How COVID-19 Affects the Brain

Authors: Maura Boldrini, MD, PhD1,2Peter D. Canoll, MD, PhD3Robyn S. Klein, MD, PhD4,5,6

COVID-19 has resulted in more than 120 million cases and 2.6 million deaths to date. Respiratory and gastrointestinal symptoms are accompanied by short- and long-term neuropsychiatric symptoms (NPs) and long-term brain sequelae.

Some patients present with anosmia, cognitive and attention deficits (ie, brain fog), new-onset anxiety, depression, psychosis, seizures, and even suicidal behavior.1,2 These present before, during, and after respiratory symptoms and are unrelated to respiratory insufficiency,1 suggesting independent brain damage. Follow-ups conducted in Germany and the United Kingdom found post–COVID-19 NPs in 20% to 70% of patients, even in young adults, and lasting months after respiratory symptoms resolved,1 suggesting brain involvement persists.

Entering through angiotensin-converting enzyme 2 receptors,2 SARS-CoV-2 can damage endothelial cells leading to inflammation, thrombi, and brain damage. Moreover, systemic inflammation leads to decreased monoamines and trophic factors and activation of microglia, resulting in increased glutamate and N-methyl-d-aspartate (NMDA)3 and excitotoxicity (Figure). These insults induce new-onset or re-exacerbation of preexisting NPs.

Brain Vascular Injury, Neurotransmitter System Dysfunction, Thrombotic Events, Neuronal Damage, and Neuropsychiatric Symptoms

A, SARS-CoV-2 invades endothelial cells via transmembrane angiotensin-converting enzyme 2 (ACE2) receptor, enabled by transmembrane protease, serine 2 (TMPRSS2). B, Cytokine elevation and microglia activation result in increased kynurenine, quinolinic acid, and glutamate, and neurotransmitter depletion. C, Coagulation cascade and elevation of von Willebrand factor (vWF) lead to thrombotic events. D, Altered neurotransmission, excitotoxicity by increased glutamate, and hypoxic injury contribute to neuronal dysfunction and loss. E, Neuropsychiatric symptoms differ depending on the Brodmann area involved. IL indicates interleukin; NMDA, N-methyl-d-aspartate; TNF, tumor necrosis factor.

Does the Virus Invade the Brain?

SARS-CoV-2 is known to penetrate the olfactory mucosa, causing loss of smell, and may enter the brain, migrating from the cribriform plate along the olfactory tract2 or through vagal or trigeminal pathways; however, definitive evidence for this is lacking. SARS-CoV-2 could pass the blood-brain barrier (BBB) because inflammatory cytokines induce BBB instability or via monocytes.4 It could reach brain tissue via circumventricular organs (CVOs), midline structures around the third and fourth ventricles, that monitor blood and cerebral spinal fluid content via fenestrated capillaries lacking the junctional proteins expressed in the BBB. Viral RNA was detected by reverse transcription–quantitative real-time polymerase chain reaction but not by in situ hybridization in medulla and cerebellum,2 located next to the area postrema, a CVO that controls emetic responses to toxins. SARS-CoV-2 protein has been found in brain vascular endothelium but not in neurons or glia.2 Thus, detected viral RNA may represent contamination by vasculature in leptomeninges and Virchow-Robin spaces. Histopathologic analysis of whole human brain showed microglial nodules and phagocytosis of neurons (neuronophagia) in brain stem and less frequently in cortex and limbic structures, associated with sparse lymphocytic infiltration, and no correlations between histopathologic findings and levels of viral messenger RNA in the same brain.5 While ageusia, nausea, and vomiting may be related to CVO and brain stem viral invasion, other short-term and long-lasting NPs are more likely due to neuroinflammation and hypoxic injury. Brain stem involvement may explain persistent autonomic abnormalities and anxiety.

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