Introduction

The worldwide dashboard of WHO registered more than 97 million confirmed cases and 2.1 million deaths due to COVID-19 as of January 24, 2021, a year after very first identified case [1]. Though it most often presents with symptoms and complications referable the respiratory system, reports of neurological manifestations continue to grow.

Several studies have reported neurological complications patients with COVID-19 [2,3,4]. Reports from Wuhan, China describe neurological complications frequently in patients with COVID-19. Those studies showed that 36.4% patients had neurological symptoms including acute cerebrovascular events, impaired consciousness and dizziness [4]. Another study showed one-third of patients with COVID-19 had neurological complications [5]. Anosmia and dysgeusia are also common neurological manifestation of COVID patients and is thought to be mediated by viral invasion of the olfactory neuroepithelium and cellular distribution of taste cells via ACE2 receptor [6, 7].

A prospective study by Frontera et al., detected neurologic disorders in 13.5% of patients with COVID-19 and indicated that neurological symptoms were associated with decreased likelihood of discharge to home and increased risk of in-hospial mortality [8]. These manifestations appear to be an amalgamation of systemic disease complications including systemic inflammatory mediators, nervous system and vasculature inflammation, or the effects of direct viral invasion. The neuroinflammation associated with COVID-19 could be either from direct viral neuroinvasion leading to inflammation and cytokine release or from delayed autoimmune dysregulation or molecular mimicry leading to autoimmune/inflammatory syndromes that is parainfectious/ postinfectious [9,10,11]. Currently, there is insufficient knowledge about the effects of SARS-CoV-2 on central nervous system (CNS) inflammation involving brain, optic nerve and spinal cord. In this review, we have retrospectively analyzed the various CNS inflammatory manifestations of COVID-19 reported to date. This includes acute myelitis, acute disseminated Encephalomyelitis (ADEM), acute hemorrhagic necrotizing encephalitis (AHNE), and cytotoxic lesion of the corpus callosum (CLOCC). We also discuss the relevant neuroimaging and cerebrospinal fluid markers (CSF) associated with CNS inflammation and COVID-19.

Methods

Study design

We conducted a thorough literature review in March 2021 using the terms “SARS-CoV-2 and neurological complication”, “SARS-CoV-2 and CNS Demyelination” for reports of myelitis, transverse myelitis (TM), longitudinally extensive transverse myelitis (LETM), neuromyelitis optica (and spectrum disorder; NMO or NMOSD), myelitis, Acute Disseminated Encephalomyelitis (ADEM), Acute Hemorrhagic Necrotizing Encephalitis/Acute Hemorrhagic Leukoencephalitis (AHNE/AHLE), Cytotoxic lesion of the Corpus Callosum (CLOCC) and Optic neuritis (ON).

We searched PubMed, Google Scholar and Scopus databases for identifying case series and case reports published between December 01, 2019 to March 15, 2021. Review articles and consensus statements were excluded from the analysis. We used the preferred reporting items for systematic reviews and meta-analyses (PRISMA) for the display of inclusions and exclusions [12]. Based on our search criteria, we found articles from PubMed (n = 189), Google Scholar (n = 1201) and Scopus (n = 55). Amongst all, 424 cases were identified as duplicates. Finally, we screened 1021 articles for title and abstracts, and reviewed full-text literatures in accordance with our study objective after removing 918 articles which were either missing clinical information or did not meet our study objective and 70 based on exclusion criteria (Fig. 1). The review was limited to articles in English.

Fig. 1
figure 1

Preferred Reporting Items for Systemic Reviews and Meta-Analysis (PRISMA) Flow Diagram

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We included 33 publications and 43 cases for review for observational analysis that met our below-mentioned inclusion criteria, out of which 15 were of acute myelitis including transverse myelitis, 10 cases of ADEM, 6 cases of CLOCC, 9 cases of AHNE/AHLE. Apart from these one case of myelitis, considered by the authors to be Clinically isolated syndrome (CIS), and two cases had MOG mediated demyelinating disease. One MOGAD patient presented with optic neuritis and one with optic neuritis and myelitis. We excluded statistical analysis of MOGAD disorders as a separate entity as well as one CIS case due to low sample size although we describe these cases in “Discussion”. Therefore 40 cases of COVID-19 and CNS inflammatory disorder were reviewed for descriptive quantitative analysis.

Inclusion criteria

The inclusion criteria for the published studies included: (1) Patient age ≥ 18 years; (2) COVID-19 diagnosis confirmed by RT-PCR nasopharyngeal or serum antibody test; (3) CSF study findings in COVID-19 and MRI imaging performed; (4) CNS specific disorders including ADEM, AHNE/AHLE, CLOCC, acute myelitis including transverse myelitis and longitudinally extensive myelitis and ON.

Exclusion criteria

The exclusion criteria from the published studies include: (1) Patient age < 18 years; (2) Duplicate articles which involved repetition of cases (3) Articles in languages other than English; (4) Studies that had no available individual patient’s data; (5) Editorials; (6) Articles and reported literature on CNS and peripheral nervous system (PNS) disorders other than acute myelitis, ADEM, AHNE/AHLE, CLOCC and ON.

Quality assessment

The critical appraisal checklist for case reports provided by the Joanna Briggs Institute (JBI) was used to perform assessment of overall quality of case series and case reports [13].

Data acquisition

Two reviewers independently performed the literature search. From the selected articles, we extracted the following data for our analysis: study type, date of publication, age, gender, clinical presentation of COVID-19, diagnostic tests for SARS-CoV-2 infection including RT-PCR nasopharyngeal, CSF SARS-CoV-2 RT-PCR and serum antibodies, CSF markers including cell count, protein, severity of COVID-19 (based on IDSA/ATS criteria), treatment, neuroimaging including MRI findings. Severity of COVID-19 was measured using IDSA/ATS criteria [14].

Data analysis

We performed demographic analysis including age, gender, severity of COVID-19 cases and outcome of the cases where provided. Pooled descriptive analyses were conducted to assess differences in these markers among groups including severe vs non-severe, fatal vs non-fatal outcomes.

Results

Based on our literature search, we found a total of 40 cases with COVID-19 diagnosed with various CNS inflammatory disorders for the descriptive quantitative analysis. These included 35 case reports and 2 case series published from 16 different countries. Of the 40 cases, 14 were from the USA, 4 cases from France,3 cases from UK, 2 each from the Italy, Qatar, India, Belgium, Iran and one each from UAE, Australia, Brazil, Germany, Spain, Moldova, Japan, Singapore and Switzerland. Summarized information of these cases is presented in Tables 1, 2, 3 and 4.

Table 1 Study Origin, Demographics, CSF, MRI findings, severity and outcomes in COVID-19 and acute transverse myelitis and MOGAD myelitis disorder
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Table 2 Study Origin, Demographics, CSF, MRI findings, severity and outcomes in COVID-19 and MOG disorder with optic neuritis and CIS
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Table 3 Study Origin, Demographics, CSF, MRI findings, severity and outcomes in COVID-19 and ADEM and AHNE/AHLE
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Table 4 Study Origin, Demographics, CSF, MRI findings, severity and outcomes in COVID-19 and CLOCC
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The demographic characteristics including severity of COVID-19, outcomes, treatment, MRI abnormality is summarized in Table 5. The main cohorts of CNS inflammatory disorder include acute myelitis including transverse myelitis (TM) /LETM and optic neuritis, ADEM including AHLE/ANHE and CLOCC. Out of the entire cohort, there were 14 patients (35%) with age < 50 years, and the remaining 26 patients (65%) were aged > 50 years. The mean age was 50.7 (SD ± 15.1) years, median age was 52.5 years, with age ranging from 21 to 75 years. Amongst the total of 40 patients in the the statistical analysis, 27 patients were male (68%) and the other 13 were female (32%). Of the 40 cases, 37% (n = 15) had transverse myelitis, 25% (n = 10) ADEM, 15% (n = 6) AHNE/AHLE, and 23% (n = 9) CLOCC/MERS. Based on IDSA/ATS criteria of either requiring vasopressor for septic shock or mechanical ventilation, 49% (n = 18) of patients were considered to have had a severe COVID infection. In our review, 19% (n = 7) were fatal (Table 5).

Table 5 General characteristics of SARS-CoV-2 patients with CNS inflammatory disorder (n = 40)
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In terms of medications received, 71% of the patients (n = 25) were given intravenous methylprednisolone (IV MP), 26% (n = 9) were given intravenous immunoglobulin G (IVIG), while 23% of the patients (n = 8) received plasma exchange/plasmapheresis (PLEX) for management of various neurological inflammatory disorders. For management for COVID-19, 6% of the patients (n = 2) were given azithromycin, 9% (n = 3) were given hydroxychloroquine (HCQ), while 14% (n = 5) received a combination of HCQ and azithromycin. No patient received tocilizumab. Abnormal contrast enhancement in MRI imaging of the spine and brain was reported in 10% (n = 4) and 23% (n = 9) respectively (Table 5).

The comparisons of severity, outcomes, and CNS manifestations (acute myelitis, ADEM, AHNE/AHLE, and CLOCC/MERS) against age, gender, CSF protein, and elevated cell count are shown in Table 6. However, a statistically significant difference was observed in the CSF cell count amongst patients with a non-severe compared to patients with severe COVID-19 infection.Seventy nine percent (11/14) of the reported elevated cell counts were in patients with a non-severe as compared to patients with severe COVID-19 infection where only 21% of cases had elevated cell counts (3/14) (p = 0.03), whereas 71% of those with transverse myelitis have elevated cell count. Elevation of the CSF protein levels among the various pathologies also showed a difference that was borderline significant. No significant differences were seen in other variables with regards to age, gender, and CSF characteristics (Table 6).

Table 6 Comparisons of COVID-19 severity, outcome and CNS inflammatory disorders for different characteristics
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Discussion

It is now well known that infection with SARS-CoV-2 causes a multi-systemic inflammatory/immunological response. Although the exact mechanism responsible for postinfectious neurological disorders is not fully understood, the diverse neurological presentations of COVID-19 have been attributed to the underlying immunological mechanisms [10, 15, 16]. It is hypothesized that in some instances the T cell and/or antibody immune reaction against the infectious agent is directed against a CNS cell or structure because of similarities between some component of the infectious agent and a protein, lipid or carbohydrate component of the CNS. This which once was called cross-reactivity is now known as molecular mimicry. Even though a strong immune response is essential for protective adaptive immunity, a prolonged and overactive immune response contributes to pathological tissue injury [17]. This immune response has garnered attention towards a phenomenon called “cytokine storm” which is associated with high fever, respiratory distress, multi-organ failure and increased mortality over the first 2 weeks in COVID-19 patients [18, 19].Currently, little is known about the lasting neurological effects of the “cytokine storm”. In this systematic review of 43 patients, 40 subjected to staitical analysis with a spectrum of CNS inflammatory disorders in COVID-19 patients, the most common presentation was that of acute myelitis, often transverse, followed by ADEM, CLOCC/MERS, and AHNE/AHLE. The timing of neuroinflammatory complications relative to initial symptoms of COVID-19 infection and the rarity of detection of SARS-CoV-2 in CSF or CNS, suggest that most of these particular CNS syndromes reviewed in this paper are parainfectious/postinfectious disorders [9, 20,21,22]. The patients in this review exhibited a wide variety of neurological symptoms of which the most common presentation in myelitis was urinary retention and lower limb weakness [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37]. ADEM mostly presented with decrease level of mentation [21, 38,39,40,41,42,43], CLOCC/MERS with altered sensorium [44,45,46,47,48,49,50,51] and AHNE/AHLE with reduced consciousness and coma [52,53,54,55,56,57].

In terms of diagnostic test for COVID-19 in our review all CNS inflammatory disorders were diagnosed with positive nasopharyngeal RT-PCR, whereas CSF RT-PCR SARS-CoV-2 was positive in two cases of ADEM [21, 41]. Serum SARS-CoV-2 IgG and IgM antibodies were positive in a case of TM [32]. It is unknown if the CNS disease is due to the direct invasion. CSF protein was found to be elevated in 11 cases of transverse myelitis including a case of myelitis, 5 cases of ADEM and 4 cases of AHNE/AHLE suggestive of underlying neuroinflammatory process and changes in blood brain or blood meningeal barriers. Similar to our reports, another study also showed increased CSF protein level in the majority of the COVID-19 patients with neurological manifestations [58]. CSF cell count analysis was reported in 30 patients among which 11 cases had elevated cell count > 5 cells/mm3 with lymphocytic predominance.

Acute myelitis including LETM plus optic neuritis

Viral infections of the CNS are uncommon but are important in the differential diagnosis of acute/subacute myelopathy [59]. Acute myelitis was the most common CNS inflammatory disorder noted in our analysis with a total of 15 cases, including cases of TM and LETM. The average latency reported in previous cases of postinfectious myelitis/encephalomyelitis was 3–20 days [11, 60]. The latency period of myelitis in this review was similar from less than 1 week [22, 23, 26,27,28, 33] to more than 1 week [25, 29,30,31, 36, 37]. The patients presented with a vast range of neurological symptoms, the most common in myelitis were urinary retention and lower limb weakness. Other less frequent symptoms were weakness in upper limb, quadriplegia; paresthesia of lower limb or upper limb or both. The MRI findings in myelitis were categorized into short segment 2 (n = 15, 13.3%) as described by Chakraborty et. al. and Munz et al. [26, 30], or long segment cord involvement of either cervical, thoracic or cervico-thoracic reported in 12 cases [23, 25, 27,28,29, 31, 33,34,35]. Interestingly abnormal enhancement of spinal cord (n = 2, 13.3%) was reported in two publications [23, 31]. Brain MRI studies were reported in 8 cases and were unremarkable in 7 cases [31,32,33,34,35]. One reported case had right pontine restriction diffusion ([36]. Zachariadis et al. reported normal spinal cord MRI in a 63-year-old man who presented with lower limb weakness and paresthesia where diagnosis for myelitis was based on clinical presentation and CSF elevated protein [32]. A case report by Zhao et al., did not have adequate investigations or their provided findings lacked essential data to fulfill all the inclusion criteria for diagnosis of acute myelitis [61].

Two cases of optic neuritis with positive serum Myelin oligodendrocytes glycoprotein (MOG) antibodies one of whom also had myelitis (MOGAD NMO) were reported by Zhou et al. and Sawalah et al. with MOG antibody titers of 1:1000 and 1:160 respectively with negative serum aquaporin 4 (AQP4) antibodies(Table 2). MOG is a protein expressed in the oligodendrocyte membrane and the outermost layer of myelin sheath. Antibodies against MOG have been involved in the pathogenesis of several neurological conditions as noted in subgroups of patients with ADEM, aquaporin-4 (AQP4) seronegative neuromyelitis optica spectrum disorders (NMOSD), monophasic or recurrent isolated optic neuritis (ON), transverse myelitis, atypical MS and ADEM [62]. The demyelination caused by MOG antibodies is attributed to encephalitogenic T cells, antibody-dependent cell toxicity (ADCC) and complement dependent cytotoxicity (CDC) and encephalitogenic T cells which cause blood brain barrier leakage, inflammation and demyelination [63, 64].

The case described by Domingues et al. 42 years woman patient presenting with hemisensory loss 3 weeks after testing positive for CSF SARS-CoV-2 by RT-PCR. A focal cervical cord lesion at C-6 was demonstrated and normal brain MRI. CSF oligoclonal bands were absent with normal CSF cell count and protein. Testing for MOG and AQP4 antibodies was not performed. This patient had an acute onset myelopathy, likely myelitis, of unknown cause. While described as case of suspected CNS demyelination as clinically isolated syndrome (CIS) the patient had a prior episode compatible with a cervical myelopathy and therefore might not meet strict criteria for CIS [65] (Table 2).

ADEM including AHNE/ANLE

ADEM is an immune-mediated generally, monophasic demyelinating disorder involving the brain and occasionally spinal cord. A number of infectious agents, mainly viruses, have been associated with ADEM [66]. In ADEM, latency periods typically vary from 0 days to 8 weeks [43]. The most common presentations were decreased responsiveness, limb weakness, paresthesia of lower limbs, and urinary retention. The most common finding seen on MRI was hyperintensity and restriction diffusion in the deep cerebral white matter. A peculiar finding of hemorrhages and hyperintense lesions within the subcortical and deep white matter of the frontoparietal lobes was noted by Langley et al. [41] and post autopsy findings of hemorrhagic white matter lesions throughout the cerebral hemispheres with surrounding axonal injury and macrophages by Reichard et al. [67]. The MRI findings of spinal cord involvement were of particular interest in 3 cases. The Zoghi et al. reported longitudinally extensive acute transverse myelitis in the thoracic and cervical segments, while Utukuri et al. reported the presence of mild T2 hyperintensities with minimal foci of non-enhancing T2 hyperintense lesions throughout the cervical and thoracic spinal cord. Novi et al. noted a single spinal cord lesion at T8 with bilateral optic nerve enhancement [21, 41, 42] (Table 3).

Acute necrotizing encephalopathy is a rare complication of influenza and other viral infections and has been related to intracranial cytokine storms, which result in blood–brain barrier breakdown but without direct viral invasion or parainfectious demyelination [68,69,70,71]. The similar and overlapping AHLE, which also includes demyelination, can be considered part of a continuum with ADEM based on clinical, pathologic and experimental evidence [67, 72, 73]. Our review revealed six cases of AHNE/AHLE associated with COVID-19. MRI findings in these cases included hyperintense T2 lesions in the thalami, cerebellum, brainstem, supratentorial gray and white matters without gadolinium-enhanced lesions with areas of restricted diffusion and microhemorrhage (Table 3). The patients predominately presented with decreased level of responsiveness. MRI findings showed hemorrhagic lesion lesions in bilateral thalami, medial temporal lobe and sub insular regions [52,53,54,55,56,57]. Outcome and severity of COVID-19 were not reported in one case [52] but the other 5 cases had severe COVID-19 based on IDSA/ATS guidelines [53,54,55,56,57]. There was two fatal outcome as reported by Dixon et al. [53, 56].

CLOCC/MERS

Cytotoxic lesions of the corpus callosum (CLOCC) is a disease entity associated with reversible lesions in the corpus callosum on MRI [74]. The MRI lesions typically resolve within a few days to weeks however the clinical recovery may take longer usually several months [75]. Our review noted 9 cases of CLOCC/MERS in patients with COVID-19[44,45,46,47,48,49,50,51]. The patients had a varied range of clinical presentations, of which the most common was altered sensorium (n = 8), aphasia (n = 2), bradyphrenia (n = 1) and limb weakness (n = 1). MRI imaging in CLOCC demonstrated diffusion restriction and non-enhancing lesions mainly in the splenium of corpus callosum with variable involvement of remaining corpus callosum and cerebral white matter as noted in our cases as well (Table 4).

Our review has several limitations. Cases included in this review were identified through a comprehensive search of databases using a systematic search strategy. However, despite the set criteria, there is a possibility of missing out new upcoming reports and studies because of the evolving nature of the COVID-19 pandemic. A second limitation associated with any review is the concern that a disproportionate number of acute myelitis and other inflammatory neurological disorders associated with COVID are more likely to be reported in case reports and series which can introduce a bias. With the rapidly growing evidence of COVID-19 and association with neurological disorders, case reports and series of atypical demyelination disorders are more likely to be published. Finally, because of the emerging nature of the pandemic, there are no suitable contemporary non-COVID-19 case studies from the institutions reporting the COVID-19 associated CNS inflammatory variants, which would be the appropriate control for comparing the differences in clinical presentations, outcomes and pathophysiology of these disorders when not associated with COVID-19. We believe further studies and reviews are warranted.

Conclusion

In this paper we have reviewed and discussed the clinical features, neuroimaging, CSF findings and outcomes in patients with various manifestations of COVID-19 associated CNS inflammation. The most prevalent CNS inflammatory disorder was acute myelitis followed by ADEM including AHNE/AHLE variant and CLOCC respectively. Our review study reveals that CNS inflammatory disorders are rare but can be associated with COVID-19 infection as they have been reported with many other viruses. Further research using MRI imaging and CSF analysis in earlier diagnosis and intervention of these disorders is warranted.