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Health condition · Clinically reviewed

Encephalitis, a neurological emergency that responds to speed and expertise.

Inflammation of the brain itself. Fast recognition, empirical aciclovir, precise antibody testing and specialist rehabilitation shape every outcome.

A radiographer guides a patient onto the bed of an advanced 3 Tesla MRI scanner in a London imaging suite

Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a registered UK clinician before publication.

  • 02

    Sourced from guidance

    Checked against ABN, Encephalitis Society, NICE and peer-reviewed sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK practice, including autoimmune antibody testing and early empirical aciclovir.

Key facts

Encephalitis at a glance.

The essentials, in plain English. What it is, the main patterns, and how it is treated in the UK today.

  • What it is

    Inflammation of the brain parenchyma itself. A neurological emergency, not simply a bad headache or a routine viral illness.

  • Main types

    Infectious (viral, bacterial, fungal, parasitic), autoimmune (NMDAR, LGI1, others), post-infectious (ADEM) and paraneoplastic.

  • Commonest sporadic

    Herpes simplex virus type 1 (HSV-1) is the leading sporadic viral cause in the UK, with a predilection for the temporal lobes.

  • Autoimmune profile

    Anti-NMDAR encephalitis often affects young women, may follow psychiatric prodrome and can be linked to an ovarian teratoma.

  • First treatment

    Empirical intravenous aciclovir is started urgently on clinical suspicion, before any confirmatory result is back.

  • Rehabilitation

    Recovery is often slow. Specialist neurorehabilitation for cognitive, physical and psychological deficits is the norm, not the exception.

Why this guide matters

An emergency with a treatable core.

Encephalitis is uncommon, but the decisions in the first hours matter for years. Three principles frame everything else on this page.

  • Suspect, then treat

    Empirical aciclovir goes in on suspicion, not on confirmation. Waiting for a positive PCR before starting antiviral therapy costs brain.

  • Test the antibodies

    Autoimmune encephalitis is now recognised, testable and treatable. Antibody panels on serum and CSF should be part of any unexplained subacute picture.

  • Rehabilitate deliberately

    Getting off ITU is a milestone, not the finish line. Specialist neurorehabilitation is where quality of life is rebuilt.

How the diagnosis is made

From first symptoms to a treatment plan.

The steps a UK acute team will follow when encephalitis is suspected, in order, so you know what is happening and why.

  1. 01

    Recognising

    Recognition and resuscitation

    Fever, reduced consciousness, seizures, focal signs or new psychiatric features prompt an urgent A to E assessment.

  2. 02

    Recognising

    Urgent CT head

    A non-contrast CT is usually the first scan, mainly to exclude a mass lesion, bleed or raised pressure before lumbar puncture.

  3. 03

    Recognising

    Bloods and cultures

    FBC, U and Es, LFTs, glucose, inflammatory markers, HIV, syphilis and Lyme serology, plus blood cultures.

  4. 04

    Confirming

    Lumbar puncture and CSF

    Cell count, protein and glucose plus viral PCR (HSV, VZV, enterovirus) and, where relevant, an autoimmune antibody panel.

  5. 05

    Confirming

    MRI brain

    Temporal lobe changes point to HSV. Multifocal white matter lesions suggest ADEM. MRI can be normal early on.

  6. 06

    Confirming

    EEG

    Temporal lobe discharges in HSV, diffuse slowing in many encephalitides, and the extreme delta brush pattern in NMDAR disease.

  7. 07

    Refining

    Body imaging and specialist input

    Where autoimmune disease is suspected, CT or PET looks for an underlying tumour, such as an ovarian teratoma in NMDAR.

Typical timeline: empirical treatment within hours, a working cause within days.

Symptoms

What encephalitis actually looks like.

A combination of infective, neurological and psychiatric features. The mix tells you which cause is most likely and how urgent the response should be.

  • Fever and headache

    A febrile illness with a severe headache is a common opener, often mistaken for a viral illness in the first day or two.

  • Reduced consciousness

    Drowsiness, confusion or a fluctuating conscious level is a red flag that separates encephalitis from ordinary infection.

  • Seizures

    New focal or generalised seizures, particularly with fever or altered behaviour, mandate urgent brain imaging and CSF sampling.

  • Focal neurological signs

    Speech disturbance, weakness, ataxia or visual changes suggest a localised inflammatory process rather than a systemic illness.

  • Behavioural and psychiatric change

    Personality change, agitation, hallucinations or catatonia. Prominent in autoimmune encephalitis, especially anti-NMDAR.

  • Movement disorders

    Orofacial dyskinesias, choreoathetosis and dystonia are hallmark features of anti-NMDAR encephalitis.

  • Autonomic instability

    Swings in blood pressure, heart rate and temperature, and hypoventilation. Common in severe NMDAR disease and needs neurocritical care.

  • Red flag - meningoencephalitis picture

    Neck stiffness, photophobia and fever with any of the above features needs immediate assessment for encephalitis and meningitis.

Treatment

How encephalitis is treated in the UK.

Empirical antivirals first, targeted antimicrobial and immunotherapy next, with neurocritical care and specialist rehabilitation running throughout.

  • Empirical IV aciclovir

    10 mg/kg three times daily, started urgently on suspicion of viral encephalitis. Continued for 14 to 21 days if HSV is confirmed.

  • Bacterial cover

    Where bacterial meningitis is a differential, ceftriaxone, amoxicillin and dexamethasone are added until CSF results clarify the picture.

  • Corticosteroids

    Methylprednisolone pulse therapy is first-line in autoimmune encephalitis. Its role in HSV encephalitis is decided case by case with specialist input.

  • IVIG and plasma exchange

    First-line immunotherapy for autoimmune encephalitis alongside steroids, particularly for anti-NMDAR, LGI1 and CASPR2 disease.

  • Rituximab and cyclophosphamide

    Second-line immunotherapy for autoimmune encephalitis when first-line treatment fails to control disease within a reasonable window.

  • Tumour resection

    For paraneoplastic disease, removing the trigger, such as an ovarian teratoma in NMDAR encephalitis, is a core part of treatment.

  • Anticonvulsants and ICU care

    Seizure control, ICP monitoring and neurocritical care as needed. Ventilation and autonomic support are often required in severe cases.

  • Neurorehabilitation

    Cognitive, physical and psychological rehabilitation, ideally in a specialist commissioned centre, shapes long-term recovery.

Specialist commissioned care

Complex encephalitis, particularly severe autoimmune disease, is often managed at specialist commissioned centres such as Queen Square (London), Birmingham, Alder Hey and Great Ormond Street for children. UKHSA notification is required where an infectious cause is suspected.

What this guide is based on

The sources behind every claim on this page.

UK national guidance, specialist society standards and international consensus, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

If you or someone you love has any of the symptoms above, ring 999 or go to A and E. Encephalitis is time-critical.

  • Association of British Neurologists (ABN) and British Infection Association. Management of suspected viral encephalitis in adults.

  • NICE guidance on meningitis and encephalitis pathways.

  • Encephalitis International (formerly Encephalitis Society). Patient and clinician resources on infectious and autoimmune encephalitis.

  • International consensus (Graus et al.) on the clinical approach to autoimmune encephalitis.

  • UK Health Security Agency (UKHSA). Notification and public-health guidance for encephalitis.

Red flags

When to escalate immediately.

Any single feature below is enough to justify urgent hospital assessment. Several together are a 999 call, not a wait-and-see.

  • Reduced or fluctuating GCS

    Any drop in conscious level with fever or seizures needs immediate hospital assessment. Time to aciclovir is a key prognostic factor in HSV encephalitis.

  • New seizures with fever

    First-ever seizures with fever, headache or altered behaviour should be treated as encephalitis until CSF and imaging say otherwise.

  • Rapid psychiatric change

    Sudden severe psychiatric symptoms in a young adult, particularly with movement disorder or seizures, raise concern for anti-NMDAR encephalitis.

  • Suspected meningoencephalitis

    Fever, neck stiffness, photophobia and altered mentation together. Empirical antibiotics and antivirals are usually started at the same time.

  • Immunocompromised patients

    Consider VZV, CMV, cryptococcus, toxoplasma and TB in HIV, transplant or chemotherapy patients. Have a low threshold for advanced imaging and specialist input.

  • Recent travel or animal exposure

    Recent travel, freshwater swimming or animal bites should prompt consideration of rabies, arboviruses, JE, WNV and Naegleria.

  • Pregnancy and post-partum

    Autoimmune encephalitis can present or relapse around pregnancy and needs a joint neurology, immunology and obstetric plan.

  • Post-viral neurological decline

    A child or young adult with new neurological symptoms after a viral illness may have ADEM and needs urgent MRI and CSF.

  • Delayed recovery or relapse

    A partial recovery that stalls or reverses is not just deconditioning. Consider ongoing inflammation, relapse or a paraneoplastic driver.

Living with it

Life after encephalitis, a longer story than the admission.

Physical recovery is often the fastest part. Cognitive fatigue, memory changes and mood take longer, and a coordinated team is what makes the difference.

A quiet reminder

Recovery is not linear.

Good weeks and quiet setbacks are part of the pattern. Rehabilitation goals sit alongside patience, sleep, mood care and gentle graded activity.

  1. 01 Recovery

    Expect a long arc

    Cognitive, emotional and physical recovery from encephalitis often takes many months. Progress is measured in seasons, not weeks.

  2. 02 Rehabilitation

    Specialist input matters

    Neuropsychology, physiotherapy, occupational therapy and speech and language therapy work best when coordinated by a specialist rehab team.

  3. 03 Support

    You are not on your own

    Encephalitis International offers helplines, information and peer support for patients, families and carers across the UK.

  4. 04 Return to life

    Work, driving and school

    Return-to-work, driving and school plans should be built around cognitive fatigue, seizure risk and any lasting deficits, with medical input at each step.

Frequently asked

Everything we get asked about encephalitis.

Quick answers on causes, urgency, testing, autoimmune disease, ADEM and long-term outlook.

  • What is encephalitis?

    Encephalitis is inflammation of the brain tissue itself. It differs from meningitis, which is inflammation of the meninges, although the two often overlap as meningoencephalitis. Causes include viral, bacterial, fungal and parasitic infections, autoimmune antibody-mediated disease, post-infectious inflammation such as ADEM, and paraneoplastic processes.

  • How urgent is treatment?

    It is a medical emergency. In the UK, empirical intravenous aciclovir is started on clinical suspicion of viral encephalitis, before CSF or MRI results are back. Delay in starting aciclovir is one of the clearest drivers of worse outcomes in HSV encephalitis.

  • What tests are done?

    A CT head is usually the first scan, followed by lumbar puncture with CSF cell count, protein, glucose, viral PCR and, where relevant, an autoimmune antibody panel. MRI brain, EEG, blood tests, HIV and syphilis serology and, in autoimmune cases, CT or PET for an underlying tumour are the mainstays.

  • What is autoimmune encephalitis?

    A group of conditions in which antibodies attack neural surface or intracellular proteins. Anti-NMDAR is the best known, often presenting with psychiatric symptoms, seizures and movement disorders in young women. LGI1, CASPR2, GABA and AMPA receptor antibodies produce their own patterns and needing tailored immunotherapy.

  • What is ADEM?

    Acute disseminated encephalomyelitis is a post-infectious, immune-mediated demyelinating illness, most often seen in children a few weeks after a viral infection. MRI typically shows multifocal white matter lesions, and treatment usually includes high-dose steroids with IVIG or plasma exchange in severe cases.

  • What is the outlook?

    Outcomes vary widely by cause, severity and speed of treatment. Some people recover fully, others live with cognitive, emotional or physical changes. Specialist neurorehabilitation, mental-health support and organisations such as Encephalitis International play a central role in the years after the acute illness.

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