Brain Complications with Influenza Infection in Children

Objectives: To summarize the characteristics and research progress of influenza-associated brain complications in children and provide references for early diagnosis and brain protection treatment. Methods: Studied published articles of influenza-associated neurocomplications in children from PubMed and summarizes them from epidemiology, clinical manifestations, diagnosis and treatment, and basic research progress. Results: Common brain complications in flu-children include febrile seizures, influenza-associated encephalopathy (IAE), acute or post-influenza encephalitis, and the most severe condition is acute necrotizing encephalopathy (ANE). However, the mechanism and relevant factors of influenza-associated brain damage have not been elucidated. Conclusion: Influenza could be accompanied by various brain lesion complications in dif ferent stages of the disease, some of which are life-threatening or leave severe neurological sequelae, such as ANE. Due to different brain injury mechanisms, specific early diagnosis and brain protection treatment for different complications are unclear or unanimous. Therefore, further classification and basic research are needed.

fection is seasonal in temperate countries, with peaks during the winter months, but it sustains activity throughout the year in tropical climates [1]. Among the brain complications of influenza, the most serious is acute necrotizing encephalopathy (ANE), which has a mortality rate of about 30%. We review the published articles of influenza-associated neurocomplications in children from PubMed, and summarizes them from epidemiology, clinical manifestations, diagnosis and treatment, and basic research progressing, in order to provide references for early diagnosis and brain protection treatment during influenza seasons.
Influenza is an acute infectious disease caused by influenza viruses, which spreads between people through droplets and contact, with fast and high transmission. It is mainly spread via children, who are generally more susceptible to infection.
Influenza outbreaks occur every year at various scales, which normally has an incubation period of 1 -4 days (2 days on average). According to the annual statistic report of infectious diseases from China CDC, the incidence of influenza has been increasing since 2014 ( Figure 1). Influenza-infected persons may exhibit typical symptoms of influenza including acute fever, headache, myalgia and discomfort, with respiratory symptoms, or not show any typical clinical features.
Symptoms in young children are characterized by high fever, febrile seizures, and gastrointestinal complaints such as nausea, vomiting, diarrhoea, and loss of appetite. Common complications of influenza can affect respiratory, digestive, muscular, cardiovascular, neurological and other systems. In the course of the disease, concomitant infection may occur by Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus-influenzae infection, and even septic shock [2].
Factors related to increased mortality rate include [3] [4]: Children under 5 years old (Children under 2 years old are prone to higher incidence of serious complications); Persons over 65 years old; Persons with the following diseases or symptoms: chronic respiratory diseases, cardiovascular diseases (excluding hypertension), kidney disease, liver disease, blood system diseases, nervous system and neuromuscular diseases, metabolic and endocrine diseases, inhibition of immune responses (including low immune function induced by the use of immunosuppressants or HIV infection); Obesity ( BMI greater than 30); Pregnancy and perinatal women; Concomitant infection by Staphylococcus aureus or Streptococcus pneumoniae [2]. It is worth noting that, omitting pediatric patients with high-risk conditions, about 1/2 of the pediatric patients in death cases have no high-risk conditions [5].

Neurologic Complications Associated with Influenza
Central nervous system (CNS) complications may occur in influenza-affected children, and with higher risk (<4 years old) in young children with basic diseases. Neurologic complications of influenza include febrile convulsion, encephalopathy, acute encephalitis, aseptic meningitis, acute cerebellar ataxia, myelitis, Guillain-Barre' syndrome, acute disseminated encephalomyelitis (ADEM), acute mental status change, and occasionally cerebrovascular disease, such as cerebral infarction [8]- [13]. Along with reduced use of aspirin in children, the incidence rate of Reye syndrome decreases.

Epidemiology and Susceptible Population
IAE is a clinical syndrome accompanied by central nervous system dysfunction during acute influenza, which is more common in young children. The incidence of IAE peaks during 1 to 5 years old children, which accounts for 81.8% of total [5], therefore 1 to 6 years old influenza patients are targeted population of prevention and treatment.

Clinical Manifestation
Patients with IAE have symptoms of convulsions, acute cognitive impairment, acute disturbance of consciousness (ADOC), and coma with varying severity [14] [15] [16]. In a recent study showed that in children under 2 years of age altered consciousness was the most frequent neurological manifestation while respiratory symptoms were present at admission; younger children also required intensive care support more frequently; and the median time from onset of respiratory signs to onset of neurological manifestations was 24 h [17].

Laboratory Inspection
There is no specific laboratory indicator of IAE. Liver dysfunction is symptom commonly seen in IAE patients, including ALT, AST, and LDH; thrombocytopenia and coagulation abnormalities can also occur. All these factors are associated with poor prognosis [9]. Hyperammonemia and hypoglycemia is presented

Pathology
Vascular injuries in nervous system were found in cadaveric tissue pathology, including intracerebral vascular occlusion, micro thrombus formation, perivascular haemorrhage and edema, but no inflammatory changes were observed in mononuclear cell infiltration [23] [24]. Clasmatodendrosis was distributed diffusely in the IAE brains in close association with synapses and was not caused by astrocyte autophagy [25].

Diagnostic Criteria, Treatment and Outcome
The diagnostic and therapeutic criteria of IAE are shown in corticosteroids, and plasmapheresis were recorded, but the evidence for its effectiveness is limited. IAE are heterogeneous with varied clinical features, and outcome related to magnetic resonance imaging changes [29].

Basic Research Progress
Extensive disruption of astrocytic projections (clasmatodendrosis) was detected in H1N1-associated deceased patients by anti-glial fibrillary acidic protein (GFAP) immunostaining of brain tissue [30]. Reduced spine density of microglia, abnormal proliferation of glial cells, demyelinating changes of nerve fiber in brain tissue pathology were also reported in previous studies, while no lymphocyte and neutrophil infiltration were found [31]; and close association with synapses and was not caused by astrocyte autophagy [25]. These pathological phenomena imply that influenza viruses can damage brain cells through non-direct damage. Serum level of neutrophil elastase was also elevated in IAE patients [32]. Research of Hosoya and colleagues indicates that elevated serum level of cytochrome c oxidase in influenza patients implies incidence of influenza-associated encephalopathy: increased level of cytochrome c oxidase above 45 ng/ml or higher threshold implies a poor prognosis of influenza-associated encephalopathy; sensitivity of 93% and specificity of 100% suggests apoptosis following virus infection [33].

Epidemiology and Susceptible Population
IANE is an acute non-inflammatory encephalopathy commonly, which is often marked by a sudden, explosive attack and occurs mostly in young children under 5-year-old.

Clinical Manifestation
Typical clinical manifestation of IANE is rapid progression to encephalopathy, coma or death within 1 to 2 days from the onset (usually within 24 hours from the onset of fever) [14] [15] [34]. Generally, the clinical stages are: 1) Prodromal stage-most children have respiratory, gastrointestinal infections such as fever, cough, vomiting, diarrhea; 2) Early-stage of acute encephalopathy-within 1 to 2 days after the prodromal stage (Usually within 24 hours after the onset of fever) rapid progress to high fever, frequent convulsions, ADOC (Acute disturbance of consciousness), and the average time from fever to the appearance of neurological symptoms is 1.7 days; 3) Acute necrotizing encephalopathy stage-deterioration of neurological symptoms occurred rapidly, such as status epilepticus, deep coma, and multiple organ failure even death; 4) recovery stage-starts from the recovery of consciousness in 6 -10 days after the onset of brain symptoms, which often lasts for several months, and very few patients can fully recover. It is possible that <4 years of age, repeated seizures, ADOC, and Babinski's sign might be the high-risk factors for IANE [4].

Laboratory Inspection
There is also no specific laboratory indicator of IANE

Imaging Examination
Patients with ANE display symmetric necrosis of the thalamus and other deep brain structures, particularly in the brain stem, surrounding white matter and cerebellar medulla. Most early-stage ANE cases have brain edema, and about 10% -20% of them show features of acute necrotizing encephalopathy; imaging shows widespread areas of restricted diffusion in white matter [36]. Generally, using contrast-enhanced CT, which is an easy-to-obtain neuroimaging test, ring-shaped enhancement of the thalamus and deep brain white matter can be detected after 3 days in the course of illness, irregular high-density shadows in the hypothalamic mottled low-density area can be observed after 7 days in the course of illness; while no abnormal lesions were found in patients who died within 30 hours [37] (Figure 2). Grey matter damage is significantly different in conventional MRI [38]: In typical cases, symmetric grey matter lesions shows decreased signal intensity in T1-weighted image (T1WI), and increased signal intensity in T2-weighted image (T2WI); 3 days after onset of encephalopathy, the thalami displays concentric-ring pattern in T1WI, that is, increased signal intensity is in the center of

Pathology
About 70% of children with ANE died or had severe neurological dysfunction.
Cadaveric brain was dissected under the light microscopy and examined with myelin and H&E staining. The lesion showed a stratified structural change from the outside to the inside, that is, the concentric structure of the thalamic lesion [16] [37]: moderately sized plasma extravasation occurred in the vicinity of the edge of the lesion, to which the arteries were generally more susceptible than veins, and lesion areas showed myelin pallor; the surrounding tissues of central vessels (arteries, veins, and capillaries) were congested, oligodendrocytes displayed acute swelling, and the brain tissues showed loosening with increasing degree from the outside to the inside; the central blood vessels were damaged, erythrocyte extravasation was accompanied with necrosis of neuron and glial cells, but not with inflammatory cell infiltration and reactive proliferation of glial cells. A post-mortem examination of one fatal case revealed vasogenic brain edema with generalized vasculopathy, suggesting that the generalized impairment of vascular endothelial cells caused by highly activated cytokines plays a central role in the pathophysiology of this disease [14]. Extensive disruption of astrocytic projections (clasmatodendrosis) was detected in H1N1-associated deceased patients by anti-glial fibrillary acidic protein (GFAP) immunostaining of brain tissue [30].
The pathological changes of the above parts can also be observed in the anatomy of patients with IAE, which suggested there might be overlap or confusion in the diagnosis between IANE and IAE.

Diagnostic Criteria, Treatment and Outcome
As reported showed, the present studies did not find indicators of IANE diagno-    [14]. Common treatments include intensive care to maintain vital signs, low brain temperature, maintain cerebral perfusion, control temperature, reduce cerebral edema, oseltamivir, immunoglobulin, glucocorticoids, and plasma exchange [40] [41] ( Table 2). Despite the consensus treatment, including immunoglobulins and intravenous steroids, the patients with IANE still had high rates of mortality (around 30%) and sequelae (33% to 50%). Oseltamivir did not help to avoid death [42].

Basic Research Progress
At present, the mechanism of nerve cell necrosis in patients with IANE is still unclear.

cron-Reactive Oxygen Metabolites，CSF d-ROM) levels were elevated in patients
with severe brain injuries, which could be valid indicators of IAE severity [48].
These phenomena indicate that influenza virus infections is accompanied by elevated levels of inflammatory cytokines; some inflammatory cytokines, such as cytochrome c oxidase, are indicative for incidence of IAE and ANE; The elevated CSF d-ROM implies that increased oxidised stress may be relevant to the pathogenesis of IAE and ANE. Nerve cell necrosis is an essential pathological change in IANE.
As necrotic nerve cells increase, necrosis-associated biomarker levels are reflected in CSF, including LDH [49] and MDA, however no related literature was reported.

Autoimmune Factors
Autoimmune encephalopathy is one of the complications of influenza-related brain injuries [8] [50]. There were tests in pathological brain tissues from autopsies and rat models were positive for antibodies to aquaporin-4 (AQP4) [25] [51]; and clasmatodendrosis was distributed diffusely in the brains tissue in close association with synapses, and was not caused by astrocyte autophagy [25]. There has been research showing that influenza viruses may promote type I T cell infiltration into CNS, thus inducing long-term exacerbation of autoimmune encephalomyelitis [52]. Neopterin concentrations in cerebrospinal fluid were elevated in patients with IAE [26], which are markers of cellular immune activation [53] [54]. These studies suggest that influenza viruses cause immune damage and persistence of neuropathy through different pathways by stimulating the formation of autoantibodies.

Vascular Endothelial Damage Factors
Imaging study showed deep cerebral venous thrombosis in ANE patients, and attenuated signal intensity and enlarged deep cerebral veins in brain T2-weighted MR [13]. Brain pathology of deceased influenza patients found vascular lesions in nervous system, such as cerebral vascular occlusions, microthrombus formation, perivascular hemorrhage [23] [24]; there also existed apoptosis of vascular endothelial cell and brain tissue [55] [56]. Studies have shown that both platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) levels were elevated in IAE patients, while elevation of PDGF was more significant and correlated with incidence and prognosis of ANE [57].

Host Gene Tendency Factors
There are reports indicating that hosts with certain genetic mutations displayed higher incidence of influenza and even ANE. Polymorphism in the ran-binding protein 2 (RANBP2) gene is associated with recurrent necrotizing encephalitis and respiratory viral infection [15]. There are some other reports indicating higher risk of severe IAE in patients with mutations in the interleukin 10 receptor, alpha subunit (IL-10RA) [58]. Researchers believe that mutations in the CYP2C9 gene are possibly involved with diclofenac-induced IAE [59]. These abnormalities are rarely seen, but their correlation with incidence of ANE cannot be ignored.

Epidemiology and Susceptible Population
There are few epidemiological reports of influenza-associated acute encephalitis.
Vulnerable populations in different countries and regions are also different. For example, most severe in France occur in adults, while in East Asia, children are more common but different in age [60].

Clinical Manifestation
The clinical manifestations of influenza-associated acute encephalitis are similar to IAE, which presenting convulsions, acute cognitive impairment, acute disturbance of consciousness (ADOC), and coma with varying severity [14] [15] [16] [60]. The two are difficult to distinguish from clinical manifestations.

Laboratory Inspection
Influenza-associated acute encephalitis should be taken into consideration when excessive high level of leukocyte is detected in the cerebrospinal fluid (CSF) [10].
Protein level in CSF is generally normal or slightly increased, while ANE is accompanied with increased protein level in CSF, and non-increasing level of leukocyte.

Imaging Examination
Brain images show changes in viral encephalitis/meningitis [22] [61], such as cerebral edema and meningeal strengthening. Changes in the brain imaging of the IAE could be observed, and no abnormalities could Changes in the brain imaging of the IAE were also observed, and no abnormalities could also be observed.

Diagnostic Criteria, Treatment and Outcome
Encephalitis was defined as encephalopathy plus two or more of the following [28] [62] (Table 3)

Epidemiology and Susceptible Population
During the influenza outbreak, abnormal behaviors such as personality changes and delirium occur in around 10% of influenza patients in the recovery period or after 2 -3 weeks since the onset of influenza, in which circumstance attention should be paid to the possible PIE [68] [69] [70]. This situation can also happen now after flu vaccination. Children and adolescents are more common.

Laboratory Inspection
Encephalitis-like cerebrospinal fluid changes could be found. Positive autoantibody test result, such as anti-NMDAR antibody-positive detection [70] [72], can be found in some PIE patients.

Imaging Examination
Typical MRI manifestations of post-infection encephalitis include multifocal white matter lesions. Long T2 signal in cortical lesions is detected by brain MRI in PIE patients.

Basic Research Progress
PIE is an autoimmune process, which is thought to be associated with demyelination and vascular disease [50]. There have been reports of one-way optic neuromyelitis (NMO) in patients with influenza A infections [73]; influenza virus infection is aggravated as neurological diseases including multiple sclerosis (MS) [74]; strengthened immunization during the epidemic seasons appears to protect patients with certain autoimmune diseases against worsening of underlying disease [75]. Autoantibodies could be detected in following patients: levels of serum antibodies against myelin-oligodendrocyte glycoprotein (MOG antibody) is elevated in patients with localized symptoms [76].

Epidemiology and Susceptible Population
Cerebral infarction or cerebrovascular disease is rare after influenza virus infection in children, but common in adults. The influenza season is usually consistent with peak mortality and morbidity due to cardiovascular diseases and pneumonia. Influenza vaccination can reduce probability of infection, and therefore prevent formation of cerebral thrombosis and incidence of cerebral infarction [77] [78] [79]. Patients with high blood pressure, coronary heart disease, diabetes mellitus, hypertriglyceridemia and hypohigh-density lipoproteinemia had higher incidence of cerebral infarction after infection with influenza virus than healthy people.

Clinical Manifestation
There are few reports on cerebral infarction or cerebrovascular disease compli-

Imaging Examination, Pathology
The neuroimaging features of deep cerebral venous thrombosis (DCVT) may sometimes be found [38] [39]. Catheter cerebral angiogram documented vasculopathy in PRES-involved regions with areas of focal vessel dilatation and string-of-bead appearance [81]. Brain imaging studies revealed that cerebral hemorrhage and cerebral infarction lesions were consistent with autopsy studies [20] [82]. Cerebrovascular damages could be found in the cadaveric brain tissues, appearing as cerebrovascular occlusion, microthrombus formation, and perivascular hemorrhage [23] [24]. Lesion damage showed stratified structural change from the outside to the inside, which is concentric structure of thalamic lesions [16] [37].

Diagnostic Criteria, Treatment and Outcome
Clinical influenza complicated with cerebral infarction is often missed or misdiagnosed. For example, the delay of anticoagulant treatment due to misdiagnosis or missed diagnosis in the time window can lead to serious adverse consequences.
The situation of patients with cerebral infarction or cerebrovascular disease after influenza virus infection is dangerous and the patient's laboratory indicators should be closely monitored and timely treated. In addition to active anti-virus, symptomatic support, brain function protection and other treatment, timely and Best treatment practice a) General treatment: Ensure adequate oxygen supply; control blood pressure and blood glucose reasonably; control the temperature and maintain internal stability b) Special treatment: Fibrinolytic therapy; platelet inhibitor therapy; anticoagulation c) Prevention and treatment of complications d) Treatment of increased intracranial pressure and brain edema effective thrombolysis and anticoagulant treatment is particularly important, but also through influenza vaccination can reduce infection to prevent cerebral thrombosis and the occurrence of cerebral infarction. As shown in Table 5, the occurrence of cerebral infarction or cerebrovascular lesions should be warned.

Conclusion
Influenza is accompanied by various brain lesion complications in different stages of the disease: Acute symptoms, such as convulsions, cognitive impairment, ADOC and coma, could occur in the acute phase; mental symptoms could be seen in the recovery period, such as deliriums, hallucinations and personality changes. Some neurological complications are life-threatening or leave severe neurological sequelae, such as acute necrotizing encephalopathy. Therefore, clinicians should be vigilant to early diagnosis and reasonable intervention. Studies have shown that in addition to viruses and hosts, there are other factors in the pathogenesis of influenza-associated brain injuries, such as inflammation, autoimmunity and cerebral vascular endothelial damage, yet the precise pathogenesis remains to be further elucidated.