Encephalitis
Encephalitis is inflammation of the brain parenchyma, most often viral (herpes simplex virus 1 being the classic treatable cause), presenting with fever, headache, altered mental status, and seizures; empiric IV acyclovir is started immediately whenever it is in the differential, pending confirmatory CSF PCR.
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What this article covers
- Overview
Definition: encephalitis is inflammation of the brain parenchyma itself, distinguishing it from meningitis (inflammation limited to the meninges), though the two frequently coexist as meningoencephalitis.
- Epidemiology
HSV-1 predominance: herpes simplex virus type 1 accounts for the largest share of sporadic (non-epidemic) viral encephalitis in immunocompetent adults across all ages.
- Risk factors
Immunocompromise: HIV infection, transplantation, and impaired cell-mediated immunity increase susceptibility to viral and opportunistic encephalitis.
- Pathophysiology
Direct viral cytolysis: HSV-1 reactivates from latency in the trigeminal ganglion, spreads along neural pathways to the temporal and frontal lobes, and causes direct neuronal injury with hemorrhagic…
- Clinical features
Fever: present in most cases and helps distinguish encephalitis from a purely structural or vascular cause of encephalopathy.
- Differential diagnosis
Viral meningitis: shares fever and headache but preserves normal mentation and lacks seizures, since inflammation is confined to the meninges rather than brain parenchyma.
- Diagnosis
Establishing basis: encephalitis is diagnosed from fever and encephalopathy together with CSF lymphocytic pleocytosis, characteristic MRI change, or EEG abnormality localizing to brain parenchyma; a…
- Management
First, start empiric IV acyclovir 10 mg/kg every 8 hours in any patient in whom HSV encephalitis is plausible, given immediately after blood cultures and lumbar puncture are obtained, and maintain hydration to avoid crystalluria.
- Complications
Status epilepticus: encephalitis, especially HSV encephalitis, is a common cause of new-onset status epilepticus and worsens overall prognosis.
- Prognosis
Untreated HSV encephalitis mortality approaches 70%, making early empiric acyclovir the single most important prognostic intervention.
- Prevention
No vaccine currently prevents HSV encephalitis, so prevention centers on early recognition and empiric treatment within hours of presentation rather than primary prophylaxis.
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