Pathophysiology
Clinical meaning
Diffuse alveolar hemorrhage (DAH) is a life-threatening pulmonary emergency characterized by bleeding originating from the pulmonary microvasculature (alveolar capillaries, arterioles, and venules) into the alveolar spaces, producing the classic clinical triad of hemoptysis, diffuse bilateral pulmonary infiltrates on chest imaging, and progressive anemia. DAH is not a single disease but a clinical syndrome with numerous underlying etiologies, all of which share the common final pathway of disruption of the alveolar-capillary basement membrane leading to hemorrhage into the alveolar compartment. The alveolar-capillary membrane is an exquisitely thin barrier (approximately 0.2-0.5 micrometers) composed of three layers: the alveolar epithelium (type I pneumocytes that facilitate gas exchange and type II pneumocytes that produce surfactant), the shared basement membrane, and the capillary endothelium. This membrane normally allows efficient gas exchange while preventing blood from entering the alveolar space. In DAH, this barrier is disrupted through one of three histopathological patterns. The most common pattern is pulmonary capillaritis, which accounts for approximately 60-70% of DAH cases. Pulmonary capillaritis is a small-vessel vasculitis affecting the alveolar capillaries, characterized by neutrophilic infiltration of the capillary walls (neutrophilic...