Pulmonary alveolar proteinosis (PAP)

In this H&E stained section of lung is shown the intra-alveolar deposition of homogeneous precipitate. The alveolar walls are normal. When stained with PAS the precipitate is positive. The precipitated material is derived from surfactant phospholipids and protein components.

PAP is a rare disease with a 4-fold greater incidence in men as compared to women. The age of onset is from 20-50 years. Chest x-ray shows bilateral perihilar infiltrates with consolidation in a 'bat-wing' configuration which mimics pulmonary edema. High-resolution CT scan will show areas of patchy ground-glass opacification with smooth interlobular septal and intralobular interstitial thickening in a pattern referred to as 'crazy paving'. When this pattern is seen one must also consider sarcoidosis, bronchioloalveolar carcinoma, and acute respiratory distress syndrome.