The Crazy-paving Sign1

Abstract

The crazy-paving sign was initially recognized in patients who had pulmonary alveolar proteinosis (1). Pulmonary alveolar proteinosis manifests as filling of the alveoli by a proteinaceous material that is positive at periodic acid–Schiff staining and is rich in lipids, in association with an inflammatory response in the adjacent interstitium (3). It is believed that pulmonary alveolar proteinosis results from an abnormality of surfactant production, metabolism, or clearance by type II alveolar cells and macrophages (4). Most cases are idiopathic. Some result from exposure to silica (silicoproteinosis) or occur in association with hematologic disorders, such as lymphoma or leukemia and, occasionally, with human immunodeficiency virus infection (4). The linear network or reticular pattern in the crazy-paving sign is believed to result from a thickening of the interlobular septa, while areas of groundglass opacity result from partial alveolar filling processes (1,5). Interlobular septal thickening can be seen from interstitial fibrosis, but the presence of areas of reticular opacity does not necessarily represent interstitial abnormalities (2). Kang et al (6), using the findings of a single case, suggested that the network in the crazy-paving sign in alveolar proteinosis may result from an accumulation of periodic acid–Schiff-positive material in the airspaces adjacent to the interlobular septa rather than to thickening of the septa. Therefore, when there is a slight increase in the severity of the alveolar filling process at the borders of unit structures such as acini or secondary pulmonary lobules, the networks in the crazy-paving sign can also be caused purely by airspace disease (2). The crazy-paving sign may also be seen in patients with a variety of other diseases (2,7). The differential diagnosis of crazy-paving sign includes Pneumocystis carinii pneumonia, mucinous bronchioloalveolar carcinoma (8), sarcoidosis, lipoid pneumonia (9), adult respiratory distress syndrome (7), and pulmonary hemorrhage syndromes (10). In a prospective study of patients showing the crazy-paving sign, a variety of causes were identified (7). These included P carinii pneumonia, alveolar proteinosis, usual interstitial pneumonia, pulmonary hemorrhage, acute radiation pneumonitis, adult respiratory distress syndrome, and drug-induced pneumonitis. Of these, P carinii pneumonia was most common. Johkoh et al (2) reported 46 patients showing the crazy-paving sign on thin-section CT images, the most common causes included adult respiratory distress syndrome (n 8), bacterial pneumonia (n 7), acute interstitial pneumonia (n 5), and, despite its rarPublished online 10.1148/radiol.2433041835