The organizing pneumonias: an update and review

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Abstract
Basic information as well as more recent concepts regarding cryptogenic organizing pneumonia and secondary forms of the disease. More recently described and less well recognized illnesses associated with organizing pneumonia, such as organizing pneumonia associated with radiation, are enumerated. In vitro studies from separate laboratories are integrated to create a proposed model of the pathogenesis and repair mechanisms that occur in organizing pneumonias. Using current criteria, we note other interstitial lung processes, in addition to organizing pneumonia, are present in some earlier reports. Cryptogenic organizing pneumonia has been reported to respond to corticosteroids with clinico-radiographic resolution in 70-80% of cases. Treatment duration is lengthy, and despite this, recurrences and late recurrences are common. Rapidly progressive, steroid resistant and poor prognostic forms of organizing pneumonia have been described and have been reported more frequently with secondary organizing pneumonia. Since other histologic interstitial patterns often coexist with organizing pneumonia, tissue sampling error or an incorrect morphologic diagnosis can be the reason for aggressive clinical behavior. Steroid nonresponsive patients have been treated with secondary non-steroidal agents. Good clinical outcomes have been reported. Inhaled antigens stimulate GM-CSF-mediated airway inflammation in organizing pneumonia. Repair requires the following: granulation tissue, upon which re-epithelialization occurs; a favorable stromal ratio of matrix metalloproteinase to tissue inhibitors of metalloproteinase; concurrent resolution of inflammation; and stromal fibroblast ingestion of collagen produced earlier in repair, reversing the initial fibrosis.