Organising pneumonia.

Organising pneumonia is defined pathologically by the presence in the distal air spaces of buds of granulation tissue progressing from fibrin exudates to loose collagen containing fibroblasts (fig 1). 2 The lesions occur predominantly within the alveolar spaces but are often associated with buds of granulation tissue occupying the bronchiolar lumen (bronchiolitis obliterans). This pathological pattern is not specific for any disorder or cause, but reflects one type of inflammatory process resulting from lung injury. It may also be a feature of the organising stage of adult respiratory distress syndrome and may be an accessory finding in other inflammatory disorders such as vasculitis. However, organising pneumonia is the particular pathological hallmark of a characteristic clinicoradiological entity called cryptogenic organising pneumonia. This terminology is preferred to the other name used for this condition—namely, idiopathic bronchiolitis obliterans with organising pneumonia (BOOP)—which may be confused with other types of bronchiolar disorders, particularly constrictive bronchiolitis obliterans which is mainly characterised by airflow obstruction.