Refining the Radiologic Recognition of Pulmonary Alveolar Proteinosis: A Crazy-Paving Pattern Approach

Abstract

Pulmonary alveolar proteinosis (PAP) is a rare airspace disease classically associated with the crazy-paving pattern on high-resolution computed tomography (HRCT). While highly suggestive, this imaging pattern is not pathognomonic and appears across a wide spectrum of pulmonary pathologies. In this review, we adopt a phenotype-first approach, using representative imaging cases to walk the reader through the differential diagnosis of crazy-paving, with attention to radiologic distribution, clinical context, and disease acuity. We emphasize distinguishing features between PAP and its mimics—including pulmonary edema, diffuse alveolar hemorrhage, organizing pneumonia, mucinous adenocarcinoma, exogenous lipoid pneumonia, acute fulminant PAP, and COVID-19 pneumonia—using side-by-side imaging and contextual pearls. Special attention is given to the radiologic clues favoring autoimmune versus secondary PAP, including geographic distribution of ground-glass opacities, subpleural sparing, and lower lobe predominance. The review concludes with a summary of diagnostic strategies, pathologic correlation, and treatment options, including insights from post-pandemic diagnostic pitfalls. This pattern-based framework is designed for the radiologist and serves as a practical guide for recognizing PAP within the broader spectrum of airspace diseases.