Histology

Histologically, AEP shows features of DAD coupled with interstitial (left) and alveolar infiltrates of eosinophils. DAD is a pattern of acute lung injury characterized in the acute phase by hyaline membranes, interstitial and intraalveolar edema, patchy type ll pneumocyte hyperplasia, and microthrombi. The acute phase forms a continuum with the organizing phase in which proliferation of interstitial fibroblasts, organizing alveolar exudates, and prominent type ll pneumocyte hyperplasia are the histologic hallmarks. AEP may be misdiagnosed as DAD when eosinophils are distributed focally (top right). Therefore, in the setting of DAD, a thorough search for eosinophils is essential to avoid this pitfall. Increased eosinophils can be detected on transbronchial biopsy specimens (bottom right), however, false negatives occur as the result of focal distribution of these cells. Eosinophils are extremely sensitive to corticosteroids, and therapy with this class of drugs can lead to rapid reduction of eosinophil content in blood and tissue. The resulting absence or reduction in eosinophils may obscure diagnosis in patients who receive corticosteroids prior to initiating the diagnostic work-up.