diffuse alveolar hemorrhage

Diffuse alveolar hemorrhage is widespread bleeding into the air sacs of the lungs. Instead of one local source of blood, countless tiny vessels across both lungs leak at once, so the alveoli that should hold air fill with blood, like rain seeping through a whole roof rather than dripping from a single crack. The flooded sacs can no longer take up oxygen.

The underlying problem is damage to the smallest lung vessels, the capillaries, often from immune-mediated inflammation of the vessel walls (the vasculitides and some connective tissue diseases), but also from severe infection, clotting problems, certain drugs, or heart and vessel conditions. The classic clues are breathlessness, falling oxygen, patchy shadowing across both lungs on imaging, and a dropping blood count.

Coughing up blood is a common but not universal sign; because the bleeding is spread thin, some patients never cough up obvious blood even while their lungs fill. The diagnosis is often confirmed by bronchoscopy, where washing out a lung segment returns progressively bloodier fluid. It is a medical emergency, and treatment is aimed at the cause, frequently with high-dose immune-suppressing drugs when inflammation is responsible.