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Neurology

Encephalitis

Encephalitis is inflammation of the brain parenchyma causing altered mental state, seizures or focal neurological dysfunction, usually from infection or autoimmunity; suspected herpes-simplex encephalitis needs immediate empirical intravenous aciclovir while investigations are arranged.

In a nutshell

Encephalitis is a brain-parenchymal inflammation syndrome causing altered mental state, seizures or focal neurology, often with fever. Suspected HSV encephalitis is a treatment emergency: start IV aciclovir immediately while arranging safe LP, CSF PCR, MRI and EEG. Cover bacterial meningitis when it cannot be excluded, and consider autoimmune or travel-related causes when the pattern is atypical or recovery is poor.

Classic presentation

A person develops fever, confusion, personality or memory change, dysphasia and temporal-lobe seizures; MRI may show medial-temporal abnormalities, but early imaging and CSF PCR can be normal.

Key points

  • Altered cognition, behaviour or consciousness, seizures or focal neurology point to encephalitis rather than isolated meningitis.
  • Start empirical IV aciclovir immediately when HSV or VZV encephalitis is possible; do not wait for LP, MRI or PCR.
  • Use weight- and renal-adjusted intravenous aciclovir under the current BNF or local NHS protocol.
  • Perform LP promptly if safe with CSF cells, protein, glucose, culture and HSV-1/2, VZV and enterovirus PCR; repeat early negative HSV PCR if suspicion remains high.
  • MRI is preferred and EEG helps identify focal or non-convulsive seizures; normal early tests do not exclude encephalitis.
  • If bacterial meningitis or sepsis cannot be excluded, start the NICE NG240 antibacterial pathway as well as aciclovir.
  • Subacute psychiatric, movement or seizure syndromes should trigger autoimmune testing and specialist neuroimmunology input, but infection must be covered or reasonably excluded before immunotherapy.
  • Travel, tick or animal exposure may require UKHSA or Imported Fever Service advice, and acute encephalitis is notifiable.

First-line investigation

Safe early lumbar puncture with CSF inflammatory, microbiological and viral PCR testing, plus MRI brain and EEG; image before LP when consciousness, seizures, papilloedema or focal neurology make it unsafe.

Management

Stabilise, admit and treat empirically

  • Use ABCDE, admit urgently, start IV aciclovir immediately and add the NICE bacterial-meningitis pathway when meningitis or sepsis cannot be excluded.1,6,4

Obtain safe CSF, MRI and EEG

  • Perform LP promptly when safe, send CSF cells/chemistry/culture and HSV-1/2, VZV and enterovirus PCR, and arrange MRI and EEG; image before LP when consciousness, seizures, papilloedema or focal signs make it unsafe.1,3

Continue or refine antiviral treatment

  • Adjust aciclovir to renal function and weight, continue it while high clinical suspicion remains, repeat early negative HSV PCR when needed and confirm clearance before stopping proven HSV treatment according to specialist and local protocol.4,3,8

Control seizures, pressure and critical illness

  • Treat seizures or status epilepticus, monitor for non-convulsive status, protect the airway, manage raised intracranial pressure with specialist input and provide rehabilitation-focused supportive care.1,2,8

Identify autoimmune and unusual infections

  • If the phenotype is atypical or recovery is poor, send serum/CSF neuronal antibodies, screen for malignancy and exposures, and involve neuroimmunology and infection specialists before specialist-directed immunotherapy.5,1,7

Rehabilitate, notify and safety-net

  • Assess cognition, behaviour, memory, mood, speech, swallowing, mobility, fatigue and seizures; arrange neurorehabilitation, epilepsy and neuropsychology follow-up, notify suspected acute encephalitis and give current DVLA advice.9,2,10

Exam traps

  • Do not delay aciclovir for LP, MRI or CSF PCR.
  • An early negative HSV PCR, normal CT, normal early MRI or bland CSF does not safely exclude HSV encephalitis.
  • Aciclovir alone is insufficient when bacterial meningitis or sepsis remains possible.
  • Do not perform LP before imaging when GCS is very low or falling, seizures are uncontrolled or recurrent, papilloedema or focal neurology is present, or mass effect is suspected.
  • Prominent psychiatric symptoms, dyskinesia, faciobrachial dystonic seizures or hyponatraemia can suggest autoimmune encephalitis, but antibodies are not a substitute for excluding infection.
  • Treat seizures and raised intracranial pressure actively; routine corticosteroids are not treatment for isolated viral encephalitis.

Illustrations

Temporal lobe involvement in herpes simplex encephalitisAn MRI showing swelling and signal change in the mesial temporal lobe, the characteristic distribution of herpes simplex encephalitis.dr Laughlin Dawes, Wikimedia Commons · CC-BY-3.0

Key sources

  1. British Infection Association, Meningitis and Encephalitis Infection Quick Reference Guide: Investigation in Adult Hospital Patients (BIA adult flowchart dated October 2024, revision due October 2029: immediate assessment, LP safety, CSF PCR panel, MRI or EEG for encephalitis, autoimmune screen, critical-care criteria and empiric coverage when meningitis with encephalitis is possible)Updated 1 Oct 2024
  2. NHS, Encephalitis and Encephalitis: treatment (NHS symptoms, urgent hospital-treatment and supportive-care information, including seizure, ventilation, intracranial-pressure, rehabilitation and immune-mediated treatment pathways)Updated 15 May 2023
  3. Management of suspected viral encephalitis in adults, Association of British Neurologists and British Infection Association national guideline (UK national guideline published in 2012 and retained by the BIA as the reference adult viral-encephalitis guideline; supports immediate empirical IV aciclovir, safe CSF sampling, repeat PCR when early results are negative and specialist management)Updated 1 Feb 2012
  4. NHS Ayrshire and Arran Medicines, Encephalitis antimicrobial guideline (Current local NHS antimicrobial guideline reviewed and updated April 2026: IV aciclovir 10 mg/kg every 8 hours, 14 days in immunocompetent and 21 days in immunocompromised adults, with renal/weight adjustment and repeat CSF PCR before stopping proven HSV treatment)Updated 1 Apr 2026
  5. Autoimmune encephalitis: proposed best practice recommendations for diagnosis and acute management (International specialist consensus used because no dedicated current UK autoimmune-encephalitis guideline was identified; supports MRI/EEG/CSF, neuronal antibody testing, infection exclusion, early specialist immunotherapy and malignancy assessment)Updated 1 Jul 2021
  6. NICE NG240, Meningitis (bacterial) and meningococcal disease: recognition, diagnosis and management (NICE guideline published 19 March 2024 for recognition, investigation and treatment of bacterial meningitis and meningococcal disease; used here for the overlapping meningitis or sepsis pathway because confirmed viral encephalitis is outside its scope)Updated 19 Mar 2024
  7. UKHSA, Tick-borne encephalitis: epidemiology, diagnosis and prevention (UKHSA guidance updated 21 May 2026: consider TBE and other imported infections after relevant travel or tick exposure, seek Imported Fever Service advice and report suspected acute encephalitis)Updated 21 May 2026
  8. BNF, current prescribing information for aciclovir, antibacterial cover, antiseizure medicines and immunotherapy (Current UK prescribing source for dose, renal adjustment, interactions and monitoring; direct access was restricted and the available browser session was unavailable, so dose claims are additionally anchored to the current NHSAAA guideline and unsupported immunotherapy doses are omitted)
  9. GOV.UK/UKHSA, Notifiable diseases and how to report them (Current UKHSA reporting guidance: suspected acute encephalitis is a routine notifiable disease and should be reported without waiting for laboratory confirmation)Updated 2 Apr 2025
  10. GOV.UK, Neurological disorders: assessing fitness to drive (DVLA neurological guidance updated 7 November 2025; seizures, cognitive or focal neurological impairment after encephalitis can affect driving and notification)Updated 7 Nov 2025

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.