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Cryptogenic organizing pneumonitis (COP) is a specific clinicopathologic syndrome of unknown cause. The incidence and prevalence of COP are unknown, but a cumulative incidence of six to seven per 100,000 hospital admissions was found at a major teaching hospital. The disease onset is typically in the fifth or sixth decades of life, with men and women being affected equally.

The clinical presentation of COP often mimics that of community-acquired pneumonia. In 50% of cases, the onset is heralded by a flulike respiratory illness that includes fever, malaise, fatigue, and cough. Almost three-fourths of patients have their symptoms for less than 2 months, and few have symptoms for more than 6 months before diagnosis. Weight loss of greater than 10 lb is seen in more than 50% of patients. Inspiratory crackles are frequently present on chest examination. A normal pulmonary examination is found in 25% of subjects.

Routine laboratory studies are nonspecific. A leukocytosis without increase in eosinophils is seen in approximately half of patients. The initial erythrocyte sedimentation rate is frequently elevated in patients with COP.

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Pulmonary function is usually impaired with a restrictive defect being most common. Resting and exercise arterial hypoxemia is common.

The radiographic manifestations are quite distinctive. Bilateral, diffuse alveolar opacities in the presence of normal lung volume constitute the characteristic radiographic appearance in patients with COP. The alveolar opacities may rarely be unilateral. High- resolution computed tomography (HRCT) scans of the lung reveal patchy airspace consolidation, ground-glass opacities, small nodular opacities, and bronchial wall thickening and dilatation. These patchy opacities occur more frequently in the periphery of the lung and are often in the lower lung zone. The computed tomography (CT) scan may reveal much more extensive disease than is expected by review of the plain chest radiographs. Recurrent and migratory pulmonary opacities are common.

Histopathologic organizing pneumonia is a specific morphologic pattern of intraluminal fibrosis that is relatively nonspecific but may be characteristic of certain clinical syndromes. The lesion is characterized by excessive proliferation of granulation tissue within small airways (proliferative bronchiolitis) and alveolar ducts associated with chronic inflammation in the sur- rounding alveoli. Foamy macrophages are commonly seen in the alveolar spaces, presumably secondary to the bronchiolar occlusion. A uniform, temporally recent appearance to the changes without severe disruption of the lung architecture is a characteristic finding. The architecture of the lung is preserved, with no remodeling or honeycombing found.

Corticosteroid therapy is the most common treatment used in these patients. It results in clinical recovery in two-thirds of the patients. Early reduction in the dose or cessation of initial orticosteroid therapy may increase the risk of relapse.