Endothelial Dysfunction in the Brain: Setting the Stage for Stroke and Other Cerebrovascular Complications of COVID-19.

The Coronavirus disease 2019 (COVID)-19 pandemic has already affected millions worldwide, with a current mortality rate of 2.2%. While it is well-established that severe acute respiratory syndrome-coronavirus-2 causes upper and lower respiratory tract infections, a number of neurological sequelae have now been reported in a large proportion of cases. Additionally, the disease causes arterial and venous thromboses including pulmonary embolism, myocardial infarction, and a significant number of cerebrovascular complications. The increasing incidence of large vessel ischemic strokes as well as intracranial hemorrhages, frequently in younger individuals, and associated with increased morbidity and mortality, has raised questions as to why the brain is a major target of the disease. COVID-19 is characterized by hypercoagulability with alterations in hemostatic markers including high D-dimer levels, which are a prognosticator of poor outcome. Together with findings of fibrin-rich microthrombi, widespread extracellular fibrin deposition in affected various organs and hypercytokinemia, this suggests that COVID-19 is more than a pulmonary viral infection. Evidently, COVID-19 is a thrombo-inflammatory disease. Endothelial cells that constitute the lining of blood vessels are the primary targets of a thrombo-inflammatory response, and severe acute respiratory syndrome coronavirus 2 also directly infects endothelial cells through the ACE2 (angiotensin-converting enzyme 2) receptor. Being highly heterogeneous in their structure and function, differences in the endothelial cells may govern the susceptibility of organs to COVID-19. Here, we have explored how the unique characteristics of the cerebral endothelium may be the underlying reason for the increased rates of cerebrovascular pathology associated with COVID-19.