Organizing pneumonia is a particular type of inflammatory reaction of the lung which gives rise to a clinico-pathological syndrome. It is called "secondary" when a cause such as an infection, a drug toxicity, or a connective tissue disease can be identified, or "cryptogenic" when no cause is identified. The clinical picture is usually characterized by the subacute onset of fever, fatigue, cough and dyspnea, with multiple subpleural areas of consolidation on thoracic imaging.
Organizing pneumonia is characterised by the presence of buds of endoalveolar connective tissue. These result from an injury to the alveolar epithelium, followed by the deposition of fibrin in the alveolar spaces, and the migration of fibroblasts which produce a myxoid endoalveolar matrix. A remarkable feature of organizing pneumonia is the complete disappearance of these endoalveolar buds with corticosteroid treatment, in sharp contrast with what is seen in pulmonary fibrosis. The clinical response to corticosteroids is usually prompt and excellent. Relapses are frequent but usually benign.
As the clinical, imaging and pathological characteristics of organizing pneumonia are now well established, many questions remain unanswered, such as the mechanisms involved in the complete reversibility of the pulmonary lesions, and the role of steroid-sparing treatments such as immunomodulatory macrolides.