Mechanisms of Neurological Disease
Multiple putative mechanisms of disease have been proposed for COVID-19 induced nervous system disorders including coagulopathies as well as virus-associated host responses. Indeed, it is probable that specific pathogenic processes underlie the individual neurological presentations associated with COVID-19 in both the CNS (Figure 1A) and the PNS (Figure 1B). We review the different proposed mechanisms next.
Cerebrovascular Disease/Bioenergy Failure
Microvascular injury characterized by thinning of the basal lamina of endothelial cells, fibrinogen leakage and microhaemorrhages has been described in the brainstem and olfactory bulb of deceased COVID-19 patients corresponding to visible MRI changes. These observations are also complemented by other neuroimaging studies in which cerebral infarction was the most common finding on conventional brain MRI. Most post-mortem analyses have shown signs of thrombotic microangiopathy and endothelial injury with minimal evidence of prototypic vasculitis. This pattern is suggestive of endotheliitis. Although there have been several case reports of CNS vasculitis associated with COVID-19, none have confirmed the diagnosis histologically. A cohort of patients with stroke and COVID-19 in Wuhan, China, showed elevated serum levels of IL-6, IL-8 and TNF-α, a finding that has been replicated in several subsequent studies. Both IL-8 and TNF-α promote the release of von Willebrand factor, a marker of endothelial damage that is elevated in both ICU and non-ICU patients with COVID-19, while IL-6 inhibits cleavage of von Willebrand factor leading to accumulation of multimers that promote platelet aggregation. These changes are bolstered by findings of damaged cerebral blood vessels or endotheliitis that was associated with extravasation of fibrinogen. These mechanisms of disease are highly plausible because of the frequency of coagulation-related events during COVID-19. Indeed, neuroimaging studies point to abnormal energy metabolism, shown by reduced FDG detection in frontal lobes of patients with acute COVID-19.
SARS-CoV-2 infects respiratory cells via engagement of the angiotensin-converting enzyme 2 (ACE2) receptor,[15,139,140] with a higher binding affinity than other coronaviruses such as SARS-CoV-1. The ACE2 receptor is present on type II alveolar and respiratory epithelial cells, cardiomyocytes, neurons and astrocytes.[140,142] This receptor is also present in pericytes and smooth muscle cells of cerebral blood vessels and is expressed in the thalamus, cerebellum and brainstem nuclei of humans.[143–145] After binding to ACE2, cleavage of the spike (S) protein of SARS-CoV-2 by transmembrane serine protease 2 (TMPRSS2) facilitates cell entry. Alternative docking receptors including neuropilin-1 (NRP1) and basigin (BSG)/CD147 are found at higher levels in the CNS. Similarly, alternative proteases including furin and cathepsin might permit viral entry in cells with low levels of TMPRSS2 expression (e.g. brain).
Several anatomic routes of neuroinvasion by SARS-CoV-2 have been proposed. The integrity of the blood–brain barrier is compromised in multiple conditions associated with mortality in COVID-19, including hypertension, diabetes, smoking and stroke. Areas of increased vascular permeability or lack of blood–brain barrier, such as the pituitary and median eminence of the hypothalamus are also rich in ACE2, NRP1 and TMPRSS2, thus representing possible portals of entry into the CNS. SARS-CoV-2 infects nasal epithelium and perhaps olfactory bulb cells, presenting another entry portal to the CNS, as suggested for other coronaviruses.[151,152] A recent post-mortem analysis of humans with COVID-19 detected SARS-CoV-2 by RT-PCR in neuroepithelium, the olfactory bulb, trigeminal ganglion and brainstem, albeit at low levels. Interestingly, olfactory nerves terminate in the frontal cortex as well as the hypothalamus and amygdala, structures that are implicated clinically, radiographically and electrographically in the neurological sequelae of COVID-19. The importance of the choroid plexus in the development of COVID-19 associated neurological disease in conjunction with neuroinflammation has been highlighted recently in a large study predicated on RNA deep sequencing of brain-derived single cell nuclei transcriptomes. The lack of evidence for productive infection of trafficking immune cells by SARS-CoV-2 to date makes a Trojan horse mechanism of neuroinvasion less likely. Nonetheless, viral proteins and RNA have been detected in CD68+ macrophages isolated from bronchoalveolar lavage of COVID-19 patients. SARS-CoV-2 RNA levels in brain tissue detected by RT-PCR are low and seemingly independent of the presence or absence of apparent neurological dysfunction and histopathological alterations.[14,132] Immunodetection of SARS-Cov-2 viral antigens in neurons from autopsied patients with COVID-19 underscores the potential for direct viral invasion as an important disease determinant.
Remdesivir, a nucleoside analogue that inhibits RNA-dependent replication of SARS-CoV-2, is the only direct antiviral agent approved for COVID-19 treatment despite preliminary results showing no impact on mortality or progression to mechanical ventilation. Molnupiravir is orally available nucleoside analogue that induces coronavirus lethal mutagenesis and is in phase 2 and 3 trials for treatment of COVID-19. A recent randomized control trial of the TMPRSS2 inhibitor, camostat mesylate, in hospitalized patients with COVID-19 did not have any impact on recovery, progression to ICU or mortality.
Host Neuroimmune Responses
Post-infectious neuro-inflammation triggered by expression of viral antigens into the CNS is another proposed mechanism of encephalitis in COVID-19. While human data supporting this hypothesis are limited, a recently published study using a murine model showed a subunit of the SARS-CoV-2 spike protein (S1) crosses the BBB via absorptive transcytosis when administered intravenously and intra-nasally. Indeed, neuropathological studies demonstrate glia activation and occasional leucocyte infiltrates in patients with COVID-19 although the associated molecular pathways (e.g. cytokine, protease, or free radical release) induced are unclear. CSF studies suggest activation of innate immune responses with elevated levels of β2-microglobulin and neopterin and the presence of dedifferentiated monocytes.[113,123] This is associated with increased levels of neurofilament suggesting neuronal injury. Autoimmune mechanisms including both antibody- as well as cell-mediated immune injury of neural tissue are also plausible, given the recognition of autoimmune processes in the systemic COVID-19 pathogenesis. The injury and loss of endothelial cells in arterioles, venules and capillaries represents another neuropathogenic avenue via disruption on the blood–brain barrier and through endothelia production of immune molecules in the lung, kidney and heart of patients with COVID-19. These latter events can be initiated by systemic immune activation as well as a coagulation diathesis. An important qualification to these mechanisms is that concurrent clinical events including systemic hypoxia-ischaemia might affect immune processes within the nervous system. Among patients with COVID-19 associated cerebrovascular disease, autoimmune processes have been directly implicated. For example, the contribution of antiphospholipid antibodies to ischaemic stroke in patients with COVID-19 is controversial. Zhang et al. described three COVID-19 patients with coagulopathy and multi-territory infarcts and anticardiolipin and anti-β2 microglobulin antibodies. Subsequent studies have reported lupus anticoagulant positivity in more than half of COVID-19 patients. Most case reports of antiphospholipid antibodies in COVID-19 do not include repeat assays 12 weeks apart, which is required for the diagnosis of antiphospholipid antibody syndrome. Transient elevation of lupus anticoagulant during systemic inflammation is common, and several infections are associated with false positive antiphospholipid assays, including HIV, hepatitis C virus and syphilis, making current reports of antiphospholipid antibodies in COVID-19 difficult to interpret.
Similarly, autoimmunity is also incriminated in COVID-19 associated GBS; anti-ganglioside antibodies implicated in autoimmune polyradiculoneuropathies such as anti-Gq1b, -GM1 and -GD1b antibodies have been reported in patients with COVID-19 presenting with cranial neuropathies, weakness, areflexia and sensory ataxia. Anti-ganglioside antibodies are most strongly associated with more aggressive axonal motor neuropathies and poorer functional outcomes compared to AIDP. The rare presence of these antibodies raises concern about potential molecular mimicry mediated by SARS-CoV-2 that could trigger autoimmune responses with important implications for vaccine safety. The spike (S) protein of SARS-CoV-2 is highly glycosylated; thus, the development of anti-glycan antibodies may be essential for an effective host immune response in COVID-19. In a microarray study of 800 human carbohydrate antigens, levels of anti-glycolipid antibodies associated with GBS, including GM1a, GD1a and GD1b significantly higher in COVID-19 patients compared to healthy controls. In this latter study, there was no direct correlation with antibody titre and clinical features of GBS. Anti-glycan antibodies are also observed in other viral and bacterial infections (HIV, EBV, Neisseria meningitidis[31,165]) as well as autoimmune diseases such as Crohn's disease, and thus may merely be a marker of systemic inflammation. Of relevance, there were no reported cases of GBS in the three major COVID-19 vaccine trials.[167–169]
While randomized control trials demonstrate dexamethasone and tocilizumab improve respiratory outcomes in hospitalized patients, their effects on neurological disease in COVID-19 is presently supported only by case reports.[67,170–172] A subset of COVID-19 associated encephalopathies are responsive to steroids and IVIg, and there is a single report of a young patient with encephalitis and SARS-CoV-2 (based on CSF lymphocytosis and T2/FLAIR hyperintensities on MRI), which resolved after treatment with IVIg and tocilizumab. In most cases with a positive response to immunosuppressive or modulatory therapy, SARS-CoV-2 was not detected in CSF, further supporting a para-infectious/immune-mediated basis for disease.
Brain. 2022;144(12):3576-3588. © 2022 Oxford University Press