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 interstitial pneumonitis), fibrinoid necrosis of the vessel walls, and destruction of the alveolar-capillary basement membrane. The infiltrating neutrophils release reactive oxygen species, proteolytic enzymes (elastase, collagenase, myeloperoxidase), and neutrophil extracellular traps (NETs) that directly damage the capillary endothelium and basement membrane, creating gaps through which red blood cells extravasate into the alveolar space. The most important causes of pulmonary capillaritis are ANCA-associated vasculitides: granulomatosis with polyangiitis (GPA, formerly Wegener granulomatosis) associated with cytoplasmic ANCA (c-ANCA/PR3-ANCA), microscopic polyangiitis (MPA) associated with perinuclear ANCA (p-ANCA/MPO-ANCA), and eosinophilic granulomatosis with polyangiitis (EGPA, formerly Churg-Strauss syndrome). Anti-glomerular basement membrane (anti-GBM) disease (Goodpasture disease) produces both DAH and rapidly progressive glomerulonephritis through autoantibodies targeting the alpha-3 chain of type IV collagen, which is present in both pulmonary and glomerular basement membranes. Systemic lupus erythematosus (SLE) causes DAH through immune complex deposition in alveolar capillaries (lupus pneumonitis) in approximately 2-5% of SLE patients, with a mortality rate of 50-90%. The second histopathological pattern is bland pulmonary hemorrhage (without capillaritis), where bleeding occurs through an intact but stressed alveolar-capillary membrane. This pattern is seen in coagulopathies (DIC, anticoagulant therapy, thrombocytopenia), mitral stenosis (chronically elevated pulmonary venous pressures cause capillary stress failure), and idiopathic pulmonary hemosiderosis (a rare condition predominantly in children where recurrent episodes of bland hemorrhage lead to iron deposition and pulmonary fibrosis). The third pattern is diffuse alveolar damage (DAD), the pathological correlate of ARDS, where the inflammatory destruction of the alveolar-capillary membrane causes hemorrhagic pulmonary edema. Regardless of the underlying mechanism, the consequences of alveolar hemorrhage are devastating. Blood filling the alveoli directly impairs gas exchange by creating a diffusion barrier for oxygen and carbon dioxide, producing ventilation-perfusion mismatch and shunt physiology. The hemoglobin released from extravasated red blood cells is converted by alveolar macrophages into hemosiderin, and the presence of hemosiderin-laden macrophages (siderophages) in bronchoalveolar lavage (BAL) fluid is a key diagnostic finding. Surfactant is diluted and inactivated by the hemorrhagic fluid, further compromising alveolar stability and promoting atelectasis. If hemorrhage continues, the patient develops progressive hypoxemic respiratory failure requiring mechanical ventilation. Recurrent episodes of DAH lead to pulmonary fibrosis from the organization of blood and inflammatory debris within the alveolar spaces.