CoVs-seropositivity is reported in association with several neurological disorders with diverse immune-inflammatory processes such as acute disseminated encephalomyelitis (ADEM)-like demyelination, multiple sclerosis, optic neuritis, and encephalitis [
18‐
21]. The capacity for neurovirulence in coronaviruses, including SARS-CoV-2, may contribute to the relatively high prevalence of neurological complications in COVID-19 patients [
22‐
25], particularly among hospitalized patients with severe or critical illnesses [
10,
26]. Early reports estimated the incidence of neurological complications to be about 37% [
10,
26‐
32]. These complications were more common among severe cases associated with greater acute innate immune-inflammatory response (e.g., pro-inflammatory cytokines, chemokines), higher levels of CRP, ferritin, and D-dimer [
7,
8,
10,
12,
31,
33]. Of note, like in many other viral infections and para-infectious diseases, an increase in neutrophil and decrease in lymphocyte counts were associated with greater disease severity and poorer clinical outcome [
14,
33]. Notably, the severity of lymphocytopenia correlates negatively with a rise in serum levels of IL-6, TNF-α, and IL-10, and positively with disease progression and activity [
34], and upregulation of T cell exhaustion markers [
14,
34]. The exhaustion markers are T cell immunoglobulin and mucin domain-containing protein-3 (TIM-3) (mainly expressed on activated CD4
+ Th1 cells and CD8
+ cytotoxic T cells) [
35], programmed cell death-1 (PD1) (mostly expressed on activated CD4
+ T cells and CD8
+ T cells [
35]). CD94/NK group 2 member A (NKG2A) mainly expressed on cytotoxic lymphocytes such as NK cells and CD8+ T cells [
14]. The concurrent upregulation of these markers suggests that the aberrantly upregulated cytokines promoted lymphocytopenia via depleting functionally exhausted T cells [
14,
34]. The normalization of lymphocyte count, together with the reversal of T cell functional exhaustion, adds support for the contribution of aberrant cytokine production to lymphocyte death [
14]. Moreover, post-mortem examination of COVID-19-positive subjects revealed atrophy and necrosis of the spleen and lymph nodes, together with lymphocytic apoptosis [
36]. SARS-CoV-2 nucleoprotein antigen was detected in angiotensin converting enzyme 2 (ACE2)-expressing CD169
+ macrophages present in the marginal zone of the spleen and in the subcapsular sinuses of the lymph nodes [
36]. These macrophages exhibited significant IL-6 upregulation. These findings suggest the infected macrophages can, via IL-6 upregulation, promote viral spread, inflammation, and lymphocyte depletion [
36]. Lymphocytic apoptosis together with the pattern of cytokine and chemokine upregulation reflective of the innate immune response indicate that SARS-CoV-2 infection primarily involves uncontrollable activation of innate immune cells (mainly neutrophils and macrophages) while impairing cells mediating adaptive immunity (i.e., CD4
+ T cells, CD8
+ T cells, B cells, and NK cells) [
14]. This can be pathogenetically relevant as these adaptive immune cells can exert immunoprotective effects to regulate harmful inflammation, protect uninfected host cells, and facilitate viral clearance [
1,
14].
The neurological manifestations can be generally divided into two categories: central and peripheral [
10,
23,
26‐
30,
37‐
41]. Central manifestations include headache, dizziness, impaired consciousness, encephalopathy, delirium, global confusion, syncope, seizures, gait difficulties, cerebrovascular events, encephalitis, and post-infectious autoimmunity. Peripheral disorders include isolated cranial nerve dysfunctions (i.e., impaired sense of smell and taste sensation), Guillain-Barré-syndrome, and myositis-like muscle injury. Although the majority of neurological symptoms develop throughout the course of illness, others such as acute strokes can be the initial presentation [
10]. In this review, we focus only on the pathophysiology of the CNS complications in COVID-19.