Histology

This pathology description is based on autopsies of eight fatal cases of SARS from Tan Tock Seng Hospital, Singapore. In those individuals with severe disease resulting in death, the predominant pathology in the lung was diffuse alveolar damage (DAD) in varying phases of organization. In these cases scattered type II pneumocytes showed marked cytologic changes including multinucleation, cytomegaly, nucleomegaly, clearing of nuclear chromatin, and prominent nucleoli. Although these changes were severe, they are within the spectrum of epithelial changes seen in other cases of DAD. Definite viral inclusions were not seen.

DAD is a pattern of acute lung injury characterized in the acute phase by hyaline membranes (photomicrograph left), interstitial and intraalveolar edema, patchy type II pneumocyte hyperplasia, microthrombi, and scant interstitial infiltrates of mononuclear cells. The acute phase forms a continuum with the proliferative or organizing phase in which proliferation of interstitial fibroblasts and prominent type II pneumocyte hyperplasia are the histologic hallmarks. The histological correlate of acute respiratory distress syndrome (ARDS), DAD has numerous possible causes including infections, inhalants, ingestants, drugs, shock, sepsis, radiation, and miscellaneous injuries such as acute pancreatitis, burns, and uremia.