PatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.
Encephalitis is inflammation of the brain parenchyma, often caused by viral infections but also by other pathogenic organisms and occasionally by other conditions, eg toxins, autoimmune disorders. Meningoencephalitis is an inflammatory process, most often due to viral infection, involving both the brain and meninges.
- There are around 4,000 new cases of encephalitis each year in the UK.
- Infections are most frequent and severe in children and the elderly.
- Herpes simplex can cause a benign lymphocytic meningitis in adults, but usually produces a severe encephalitis in neonates. Infection in adults can also be very severe.
- Post-infectious encephalitis is the most common demyelinating condition and is most often seen in children, as it may complicate the common childhood exanthemas.
- HIV infection is of increasing importance; toxoplasmic meningoencephalitis was one of the first opportunistic infections to be described in HIV-infected patients.
- Viral encephalitis may be due either to:
- Acute viral encephalitis (caused by a direct viral infection of the brain).
- Post-infectious encephalitis (also called acute disseminated encephalomyelitis) which is an autoimmune process, following a viral infection elsewhere in the body.
- Most viral infections in childhood are able to cause encephalitis.
- Herpes simplex virus (HSV) encephalitis is recognised worldwide as the most frequent infectious encephalitis, and the only one with a validated specific treatment.
- Most HSV encephalitis is due to HSV-1. Around 10% are caused by HSV-2.
- Other common viral causes are herpes zoster, Epstein-Barr virus, mumps, measles and enteroviruses.
- Other viral causes include cytomegalovirus, adenovirus, influenza virus, poliovirus, rubella, rabies, arbovirus (eg California virus, Japanese B encephalitis, St Louis encephalitis, West Nile encephalitis, Eastern and Western equine encephalitis), reovirus (Colorado tick fever virus), parvovirus B19.
- Viral encephalitis may be due either to:
- Bacterial causes: tuberculosis (TB), mycoplasma, listeria, Lyme disease, Bartonella henselae (cat scratch fever), leptospira, brucella, legionella, neurosyphilis, all causes of bacterial meningitis.
- Rickettsial: Rocky Mountain spotted fever, endemic typhus, epidemic typhus, Q fever, human monocytic ehrlichiosis.
- Fungal: cryptococcosis, coccidiomycosis, histoplasmosis, North American blastomycosis, candidiasis.
- Parasitic: African trypanosomiasis, toxoplasmosis, echinococcus, schistosomiasis.
- Tick-borne encephalitis is a rapidly growing public health problem in Europe and other parts of the world. It is caused by tick-borne encephalitis virus, a member of the family Flaviviridae. (See separate Tick-borne encephalitis article for more information.)
- Primary amoebic meningoencephalitis is caused by infection from an amoeba such as Naegleria fowleri or Balamuthia mandrillaris.
The disease is very rare but has a very high mortality rate.
The clinical hallmark of acute encephalitis is the triad of fever, headache, and altered mental status.
Most patients with viral encephalitis present with the symptoms of meningitis (fever, headache, neck stiffness, vomiting) followed by altered consciousness, convulsions, and sometimes focal neurological signs, signs of raised intracranial pressure, or psychiatric symptoms. There may be an association with a history of infection elsewhere in the body.
- Encephalitis may begin with a flu-like illness or with a headache, followed by a rapid development of altered consciousness, with confusion, drowsiness, seizures and coma.
- Symptoms may also include symptoms of increased intracranial pressure, such as severe headache, vertigo, nausea, convulsions and mental confusion. Other possible symptoms include photophobia, sensory changes and neck stiffness.
- Epilepsy, focal neurological signs and cognitive impairment may develop.
- Subacute sclerosing panencephalitis is a late complication of measles and presents four to ten years after the initial infection. Progression may be slow or rapid with personality change, dementia, seizures, ataxia and death. Progressive rubella panencephalitis is similar.
- Behçet's disease
- Systemic lupus erythematosus
- Post-vaccine encephalomyelitis
- Multiple sclerosis
- Intracerebral tumour
Clues to the origin of the encephalitis may be ascertained from the presentation; for example:
- A history of recent foreign travel, contact with animals, insect bites, immune status and occupation may all give a clue to the infecting organism.
- Chickenpox encephalitis often has cerebellar involvement.
- Herpes simplex encephalitis is often associated with temporal lobe epilepsy.
The investigations and management of viral encephalitis should be as detailed in current guidelines for adults and children.
Laboratory diagnosis of viral encephalitis is ideally based on examination of CSF for cells, protein and glucose, lactate and virology PCR. All patients with suspected encephalitis should have a CSF PCR test for HSV (1 and 2), varicella-zoster virus (VZV) and enteroviruses, as this will identify 90% of cases due to known viral pathogens.
- Viral encephalomyelitis leads to a lymphocytosis with normal CSF/Plasma glucose ratio. Opening pressure is usually normal or high and the CSF is clear.
- CSF protein levels are usually normal or high.
- Elevated CSF specific antibody levels relative to serum indicate CNS infection with the respective organism.
- All patients with suspected encephalitis should have a CSF PCR test for HSV (1 and 2), VZV and enteroviruses, as this will identify 90% of cases due to known viral pathogens.
- PCR analysis of the CSF can be useful to diagnose several viral infections, including herpes simplex, Epstein-Barr, varicella zoster, cytomegalovirus, HIV, rabies and TB.
- FBC and film: leukocytosis. May indicate atypical lymphocytes in Epstein-Barr viral infections, morulae of Ehrlichia, trypanosomes in trypanosomiasis, borreliae in relapsing fever, or the gametes of Plasmodium falciparum in malaria.
- Other blood tests should include blood cultures, renal function and electrolytes, LFTs, glucose, ESR and CRP.
- Other cultures, eg throat swabs and stool cultures, may be indicated.
- CT scan:
- Can help to rule out space-occupying lesions, strokes, basilar fractures of the skull, and detect CSF leaks to localise fracture sites.
- CT scan is also used to identify raised intracranial pressure, which will then need to delay a lumbar puncture.
- MRI scan:
- Provides a sensitive detection of demyelination and can provide evidence of oedematous changes that occur in the early stage of encephalitis.
- Electroencephalogram (EEG):
- Frequently abnormal (diffuse slowing with periodic discharges) in chronic and acute herpes simplex encephalitis and can sometimes help to determine the localisation in the early stages.
- Is more useful than a CT scan in the first week.
- Urgent hospital admission.
- Immediate parenteral antibiotics for possible diagnosis of meningitis. Intravenous or intramuscular benzylpenicillin should be given as long as the patient is not allergic to penicillins.
- In any case of possible encephalitis, prompt treatment with aciclovir by intravenous infusion, to cover herpes simplex. Aciclovir can be life-saving but must be started immediately the diagnosis is suspected. Aciclovir has been shown to greatly improve the prognosis if given before coma develops but any delay in starting treatment leads to a much worse prognosis.
- There is no specific treatment for other viral causes and the emphasis of treatment is supportive.
- The role of steroids in the treatment of HSV encephalitis is not established.
- Intravenous fluids need to be given very carefully in order not to aggravate cerebral oedema.
- Other treatments may include anticonvulsants and sedatives (to reduce agitation).
- Intensive care, including ventilation, may also be necessary in severe cases, to reduce brain swelling.
- Intravenous broad-spectrum antibiotics may be given to treat secondary bacterial infections.
- Amphotericin is usually given for primary amoebic meningoencephalitis.
- Inappropriate antidiuretic hormone secretion.
- Disseminated intravascular coagulation.
- Cardiac and respiratory arrest.
- There is a broad range of potential neuropsychiatric impairments. Significant changes may occur in personality and in the ability to function, even if there is a complete physical recovery. Residual impairment may be cognitive, behavioural or emotional and vary greatly in severity. Severe amnesic syndrome caused by profound damage to the temporal lobes may occur.
- Physical problems include mild balance, co-ordination and dexterity problems or major neurological problems, with speech and swallowing problems and total dependency.
- The prognosis depends on the age of the patient and the underlying aetiology.
- The poorest prognosis for viral encephalitis occurs in patients with untreated herpes simplex encephalitis and subacute sclerosing panencephalitis.
- Some encephalitides can be reliably prevented by vaccination, eg Japanese encephalitis and rabies.
- Vector control is the main method of prevention for some pathogens, eg arboviruses.
Further reading & references
- Dedicoat M, Livesley N; Management of toxoplasmic encephalitis in HIV-infected adults (with an emphasis on resource-poor settings).; Cochrane Database Syst Rev. 2006 Jul 19;3:CD005420.
- Stahl JP, Mailles A, Dacheux L, et al; Epidemiology of viral encephalitis in 2011. Med Mal Infect. 2011 Sep;41(9):453-64. Epub 2011 Jul 29.
- Suss J; Tick-borne encephalitis 2010: epidemiology, risk areas, and virus strains in Ticks Tick Borne Dis. 2011 Mar;2(1):2-15. Epub 2010 Dec 17.
- Long SS; Encephalitis diagnosis and management in the real world. Adv Exp Med Biol. 2011;697:153-73.
- Management of suspected viral encephalitis in adults, Association of British Neurologists and British Infection Association National Guidelines (November 2011)
- Management of suspected viral encephalitis in children, Association of British Neurologists and British Infection Association National Guidelines (November 2011)
- Immunisation - The Green Book; Dept of Health
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Dr Colin Tidy
Dr Louise Newson
Prof Cathy Jackson