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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 (fig1).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.
Aetiology of organising pneumonia
Because organising pneumonia is a non-specific inflammatory pulmonary process, it may result from a number of causes. Pathologists may report features of organising pneumonia in association with conditions such as infectious pneumonia, lung abscess, empyema, lung cancer, bronchiectasis, broncholithiasis, chronic pulmonary fibrosis, aspiration pneumonia (giant cells and foreign bodies usually are present), adult respiratory distress syndrome, pulmonary infarction, and middle lobe syndrome.3 4
Organising pneumonia may be classified into three categories according to its cause: organising pneumonia of determined cause; organising pneumonia of undetermined cause but occurring in a specific and relevant context; and cryptogenic (idiopathic) organising pneumonia. Several possible causes and/or associated disorders may coexist in the same patient. There are no clear distinguishing clinical and radiological features between cryptogenic and secondary organising …
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