesions are usually studied in lungs resected for cancer, in which case patients with severe chronic obstructive lung disease are excluded.

In these specimens the lesions are mild, and chronic bronchiolar inflammation is the important association with chronic airflow obstruction.

The inflammation is often mild, consisting of an increased number of lymphocytes, which are plasma cells with an occasional neutrophil.

The mechanism by which inflammation produces airflow obstruction is unknown.

Some have postulated that the inflammatory exudates displaces surfactant. Others have suggested that mediators of inflammation produce direct or reflex constriction of bronchioles.

 Inflammation may induce fibrosis and narrowing of bronchioles, further increasing airflow obstruction.

Although goblet cell metaplasia is well related to airflow obstruction, it should be regarded as a consequence, not a cause, of inflammation.

Increased bronchiolar muscle in smokers with chronic airflow obstruction reflects hyperplasia as a response to inflammation, similar to the situation in asthma.

 Respiratory bronchiolitis is also a significant cause of mild chronic airflow obstruction.

Severe Chronic Airflow Obstruction:

Patients with severe chronic airflow obstruction are usually studied at autopsy, and the lesions are different from those in persons with mild chronic airflow obstruction.

The consequences of inflammation, like severe narrowing of the airways with an excess of very small bronchioles (less than 400 micrometer in diameter), become more important.

Goblet cell metaplasia is also important, but inflammation, fibrosis, and increased muscle are less significant.

In patients with severe emphysema, the bronchioles become distorted by irregular narrowing, a condition that results in airway obstruction.

Severe Forms of Bronchiolitis:

Certain agents other than tobacco smoke produce bronchiolitis, including toxic gases -  Eg. oxides of nitrogen, ozone, and ammonia.

The first two, which result from photochemical pollution, produce bronchiolitis - in the case of ozone, primary respiratory bronchiolitis.

Most occupational accidents involving nitrogen dioxide are related to the burning of paper or x-ray film.

Rheumatoid arthritis is occasionally associated with mild bronchiolitis, but sometimes severe, progressive airflow obstruction occurs.

In these circumstances, the smallest bronchi and bronchioles are occluded by loose granulation tissue in a spotty distribution.

Other, rare causes of bronchiolitis are graft-versus-host reaction and rejection in heart-lung transplants.

Viral infections are an important cause of bronchiolitis.

Although inflammation is mild, it may be a life-threatening disease, particularly in children.

 Adenovirus and measles bronchiolitis may result in bronchiolar obliteration and bronchiectasis.