Dengue Fever and Neurology: Well Beyond Hemorrhage and Strokes

Dengue virus an arbovirus is endemic in an area that comprise almost half of the world's population, contrary to past beliefs that dengue virus differentiate from other neuroinvasive arbovirus due to its lack of neurological invasion and disease related neurological complications excluding hemorrhagic and thromboembolic , the body of evidence have grown to demonstrate a series of neurological manifestations linked to dengue virus with possible mechanisms involving direct virus invasion of the nervous system or immune mediated complications. In this review we provide a wide approach to this neglect but not so rare manifestations of a very common disease.


MANUSCRIPT Introduction
Arboviruses are viral diseases transmitted by the bite of hematophagous arthropods, especially mosquitoes 1 . These mosquitoes are vectors of diseases such as Dengue, Chikungunya, Malaria, Yellow Fever, and Zika virus. Its ability to transport and spread the disease to humans causes millions of deaths each year 2 . In 2019 (by the end of October), Dengue alone registered 2.7 million cases worldwide, including 22,127 of severe cases and 1,206 deaths; 13% higher than the 2015 numbers -the year of the last outbreak 3 . Arboviruses have been a major public health challenge, especially with regard to disease control and its transmission chain.
Dengue is a mosquito-borne viral disease caused by one of four dengue virus serotypes (DENV 1-4), that are genetically similar but antigenically distinct 4,5 . DENV is a small, spherical, lipid-enveloped viruses with a genome composed of a single-stranded, positivesense RNA virus of the genus Flavivirus, family Flaviviridae 6,7 . The disease is spread mainly by the female Mosquito Aedes aegypti, a vector that can be found in all tropical and subtropical regions, and by the Mosquito Aedes albopictus, in regions of temperate climate 5 .
The spectrum of symptoms caused by DENV infection range from an influenza-like disease known as dengue fever (DF) to a severe, sometimes fatal disease characterized by hemorrhage dengue hemorrhagic fever (DHF) and/or shock (dengue shock syndrome [DSS]) 8 . DF symptoms included acute fever, headaches, muscle ache, joint pain, gastroenteritis, and skin rashes 6 . Less than 5% of patients progress to severe life-threatening manifestations, usually seen in those who were previously exposed to heterotypic DENV infection 7 . In fact, the clinical outcome of dengue infection depends on several factors, both in the patient, in the vector, and in the virus itself 9 .
Interestingly, the dengue virus has also become a relevant agent for pathologies of the nervous system. Neurological manifestations in dengue virus-infected patients range from 0.5% to 21% in different studies 4 . The most common subtype associated with neurological disorders appears to be DENV-3 10 . In fact, in dengue disease, the hemorrhagic manifestations in the central nervous system (CNS) are the most known and reported in studies. However, here we discuss neurological manifestations apart from bleeding and heart attacks.

Search Methods
We searched PubMed for articles with no time restriction for articles published in English, using the term "dengue fever", "myelitis", "encephalitis", "ADEM", "new daily persistent headache", "guillain barre syndrome", "neurological complications". We emphasized in clinical studies, case reports, systematic reviews with or without meta-analysis and pertinent narrative reviews to write this review treating about the neurological complications not caused by hemorrhagic or thromboembolic events. The articles selected for the discussion, with clinical data, in this review are presented in Table 1 with a short description of its main findings.

Discussion
Neurological events are uncommon, but an increase in these reports has occurred in recent years 7 . Sahu and colleges (2014) enrolled 486 patients and found an incidence rate of 9.26% of neurological complications in dengue fever patients 10 . Nonetheless, in a prospective study involving 116 patients, 79% of them presented some neurological complications: 34% presenting with encephalitis or encephalopathy, and 45% with muscular alterations 11 . The pathogenesis of dengue involvement of the nervous system is not quite clear 4 . Such manifestations can result from the direct viral invasion of the nervous system due to its neurotropism, immune mediated mechanisms or vascular and metabolic abnormalities, as intracranial hemorrhage, cerebral edema, hyponatremia, hypokalemia, cerebral anoxia 7 .
In vitro and in vivo studies point to an involvement of glial cells and innate immune response in DENV infection 15,16 , in one study with rhesus macaques there were no apparent brain histological alteration but a series of virus induced astrocytes alterations, decreasing its number, inducing its activation 15 . There is also evidence of the virus in the cerebrospinal fluid (CSF) of patients 17 .
In terms of the immune response, a study showed a similarity between the release of the Another study in mice showed behavior similar to anxiety associated with an increase in inflammatory cytokines such as IL-6 and tumor necrosis factor alpha (TNF) and neuronal loss in the hippocampus 22 . These findings are corroborated by another study with the dengue virus 3, inducing meningoencephalitis, and behavioral changes in mice 23 .

Guillain-Barre Syndrome
Guillain-Barré syndrome (GBS), also known as acute idiopathic polyradiculoneuropathy, is an acute, inflammatory, demyelinating, and immune-mediated syndrome. The main findings are areflexia, ascending motor paralysis, and an elevated protein concentration, without pleocytosis (cytological albumin dissociation) in the cerebrospinal fluid 4  GBS and its variants represent 5% of neurological complications in pediatric patients who had DF 13 , the period between the onsets of the illnesses of approximately 10,9 days 26 . Acute motor and sensitive axonal neuropathy (AMSAN) was the most common GBS subtype in these patients, followed by acute motor axonal neuropathy (AMAN), acute inflammatory demyelinating polyradiculoneuropathy (AIDP) and Miller Fisher syndrome (MFS) 12 .
MFS is characterized by the triad of ophthalmoplegia, areflexia, and ataxia, which can also present bulbar paralysis in 60% of the cases. GBS and MFS are being linked as an immune-mediated complication of DF 27 . Another variant of GBS that has been reported with dengue disease is the pharyngeal-cervical-brachial variant 28 .
In a interesting report, the axonal variant of the GBS has been described in two brothers simultaneously and associated with dengue virus infection although none of the two brothers had dengue fever typical symptoms 29 , this report corroborates with a possible genetic predisposition associated with this complication of dengue.
As most cases of GBS occur after the acute stage of dengue, there is a tendency to believe that there is an immunological origin for such manifestations 27 . This is because evidence supporting this hypothesis is related to the same pro-inflammatory substances that participate in both diseases, either in the immune response against the dengue virus or in the pathogenesis of GBS. TNF, interleukins, and complements are substances that might have an important role in the pathogenesis. Also, the immune response that needed to fight the dengue virus may cross-react (molecular mimicry) and attack myelin or peripheral nerve axons 24,30 .

Myopathy
Myalgia is a well-known symptom of dengue disease. In some studies, 90% of the patients with dengue fever had myalgia as a complaint 31 . Muscular alterations in biopsy found 12/15 patients without muscle weakness complaints suggesting that muscular alterations occur even in asymptomatic patients 32 .
Myopathy is also a common presentation in dengue infection: 34 out of 116 patients presented with muscle weakness and high creatinine kinase serum levels 11 . In another study 14 in 30 patients with acute myopathy had the diagnosis of dengue fever associated, 9 of those with normokalemia 33 .
The severity and clinical presentation have great variation among patients with acute myopathy and dengue fever 34,35 , varying from sub-clinical creation kinase (CK) elevation to severe muscle weakness and even rhabdomyolysis or myocarditis can occur simultaneously with dengue acute myopathy 11,34,35,36,37 . Electromyography (EMG) study when conducted does not show characteristics of inflammatory myositis 34,38 .
The myopathy is usually self-limited and doesn't leave sequels 34,38 . However, in 2006, Finsterer and colleagues described a case of a 38-year-old man that contract dengue fever on a holiday in Thailand and presented with fever, headache, and sore eyes associated with an intense 10/10 myalgia pain on a visual scale, which persisted for more than 60 days. Upon electromyography examination, the patient showed spontaneous activity in the subscapularis muscle. After 3 weeks of treatment with corticosteroids, the myositis resolved 39 .

Encephalitis
While encephalopathy as a dengue fever manifestation is more commonly secondary to multisystem disorder from the infectious affection like shock, hepatitis, coagulation disturbs, and even concomitant bacterial infection; when it comes to encephalitis, this mechanism is different 40 .
In the study of Bhushan et al. 9 patients out of the 79 with immune mediated neurological complications presented with acute disseminated encephalomyelitis (ADEM), 3 had cerebellar demyelination only and 1 patient had limbic encephalitis 12 . Considering these three manifestations, 0,8% of the patients with dengue fever had encephalitis associated with dengue virus infection a higher number was found in a pediatric study although with a smaller sample n=71 patients, 8% of those children had encephalitis related with dengue fever and another interesting data was that of those 4 children that died 3 of them had encephalitis 13 .
Although rarer, dengue encephalitis is due to direct neuronal infiltration, inducing the neurological riot by the virus itself in immunological means 40,41 . Whilst Dengue is not considered a neurotropic virus, there are increasing evidences that concern this component in encephalitis 40 . It mainly occurs once the encephalopathy is not regarding to other dengue features, and when the CSF contains the dengue virus and IgM antigen 42 . Also, the encephalitis take place in the viremic stage of the disease, while other encephalopathies happen later. There is evidence that implies the DENV-2 and DENV-3 as the serotypes most verified in neurotropism 43 .
In cases that encephalitis is presented, the manifestation comprises from headache, confusion, dizziness, disorientation, behavioural symptoms and drowsiness 4 . Occasionally, reaching cranial nerve palsy, diminished deep tendon reflexes, leading to hypotonia and hemiparesis.
Seizures are not infrequently seen and the neurological affection can attain up until coma 43 . Though not specific, the electroencephalogram (EEG) might present generalized slow waves in encephalitis, despite of being often imputable to seizures, intracranial hemorrhage and viral infection 43 .
As concerns to laboratory analysis, dengue NS-1 antigen trial has to be positive in serum.
CSF was descripted as colorless and clear in most cases, also presenting lymphocytic pleocytosis and testing negative for other infectious serology 42,43 . IgM testing in the CSF by enzyme-linked immunosorbent assay (ELISA) for dengue antibody, which has both high sensitivity (92%) and specificity (99%), is widely applied and reliable in this diagnosis investigation 43,46 . The gold standard would be the isolated virus in cell culture with antibody identification by fluorescence, however, the appliance of this method is scarcely available 46 .
It is equally important testing the serum and CSF for herpes simplex virus, Mycobacterium tuberculosis, cytomegalovirus , human T cell lymphotropic virus type-1 , Epstein-Barr virus and varicella zoster virus in order to decline this potential causes once the respective results are negative 12 .
Moreover, the encephalitis must be distinguished from encephalopathy of other etiology, in which the detection of dengue virus, NS1 antigen or IgM dengue virus-specific antibodies in CSF will cooperate 4 . Nonetheless, other probable infectious disease testing negative is important to differential diagnosis. Through the CSF sample, it is possible to verify the presence or absence of japanese encephalitis virus IgM, whereby it is proven or rejected the Japanese Encephalitis hypothesis, that is one of the main differential options 50 . Always aware to the possibility of cross-reactivity and considering the epidemiology context, the PCR exam is able to determine the concomitant infection of both viruses 51 .
In the matter of treatment, although there is no effective antiviral agent for dengue virus, acyclovir, corticosteroids and sodium valproate are occasionally used in dengue encephalitis management 43 . Despite that, the supportive care is always necessary, in which antipyretic drugs, analgesics and oral fluids might be useful. On the other hand, it is contraindicated the use of most non-steroidal anti-inflammatory, mainly acetyl-salicylic derivatives 4 .
Post-mortem analysis exhibit that the predominant features found of these patients' brains consists in non-specific oedema lesions 4 . In cases the autopsy was performed, the histopathological patterns were cerebral edema, vascular congestion, hemorrhage, perivascular lymphocytes infiltration, inflammation and also brain matter necrosis 4,43,52 .
Through immunoperoxidase stain it may as well be verified the virus antigen in brain parenchymal cells 43 . When present, TM may be associated with difficult clinical recovery conditions that often promote long-term disability 53 . When a spinal injury affects more than 3 vertebral segments, the disease can be defined as Longitudinally Extensive Transverse Myelitis (LETM), an even rarer condition which leads to severe morbidity 54 .

Myelitis
Some related symptoms to TM are weakness or paralysis of upper and lower limbs, urinary retention, and sensory alterations 55 . It is proposed that temporal factors play a significant role in pathology presentation, considering that in the acute phase patients mainly manifest flaccid paralysis, and in the post-infectious stage, which can begin 1-2 weeks after the initial symptoms, they usually present spastic weakness 56,57 .
According to Mota et al. (2017), the initial signs of TM associated with DF may be twofold. There are still controversies about TM's treatment decisions. Even with proper therapy, many patients take months to restore neurological functions, and some of them remain with residual symptoms. There is no high-level evidence to support the use of intravenous corticosteroids, however, pulse therapy with methylprednisolone is still the main option. Therapeutic plasma exchange can also be used as a second choice 57 . A case report of a 24-year-old woman, who tested positive for dengue IgM/IgG antibodies and presented acute myelitis, showed that even being treated with IV pulse methylprednisolone immunoglobulin plasmapheresis and physiotherapy, it was not possible to obtain a complete recovery in her motor and sensory deficits after 5 months 63 .
Even though myelitis is rarely associated with DF, it is important that clinicians are aware of its existence as a possible complication of the disease, to investigate it in the face of neurological symptoms in patients typical dengue symptoms 56 .

Acute disseminated encephalomyelitis
As previously approach, myelitis and encephalitis can be individual entities in the dengue fever neurological complications spectrum disorders. Another neurological complication linked to dengue fever ADEM present with both encephalitis and myelitis signs.
Bhushan and colleagues found that 11% of the patients with dengue fever immune mediated neurological complications had ADEM 12 , in another study Sil et al. evaluating a pediatric population found 5% of pediatric patients with dengue fever neurological complications had ADEM associated to 13 . A meta-analysis of 2017 encountered a prevalence of 0.4% of ADEM among dengue fever patients 64 .
The onset of neurological manifestations rangended from 3 to 19 days after the first symptoms the most common neurological manifestations was: altered mental status (58%); seizures and urination problems (35%); vision problems (31%); slurred speech (23%); walking problems (15%) and ataxia (12%) 64 .In Kamel study there are few informations on the spinal MRI of the patients but of 9 patients with ADEM in Bhushan research's 7 had a LETM as a featured in image study's 12 , another review that presented 22 cases points to these same direction stating that most patients had extensive myelitis with preference to the cervical and thoracic segment 65 .
More recently 3 peculiar cases of ADEM associated with dengue fever had been described.
Firstly, a patient with a 5-day history of fever that was found in a post vital state by its sister and presented in the emergency service with Glasgow coma scale of 5 needing mechanical ventilation, he was later treated with intensive care and steroids and recovered 66 . The other 2 cases showed two patients with dengue fever associated ADEM presenting with MRI images mimicking multiple sclerosis 67 . Treatment used, varied from intravenous immunoglobulin, steroids, plasmapheresis and other immunotherapy 64,65 . In terms of prognosis of the 29 cases analyzed in Kamel et al. metaanalysis 3 patients died, 7 had partial recovery and 16 had complete recovery. Patients with complete recovery had lower body temperature when compared to the partial recovery group and bad outcome group 64 .

New Daily Persistent Headache
New Daily Persistent Headache (NDPH) is a primary headache of abrupt onset and daily persisting without remission 68 . Although viral infections, surgical procedures, and stressful lives are punctuated as triggers, the pathogenesis remains unclear 69 . Studies have already shown an association between EBV infection and NDPH 68 . Admirably, the association between dengue and NDPH was also recently described 69 In 2017, Bordini and colleagues (2017) reported two cases of NDPH after dengue fever. The first one was a 23-year-old Caucasian male with a two-year history of daily headache. Pain was bilateral, in pressure, and severe in most of the time. Also, it was refractory to Amitriptyline, Divalproex, and Topiramate; and temporary relief was obtained after nerve blockade. The second one was a 42-year-old caucasian woman that presented with bilateral pressure headache the week after the onset of dengue fever (with positive serology). Pain was moderate to severe and nausea, photophobia and phonophobia also occurred. Pain persisted for the next seven months, but relief was obtained with a 10-day course of dexamethasone 69 .

Others
The "others" neurological manifestations of dengue fever deserve a review on its own due to the plurality of disorders associated with the virus in the literature. In fact, these unusual associations range from rapid progressive dementia in an elderly patient 71

Conclusion
In conclusion, it is important to clarify that dengue fever is not an emerging infectious disease but a well-established illness with almost half of the world's populations living in endemic areas and susceptible to the virus.
In addition to what was believed, the dengue virus has a diverse number of associated neurological complications, which are not necessarily correlated with hemorrhagic or thrombotic events, as seen briefly in this review. The diagnosis is made by routine cerebrospinal fluid (CSF) test, which shows an inflammatory reaction, protein-cytological dissociation, cerebromeningeal bleeding in some cases or ischemic stroke, presence of specific viral antigens and/or antibodies and exclusion of other infectious diseases 7 .
In endemic areas, especially in the most important contagion seasons such as summer, an unresolved neurological case, the diagnosis of dengue should not be ruled out. The disease must be considered, tested, and properly managed. This is because dengue can present with severe neurological conditions, but which can follow self-limited processes and with a good prognosis.  These report 10 cases of DF and GBS, with variable and severe symptoms, and acute motor-sensory axonal neuropathy being identified in all cases. The average days between DF and GBS was 10,9 days. All patients were treated equally with immunoglobulin, and full recovery varied from nine days to one year.
Simon et al. 6 2016 New Caledonia 3 It reports 3 cases of early GBS and dengue fever. All these three patients had de neurological disease and its regression, dengue fever, and the serum diagnosis within one week, suggesting an infectious origin. Contrasting with the majority of the cases, which are considered as a post-infectious disease. It reports a case of a six years old girl who had DF and 20 days later developed GBS. She was treated properly but some neurological sequels remained. It tries to explain the correlation in the pathogenesis between these two diseases, being the main factor the similarity of the immune response presented by both.