Encephalitis isinflammation of thebrain.^[5] The severity can be variable with symptoms including reduction or alteration in consciousness,aphasia,headache,fever,confusion, astiff neck, andvomiting.^[1][6] Complications may includeseizures,hallucinations, trouble speaking,memory problems, and problems withhearing.^[1]

Encephalitis
MRI scan image shows high signal in the temporal lobes and right inferior frontal gyrus in someone withherpes simplex encephalitis.
Specialty	Neurology,infectious disease
Symptoms	Headache,fever,confusion, stiff neck,vomiting[1]
Complications	Seizures, trouble speaking,memory problems, problemshearing[1]
Duration	Weeks to months for recovery[1]
Types	Herpes simplex,West Nile,rabies,Eastern equine, others[2]
Causes	Infection, autoimmune, certain medication, unknown[2]
Diagnostic method	Based on symptoms, supported byblood tests,medical imaging, analysis ofcerebrospinal fluid[2]
Treatment	Antiviral medication,anticonvulsants,corticosteroids,artificial respiration[1]
Prognosis	Variable[1]
Frequency	4.3 million (2015)[3]
Deaths	150,000 (2015)[4]

Causes of encephalitis includeviruses such asherpes simplex virus andrabies virus as well asbacteria,fungi, orparasites.^[1][2] Other causes includeautoimmune diseases and certain medications.^[2] In many cases the cause remains unknown.^[2] Risk factors include aweak immune system.^[2] Diagnosis is typically based on symptoms and supported byblood tests,medical imaging, and analysis ofcerebrospinal fluid.^[2]

Certain types are preventable withvaccines.^[5] Treatment may includeantiviral medications (such asacyclovir),anticonvulsants, andcorticosteroids.^[1] Treatment generally takes place in hospital.^[1] Some people requireartificial respiration.^[1] Once the immediate problem is under control,rehabilitation may be required.^[2] In 2015, encephalitis was estimated to have affected 4.3 million people and resulted in 150,000 deaths worldwide.^[3][4]

Adults with encephalitis present with acute onset offever,headache,confusion, and sometimes seizures. Younger children orinfants may present with irritability,poor appetite and fever.^[7]Neurological examinations usually reveal adrowsy or confused person.Stiff neck, due to the irritation of themeninges covering the brain, indicates that the patient has eithermeningitis ormeningoencephalitis.^[8]

Limbic encephalitis refers to inflammatory disease confined to thelimbic system of the brain. The clinical presentation often includesdisorientation,disinhibition,memory loss,seizures, and behavioral anomalies.MRI imaging reveals T2hyperintensity in the structures of the medial temporal lobes, and in some cases, other limbic structures. Some cases of limbic encephalitis are of autoimmune origin.^[9]

Encephalitis lethargica is identified by high fever,headache, delayed physical response, andlethargy. Individuals can exhibit upper bodyweakness,muscular pains, andtremors, though the cause of encephalitis lethargica is not currently known. From 1917 to 1928, an epidemic of encephalitislethargica occurred worldwide.^[10]

In 30%-40% of encephalitis cases, the etiology remains unknown.^[11]

Viral infections are the usual cause of infectious encephalitis.^[11] Viral encephalitis can occur either as a direct effect of an acuteinfection, or as one of thesequelae of alatent infection. The majority of viral cases of encephalitis have an unknown cause; however, the most common identifiable cause of viral encephalitis is fromherpes simplex infection.^[12] Other causes of acute viral encephalitis arerabies virus,poliovirus, andmeasles virus.^[13]

Additional possible viral causes arearboviralflavivirus (St. Louis encephalitis,West Nile virus),bunyavirus (La Crosse strain),arenavirus (lymphocytic choriomeningitis virus),reovirus (Colorado tick virus), andhenipavirus infections.^[14][15] ThePowassan virus is a rare cause of encephalitis.^[16]

It can be caused by abacterial infection, such as bacterialmeningitis,^[17] or may be a complication of a current infectious disease such assyphilis (secondary encephalitis).^[18]

Other bacterial pathogens, likeMycoplasma and those causingrickettsial disease, cause inflammation of the meninges and consequently encephalitis.Lyme disease orBartonella henselae may also cause encephalitis.^{[citation needed]}

Certainparasitic orprotozoal infestations, such astoxoplasmosis andmalaria can also cause encephalitis in people withcompromisedimmune systems.

The rare but typically deadly forms of encephalitis,primary amoebic meningoencephalitis andGranulomatous amoebic encephalitis, are caused by free-livingamoeba.^[19]

Autoimmune encephalitis signs can includecatatonia,psychosis,abnormal movements, and autonomicdysregulation. Antibody-mediated anti-N-methyl-D-aspartate-receptor encephalitis andRasmussen encephalitis are examples of autoimmune encephalitis.^[20]

Anti-NMDA receptor encephalitis is the most common autoimmune form, and is accompanied byovarianteratoma in 58 percent of affected women 18–45 years of age.^[21]

Another autoimmune cause includesacute disseminated encephalitis, ademyelinating disease which primarily affects children.^[22]

People should only be diagnosed with encephalitis if they have a decreased or altered level of consciousness, lethargy, or personality change for at least twenty-four hours without any other explainable cause.^[23] Diagnosing encephalitis is done via a variety of tests:^[24][25]

• Brain scan, done byMRI, can determine inflammation and differentiate from other possible causes.
• EEG, in monitoring brain activity, encephalitis will produce abnormal signal.
• Lumbar puncture (spinal tap), this helps determine via a test using the cerebral-spinal fluid, obtained from the lumbar region.
• Blood test
• Urine analysis
• Polymerase chain reaction (PCR) testing of thecerebrospinal fluid, to detect the presence of viralDNA which is a sign of viral encephalitis.

Vaccination is available againsttick-borne^[26] andJapanese encephalitis^[27] and should be considered for at-risk individuals. Post-infectiousencephalomyelitis complicatingsmallpox vaccination is avoidable, for all intents and purposes, as smallpox is nearly eradicated.^[28]Contraindication toPertussis immunization should be observed in patients with encephalitis.^[29]

An ideal drug to treat brain infection should be small, moderately lipophilic at pH of 7.4, low level of plasma protein binding, volume of distribution of litre per kg, does not have strong affinity towards binding withP-glycoprotein, or other efflux pumps on the surface ofblood–brain barrier. Some drugs such as isoniazid, pyrazinamide, linezolid, metronidazole, fluconazole, and some fluoroquinolones have good penetration to blood brain barrier.^[30] Treatment (which is based on supportive care) is as follows:^[31]

Pyrimethamine-based maintenance therapy is often used to treat toxoplasmic encephalitis (TE), which is caused byToxoplasma gondii and can be life-threatening for people with weak immune systems.^[32] The use ofhighly active antiretroviral therapy (HAART), in conjunction with the established pyrimethamine-based maintenance therapy, decreases the chance of relapse in patients with HIV and TE from approximately 18% to 11%.^[32] This is a significant difference as relapse may impact the severity and prognosis of disease and result in an increase in healthcare expenditure.^[32]

The effectiveness ofintravenousimmunoglobulin for the management of childhood encephalitis is unclear.Systematic reviews have been unable to draw firm conclusions because of a lack ofrandomiseddouble-blind studies with sufficient numbers of patients and sufficient follow-up.^[33] There is the possibility of a benefit of intravenous immunoglobulin for some forms of childhood encephalitis on some indicators such as length of hospital stay, time to stop spasms, time to regain consciousness, and time to resolution of neuropathic symptoms and fever.^[33] Intravenous immunoglobulin forJapanese encephalitis appeared to have no benefit when compared withplacebo (pretend) treatment.^[33]

Identification of poor prognostic factors includecerebral edema,status epilepticus, andthrombocytopenia.^[34] In contrast, a normalencephalogram at the early stages of diagnosis is associated with high rates of survival.^[34]

Thenumber of new cases a year of acute encephalitis in Western countries is 7.4 cases per 100,000 people per year. In tropical countries, the incidence is 6.34 per 100,000 people per year.^[35] The number of cases of encephalitis has not changed much over time, with about 250,000 cases a year from 2005 to 2015 in the US. Approximately seven per 100,000 people were hospitalized for encephalitis in the US during this time.^[34] In 2015, encephalitis was estimated to have affected 4.3 million people and resulted in 150,000 deaths worldwide.^[4][3] Herpes simplex encephalitis has an incidence of 2–4 per million of the population per year.^[36]

Encephalitis withmeningitis is known asmeningoencephalitis, while encephalitis with involvement of the spinal cord is known asencephalomyelitis.^[2]

The word is fromAncient Greekἐγκέφαλος,enképhalos 'brain',^[37] composed ofἐν,en, 'in' andκεφαλή,kephalé, 'head', and the medical suffix-itis 'inflammation'.^[38]

• WHO: Viral Encephalitis