- Chronic Bronchitis is a clinically-defined entity characterized by a productive cough for at least 3 consecutive months in at least 2 consecutive years. In individuals where chronic bronchitis results in a pattern of obstructive lung function, it is then considered a component of Chronic Obstructive Pulmonary Disease (COPD). Below we discuss the etiology and morphology of chronic bronchitis and refer the patient to the COPD page for a discussion of the clinical consequences of this disease which often occurs together with emphysema.
- Chronic bronchitis is caused by repeated and frequent exposure to inhaled irritants, particularly cigarette smoke and urban smog
- Consequently, chronic bronchitis is most often observed in smokers and urban dwellers. Irritation of the airway mucosa results in hypersecretion by the glands of the respiratory submucosa as well as goblet cells of the respiratory epithelium. Together, these secretions narrow the airway of bronchi and bronchioles, leading to increased airflow resistance, and in many cases overt obstructive lung disease.
- Chronic bronchitis is most characterized by hypertrophy and hyperplasia of the glands within the respiratory submucosa, especially in the trachea and large bronchi. These changes characteristically increase the "Reid Index" of the affected airways, defined as the ratio of the thickness of the submucosal layer to that of the entire airway wall.
- In addition, the respiratory epithelium often displays increased numbers of goblet cells while the respiratory lamina propria is infiltrated with inflammatory cells, typically mononuclear in population. Importantly, eosinophils are lacking in this infiltrate, distinguishing the inflammation of chronic bronchitis from that of asthma. Smaller airways such as bronchioles also show morphological changes such as inflammation and fibrosis which, together with mucous hypersecretion, may narrow their luminal diameter. Indeed, narrowing of these smaller airways may be the primary source of the increased airflow resistance characteristic of chronic bronchitis.