Pathological manifestations of inflammatory microthrombosis in COVID-19

Abstract

Background. COVID-19 is a predominantly respiratory disease caused by the SARS-CoV-2 virus that has spread as a pandemic since 2020; characterized by high mortality and unexplored long-term outcomes. Aim. To describe the morphological changes in the lungs and other organs in severe forms of COVID-19. Material and methods. A macro- and microscopic examination of the lungs, kidneys, liver, brain and heart of 45 patients who died from complications of COVID-19 was carried out. Histological preparations were stained with hematoxylin and eosin, by the Picro-Mallory stain, as well as with fluorescent antibodies and dyes to visualize neutrophil extracellular traps. Results. The leading pathomorphological manifestations of COVID-19 were inflammation and microthrombosis in the lungs. At the same time, diffuse alveolar damage often developed as a manifestation of acute respiratory distress syndrome in an adult. Diffuse leukocyte infiltration of the lungs was combined with the formation of neutrophil extracellular traps in the lumens of the blood vessels. Diffuse microthrombi were found in most lung tissue samples (87%), while macroscopic thrombi were much less common (16%). In single cases, the cause of death was thromboembolism of the branches of the pulmonary artery, originating from deep vein thrombi of the lower extremities or the right atrial appendage. In the tissues of the liver, heart, kidneys and brain, moderate alterative (degenerative and necrobiotic) microscopic changes were observed, as well as morphological signs of concomitant chronic diseases. Conclusion. COVID-19 is characterized by pathogenically interrelated morphological manifestations of inflammation and primary pulmonary (micro)thrombosis in the form of accumulation of leukocytes both in blood clots and in the alveoli, while regional pulmonary immunothrombosis is associated with diffuse damage to the alveoli; in the heart, brain, liver and kidneys, predominantly dystrophic and necrobiotic changes occur, which are typical of secondary injuries and/or concomitant pathology.