Small Airway Diseases

The category of small airway diseases applies to three major groups of lung diseases. These include respiratory bronchiolitis, follicular bronchiolitis, and constrictive bronchiolitis. Though many inflammatory diseases of the lung have small airway pathology as a secondary component and descriptions of those will be found in other chapters (interstitial lung diseases, pneumoconiosis, and infections), in these three diseases, the small airway pathology represents the primary pathology seen in the lung. These pathologic diseases are characterized by the common clinical features of shortness of breath, cough, decreased expiratory volumes, and/or hypoxia. These are found in multiple clinical settings including smoking/environmental exposures, immune deficiencies, and transplantation, the latter of which includes both primary rejection of the lung allograft and graft-versus-host disease in the lung secondary to bone marrow transplantation. Though cytopathology evaluation in these diagnoses is limited, both imaging and small biopsies specimens can be helpful, if evaluated within the appropriate clinical context.