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Viral encephalitis
Encephalitis is fever with altered consciousness, behaviour change or seizures — and the most important rule is to start intravenous acyclovir at once on suspicion, because herpes simplex encephalitis is lethal untreated and treatable.
Why it matters in the exam
Questions present a patient with fever, confusion, odd behaviour, smell hallucinations or a seizure, and ask for the next step: acyclovir at once, imaging before lumbar puncture when there is a focal deficit or reduced consciousness, and CSF herpes PCR. Typical herpes findings: temporal lobe involvement on MRI, one-sided periodic discharges on EEG and red cells in the CSF. In Israel remember West Nile virus in summer, mainly in older adults, sometimes with flaccid paralysis. Common distractors: waiting for the PCR result before treating, stopping acyclovir because PCR is negative in the first days, or forgetting hydration that prevents kidney injury from the drug. After herpes encephalitis, worsening weeks later may come from autoimmune anti-NMDA receptor encephalitis.
Key points
- Fever with altered consciousness, behaviour or a seizure is encephalitis until proven otherwise.
- Acyclovir 10 mg/kg intravenously every 8 hours starts at once, before results.
- Herpes hits the temporal lobe: MRI, EEG and red cells in the CSF.
- A negative PCR in the first days does not exclude it; repeat the puncture.
- In Israel — West Nile virus in summer, mainly in older adults.
Summary table
| Aspect | Finding or rule | Note |
|---|---|---|
| Presentation | Fever, confusion, behaviour change, memory and speech disturbance, seizures, focal deficit | Meningitis alone does not impair consciousness and brain function to the same degree |
| CSF | Lymphocytic pleocytosis, raised protein, normal glucose; in herpes sometimes red cells | Herpes PCR may be negative in the first 72 hours — repeat the puncture |
| Imaging and EEG | MRI: high signal in the medial temporal lobe and insula, usually asymmetric; EEG: one-sided periodic discharges | CT before puncture with a focal deficit, papilloedema, reduced consciousness, immunosuppression or a new seizure |
| Treatment | Acyclovir 10 mg/kg intravenously every 8 hours for 14 to 21 days | Adjust to kidney function and keep well hydrated to prevent kidney injury |
| Other viruses | West Nile virus (summer, older adults, flaccid paralysis), varicella zoster (immunosuppressed), enteroviruses, rabies | No specific treatment for West Nile; supportive care |
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Sources
- IDSA Clinical Practice Guidelines: The Management of Encephalitis, 2008 — Diagnosis and empirical acyclovir
- Management of Suspected Viral Encephalitis in Adults — ABN/BIA National Guidelines (Solomon et al., J Infect), 2012 — Lumbar puncture, imaging and treatment
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.