Pulmonary histopathological findings of COVID-19

Since the beginning of the COVID-19 pandemic, three clinical stages of disease severity have been described. All these stages manifest with diverse clinical scenarios, ranging from an asymptomatic form to a severe form characterized by acute respiratory distress syndrome with multiple organ dysfunction. At least three primary histological patterns of lung injury have been recorded: 1) epithelial pattern, characterized by diffuse alveolar damage with varying degrees of organization, denudation, hyperplasia of pneumocytes, as well as possible cytopathic changes; 2) vascular pattern, characterized by diffuse intra-alveolar fibrin and/or the presence of microvascular thrombi (fibrin); and 3) fibrotic pattern, which includes fibrotic diffuse alveolar damage and/or interstitial fibrosis. In this study, we describe the post-mortem histopathological findings of COVID-19, including our experience at the INER with 40 patients, 28 men and 12 women. The most frequent patterns were acute alveolar damage (n = 23), chronic cellular infiltrate (n = 10), and minimal change pattern (n = 7). The fibrotic pattern was found in 38 patients, involving < 20%; the alveolar filling pattern was described in all patients, in < 30% of the tissue; and the nodular pattern was present in 7 patients, in < 10% of the tissue. The connection between these pathological findings and the clinical course of COVID-19 suggests the possibility that pathogenesis follows a sequential pattern. In the initial phase of viral infection, respiratory epithelial cells become infected; therefore, epithelial changes predominate with evidence of viral activity.