Comparison of clinical and pathological features between severe acute respiratory syndrome and coronavirus disease 2019

Severe acute respiratory syndrome (SARS) and coronavirus disease 2019 (COVID-19) shared similar pathogenetic, clinical and pathological features. Fever and cough were the most common symptoms of both diseases, while myalgia and diarrhea were less common in patients with COVID-19. Acute respiratory distress syndrome (ARDS) was the most severe pulmonary complication that caused high mortality rate. Histologically, diffuse alveolar damage (DAD) was the most characteristic finding in non-survivors with either SARS or COVID-19. Cases of patients died less than 10-14 days of disease duration demonstrated acute-phase DAD, while cases beyond 10-14 days of disease duration exhibited organizing-phase DAD in SARS. Meanwhile, organization and fibrosis were usually accompanied by exudation. Coronavirus was mostly detected in pneumocytes, but less in macrophages and bronchiolar epithelial cells. Hemorrhagic necrosis and lymphocyte depletion were found in lymph nodes and spleen in both SARS and COVID-19, indicating a pathological basis of lymphocytopenia. Thrombosis was commonly observed in small vessels and microvasculaturr in lungs accompanying DAD. Microthrombosis was also found in extrapulmonary organs in COVID-19, that was less reported in SARS. Damages in multiple extrapulmonary organs were observed, but coronavirus was not detected in some of those organs, indicating an alternative mechanism beyond viral infection, such as hypoxemia, ischemia and cytokine storm induced immunological injury. DAD due to viral infection and immunological injury, as well as multi-organ dysfunction and extensive microthrombus formation, brought huge challenge to the management of patients with severe SARS or COVID-19.