Bleeding into the alveolar spaces characterizes the syndrome of diffuse alveolar hemorrhage (DAH) and is due to disruption of the alveolar-capillary basement membrane. This is caused by injury or inflammation of the arterioles, venules, or alveolar septal (alveolar wall or interstitial) capillaries. Hemoptysis is the usual presenting symptom; however it is not always present, even when hemorrhage is severe [1-3].
An overview of the diffuse alveolar hemorrhage syndromes will be presented here. The evaluation and treatment of hemoptysis are discussed separately. (See "Etiology and evaluation of hemoptysis in adults" and "Massive hemoptysis: Initial management".)
ETIOLOGY AND HISTOLOGY
A variety of diseases is associated with the development of the diffuse alveolar hemorrhage syndrome (table 1) [1,2,4-20]. One of three different histologic patterns may be seen:
Pulmonary capillaritis — This pathologic pattern, also referred to as alveolar capillaritis, is characterized by neutrophilic infiltration of the alveolar septa (lung interstitium) (picture 1A-B). This sequentially leads to necrosis of these structures, loss of capillary structural integrity, and pouring of red blood cells into the alveolar space and interstitium [1,4-6,21-24].
Many of the neutrophils undergo fragmentation and eventually become pyknotic, findings which support a pathogenetic role for neutrophil by-products (toxic oxygen radicals and proteolytic enzymes) in this form of lung injury. The fragmented neutrophils, nuclear dust, and fibrin also enter the alveolar spaces, and true fibrinoid necrosis of the interstitium may be seen.