pulmonary alveolar proteinosis

**Definition:** Impaired surfactant clearance leads to filling of alveoli with periodic acid–Schiff (PAS)-positive, diastase-resistant lipoproteinaceous material (surfactant lipids and proteins SP-A, SP-B, SP-C, SP-D). Alveolar architecture is preserved, with little or no inflammation or fibrosis.

**Pathophysiology:** - GM-CSF signaling (via the GM-CSF receptor on alveolar macrophages) drives the transcription factor PU.1, which is needed for macrophage maturation and surfactant catabolism. - Without it, macrophages become foamy and dysfunctional, and surfactant builds up in the alveoli.

**Types:** 1. **Autoimmune (primary, ~90%):** Neutralizing IgG autoantibodies against GM-CSF. Typically affects ages 30–50, often in smokers or people with dust exposure. Serum anti-GM-CSF antibody is highly sensitive and specific. 2. **Secondary (~5–10%):** Reduced number or function of macrophages. Causes include hematologic malignancy (e.g., MDS, AML, CML), immunodeficiency, and inhaled toxins (silica, aluminum, titanium, indium). It can also follow infections such as Nocardia, Pneumocystis, or mycobacteria. 3. **Congenital/hereditary:** Mutations in the GM-CSF receptor α or β chain (CSF2RA, CSF2RB), or in surfactant protein genes (SFTPB, SFTPC) and ABCA3, which present in neonates and infants. Also NKX2-1 and MARS1 mutations.

**Clinical features:** - Insidious dyspnea on exertion, nonproductive or mildly productive cough, fatigue, and weight loss. Low-grade fever or hemoptysis suggests superinfection. - Often little to no abnormality on exam. Crackles are uncommon, and clubbing is rare. - Increased risk of secondary infection (Nocardia, mycobacteria, fungi, Pneumocystis) due to impaired macrophage function.

**Diagnosis:** - **HRCT:** Bilateral ground-glass opacities with smooth interlobular septal thickening ("crazy-paving"), with sharp demarcation from normal lung (geographic pattern). - **CXR:** Bilateral perihilar "bat-wing" alveolar opacities. - **BAL:** Milky or opaque fluid. Cytology shows foamy macrophages and extracellular PAS-positive, diastase-resistant globules (acellular material). - **Serum:** Anti-GM-CSF antibodies. LDH, CEA, and surfactant proteins A and D may be elevated. - **PFTs:** Restrictive pattern with markedly reduced DLCO and an increased A–a gradient (hypoxemia from shunt physiology). - Biopsy is rarely needed. It shows intra-alveolar eosinophilic granular material with preserved lung architecture.

**Treatment:** - **Whole-lung lavage** (under general anesthesia, one lung at a time) is the standard of care for symptomatic patients with significant hypoxemia. - **Inhaled or subcutaneous GM-CSF** (sargramostim, molgramostim) is used in refractory or mild cases. Inhaled molgramostim has shown benefit in autoimmune PAP. - **Other options:** Rituximab or plasmapheresis in refractory cases. Lung transplant is rare and mainly for hereditary forms. Treat the underlying cause in secondary PAP. - Smoking cessation and avoidance of dust exposure.

**High-yield associations and differentials:** - "Crazy-paving" is also seen in Pneumocystis pneumonia, ARDS, lipoid pneumonia, hemorrhage, and bronchioloalveolar carcinoma, but the milky BAL and PAS+ material point to PAP. - Autoantibody against GM-CSF is the classic board answer for adult PAP. - Compare with silicosis: acute silicoproteinosis after heavy silica exposure resembles PAP. - Complication: infection and, less commonly, progressive respiratory failure or pulmonary fibrosis.

**Boards pearl:** A middle-aged smoker with progressive dyspnea, "crazy-paving" on CT, and milky BAL fluid with PAS+ material → pulmonary alveolar proteinosis (anti-GM-CSF antibodies) → whole-lung lavage.