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Textbook/Neurology/Encephalitis and HSV encephalitis/Rapid
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Encephalitis and HSV encephalitis

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2 min synopsisUK scopeSources checked 27 Aug 2026Clinical 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.

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.
Diagnostic coreCombine fluid, MRI and EEG evidence

The patient is stable enough for targeted neurological investigation.

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.
LevetiracetamLoading and maintenance doses for acute seizures follow the local status-epilepticus protocol, renal function and specialist direction rather than a universal encephalitis schedule.
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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.

Authoring stateRapid draftClinical stateAwaiting reviewJurisdictionUnited Kingdom