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.

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].

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 in around 11% of IAE cases. No viruses could be detected in CSF in most cases, and cell numbers and proteins level of CSF are normal in around 90% of IAE cases. In most cases, IAE are caused by influenza A virus infections or mixed infections of influenza A and influenza B viruses, while cases infected merely by influenza B viruses only count for 10%.

Brain imaging examinations were conducted in IAE patients, including computed tomography (CT) scan and magnetic resonance imaging (MRI). Imaging changes were found such as cerebral edema, hemorrhage, or bilateral thalamic lesions, and these neuroimaging abnormalities are associated with poor prognosis [18] [19]. Mild encephalopathy patients were also diagnosed with reversible corpus callosum lesion (MERS) [20] [21]. According to MRI and CT results, Kimura et al. divided influenza related brain lesions into 5 categories [22]: normal (category 1); diffuse involvement of cerebral cortex (category 2); diffuse brain edema (category 3); symmetrical involvement of the thalamus (category 4); and postinfectious focal encephalitis (category 5).

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].

The diagnostic and therapeutic criteria of IAE are shown in Table 1 [18] [23] [24], include: 1) Acute onset with positive detection of influenza virus; 2) Accompanied by sudden neurological symptoms; 3) Most cerebrospinal fluid cells are normal, or with slight protein elevation; 4) Imaging supports encephalopathy change, Including head CT and MRI, CT or MRI can find brain edema, bleeding or bilateral thalamic lesions and other imaging changes; diffuse slow wave in EEG. There was a study inclusion the patient with IAE included either presented with status epilepticus (for 30 min or longer) or reached level 2 diagnostic certainty on the Brighton encephalopathy score [26] [27]. Another author defines encephalopathy as altered mental status lasting ≥ 24 hours [28]. Commonly used treatment options are oseltamivir, empiric 3rd generation cephalosporins, IV-Ig,

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].

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].

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.

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].

There is also no specific laboratory indicator of IANE [8] [16] [35]. Blood tests show increased white blood cells, decreased platelets, increased CRP and ESR, prolonged APTT, and decreased FIB. Biochemical indicators show liver enzymes (ALT, AST and LDH) significantly increased and hypoalbuminemia. CSF protein levels are abnormally elevated. However, these indicators are not specific for diagnosis. A study by Ashley pointed out that the independent factors associated with death included blood AST > 500 U/L, glucose > 150 mg/dL, hematuria or proteinuria, and positive RANBP2 mutation detection [15].

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

the lesion and decreased signal intensity is in ring shape around the center, besides, increased signal intensity stilled remains in T2WI; in the second week, T1WI reveals ring-shaped increased signal intensity in the thalami, indicating subacute hemorrhage. Diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) map show concentric pattern in acute phase of typical cases [39], as shown in Figure 3 and Figure 4.

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.

As reported showed, the present studies did not find indicators of IANE diagnosis from laboratory indicator [16] [35]. Mizuguchi et al. proposed diagnostic criteria for ANE for clinical reference [16] (Table 2): 1) Determination of influenza; 2) Clinical manifestations of acute encephalopathy immediately after influenza infection, including convulsion, rapid cognitive impairment, mental status changes, decrease in consciousness, and coma; 3) Normal number of cells and increased amount of proteins (>0.4 g/L) in cerebrospinal fluid; 4) Symmetric multiple

lesions in neuroimaging test in areas including thalamus and possibly involving upper brainstem tegmentum, peri-ventricular white matter, cerebellar medulla, internal capsule, and putamen nucleus, but not other areas in the central nervous system; 5) Increase of serum transaminase with varying degrees, with possible absence of hyperammonemia and hypoglycemia; 6) Brain dysfunction that cannot be explained by other diseases, including 1) Diagnosis according to clinical manifestations: overwhelming bacterial and viral infections, fulminant hepatitis, toxic shock, hemolytic-uremic syndrome, Reye’s syndrome, heat stroke and encephalopathy syndrome, and 2) Diagnosis based on imaging examination: subacute necrotizing encephalopathy, glutaric acidemia, infantile striatal necrosis, Wernicke’s encephalopathy, carbon monoxide poisoning, acute disseminated encephalomyelitis (ADEM), acute necrotizing hemorrhagic encephalopathy and others types of encephalitis and vasculitis. Mortality rate of ANE is around 30%, and one third of survivals have neurological sequelae [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].

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

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].

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.

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.

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.

Encephalitis was defined as encephalopathy plus two or more of the following [28] [62] (Table 3): 1) having laboratory-confirmed novel influenza virus infection with seizures, encephalopathy, or encephalitis within 5 days of influenza-like illness symptom onset, without evidence of an alternative etiology; 2) Encephalopathy was defined as altered mental status lasting ≥ 24 hours; 3) Encephalitis was defined as encephalopathy plus two or more of the following: fever ≥ 100.4˚F (≥38.0˚C), focal neurologic signs, cerebrospinal fluid pleocytosis (WBC ≥ 5/uL), an electroencephalogram indicative of encephalitis, or abnormal neuroimaging indicative of infection or inflammation. In addition to anti-influenza virus treatment, symptomatic treatment are generally given, such as close monitoring and evaluation, stabilization of water and electrolyte balance, control of convulsions, and relief of cerebral edema. Based on the available evidence, it is not recommended to routinely use adjuvant therapy to treat children with encephalitis, including glucocorticoids, plasma exchangwe, intravenous immunoglobulins, interferon

alpha, and therapeutic hypothermia [63] [64] [65]. Most of the patients with Influenza-associated acute encephalitis have good recovery. The poor prognosis is related to the following factors: coma, convulsions, or focal neurological manifestations during the acute phase; younger (<5 years); needs intensive care; MRI shows limited diffusion [66] [67].

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.

PIE can be found in a variety of clinical manifestations such as autoimmune encephalitis, limbic encephalitis, and acute disseminated encephalomyelitis (ADEM); some patients may develop Guillain-Barre syndrome [71]. After 2 - 3 weeks since the onset of influenza, most PIE patients without fever, and display convulsions, delirium, hallucinations, gibberish, personality changes, limb weakness or dyskinesia autonomic disorders, hypoventilation and other first-episode neuropsychiatric symptoms.

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.

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.

Children and adolescents with psychiatric symptoms, dyskinesias, seizures, autonomic disorders, and hypoventilation in 2 weeks after the occurrence of influenza or vaccination need to consider this disease. Most patients can be treated initially with intravenous methylprednisolone and intravenous immunoglobulin (IVIG) (e.g., 400 mg/kg daily for 5 days) or plasma exchange. PIE currently has no definite diagnostic criteria; the diagnostic conditions and treatment measures can be referred to Table 4.

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].

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.

There are few reports on cerebral infarction or cerebrovascular disease complicated by influenza in children. In clinical practice, if patients with influenza infection appear facial paralysis, limb movement disorders, sensory disorders, aphasia, consciousness disorders and/or accompanied by hypertension, diabetes, and the laboratory is clear influenza virus infection, at the same time with increased fibrinogen, attention should be paid to the occurrence of cerebral infarction or cerebrovascular disease. Some children with influenza also experienced complications of thrombocytosis, including cerebrovascular accident, acute coronary syndrome, deep venous thrombosis, pulmonary embolus, mesenteric thrombosis and arterial thrombosis and also hemorrhagic complications [80].

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].

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

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.