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Educational draft · awaiting clinical reviewUse Rapid for revision, not patient-care decisions. Check current national and local guidance and the BNF or BNFC before acting.

Encephalitis and HSV encephalitis

Neurology · UK scope · Sources checked 27 Aug 2026 · Clinical review pending

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Escalate

Encephalitis is a neurological emergency. New altered behaviour or consciousness with fever, focal deficit or seizure requires immediate ABCDE assessment, intravenous aciclovir when HSV or VZV is possible, empirical antibacterial cover until meningitis is excluded, seizure control and early neurology, infection and critical-care involvement. Do not wait for MRI or PCR before treating suspected HSV encephalitis.

Synopsis

Recognise acute brain inflammation, start time-critical aciclovir for possible herpes simplex encephalitis, and integrate cerebrospinal fluid, MRI and electroencephalography while pursuing infectious and autoimmune alternatives.

  • Encephalitis is brain parenchymal inflammation causing altered mental status, behaviour or cognition, usually lasting at least 24 hours, with supporting fever, seizure, focal findings, cerebrospinal-fluid inflammation, MRI change or EEG abnormality.
  • HSV encephalitis commonly affects temporal and inferior frontal regions, producing fever, personality change, memory disturbance, aphasia, olfactory phenomena or focal seizures, but early disease can be non-specific.
  • Start intravenous aciclovir promptly when viral encephalitis is suspected; its benefit is greatest before extensive necrotic injury and treatment should not await diagnostic confirmation.
  • Obtain lumbar puncture as soon as safely possible for cells, protein, paired glucose, bacterial studies and HSV or VZV PCR, but stabilisation and treatment come first.
  • An early HSV PCR can be falsely negative, especially very soon after symptom onset; continue aciclovir and repeat cerebrospinal-fluid testing when clinical or MRI suspicion remains high.
  • MRI is more sensitive than CT for temporal inflammation, haemorrhage and alternative diagnoses; CT remains useful when urgent mass effect, haemorrhage or lumbar-puncture safety is being assessed.
  • EEG can reveal temporal periodic discharges, focal seizures or non-convulsive status and is essential when consciousness fails to improve despite no visible convulsions.
  • Autoimmune encephalitis, including anti-NMDA-receptor disease, may resemble viral infection; psychiatric symptoms, dyskinesia, autonomic instability and refractory seizures should prompt paired serum and cerebrospinal-fluid specialist testing.
  • Adjust aciclovir for kidney function, provide adequate hydration and monitor creatinine because crystal nephropathy can cause both kidney injury and drug neurotoxicity.
  • Survivors often have memory, language, epilepsy, fatigue, mood and behavioural sequelae; rehabilitation and family support should begin during admission rather than after crisis discharge.
  • In England, suspected acute encephalitis is a notifiable disease: report it through the online service within 3 days without waiting for PCR or serology, and telephone the local health-protection team urgently if another urgent disease or an immediate public-health risk is suspected; use equivalent devolved-nation routes elsewhere.

Key red flags

Raised-pressure complication

Falling consciousness, unequal pupils, Cushing physiology or new extensor posturing indicates possible cerebral oedema or herniation and requires immediate critical-care and neuroimaging action.

Investigation priorities

01
MRI brain with appropriate sequencesFirst step

Detect inflammatory brain lesions, haemorrhage and important mimics.

Management branches

First six hoursProtect brain and start empirical treatment

Acute altered cognition or consciousness is accompanied by fever, seizure or focal findings.

  1. Stabilise airway, breathing and circulation, check glucose, treat convulsive status promptly and obtain blood cultures and metabolic samples without delaying therapy.
  2. Give intravenous aciclovir at the encephalitis regimen and empirical antibacterial meningitis cover as indicated, recording symptom and administration times.
  3. Involve neurology, infection and critical care early, using urgent CT only for emergency pathology or lumbar-puncture safety while arranging MRI as the preferred brain study.
  4. Report suspected acute encephalitis through the statutory health-protection route without awaiting diagnostic confirmation; urgent telephone contact is additionally required when another urgent notifiable disease or immediate public-health risk is suspected.
Diagnostic coreCombine fluid, MRI and EEG evidence

The patient is stable enough for targeted neurological investigation.

  1. Perform lumbar puncture when safe, retaining enough cerebrospinal fluid for bacterial culture, cells, chemistry, HSV and VZV PCR and later specialist tests.
  2. Obtain MRI promptly and EEG urgently for seizure, fluctuation or unexplained impaired consciousness, then interpret all three modalities against timing.

Key medicines

Aciclovir intravenousUse the current BNF encephalitis regimen, commonly 10 mg/kg intravenously every 8 hours in adults, with renal and body-size adjustment agreed locally.Start without diagnostic delay, ensure hydration and monitor renal function; accumulation can cause confusion, tremor or myoclonus that resembles worsening encephalitis.
LevetiracetamLoading and maintenance doses for acute seizures follow the local status-epilepticus protocol, renal function and specialist direction rather than a universal encephalitis schedule.Review renal dose, sedation, irritability and behavioural change; acute use does not automatically justify lifelong treatment after recovery.
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Sources and review status5 sources · checked 27 Aug 2026 · clinical review pending+
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.

  • ABN/BIA national guideline for suspected viral encephalitis in adultsUK adult viral encephalitis guidance covering empirical aciclovir, imaging, cerebrospinal-fluid testing and repeat PCR.
  • Association of British Neurologists guideline hubSpecialist UK neurology guidelines including acute encephalitis resources.
  • BNF aciclovir monographIntravenous dosing, renal adjustment, administration and adverse effects.
  • NICE bacterial meningitis recommendationsParallel management while bacterial meningitis remains in the initial differential.
  • UKHSA notifiable diseases and reportingStatutory suspicion-based reporting and timescales for acute encephalitis in England; use equivalent devolved-nation routes elsewhere.

Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes

Authoring stateRapid draftClinical stateAwaiting reviewJurisdictionUnited Kingdom
  • If an early HSV PCR is negative but suspicion remains, continue aciclovir and plan repeat lumbar puncture with virology advice rather than declaring exclusion.