encephalitis
in review
3 min read
Updated 2026-09-29
encephalitis
Altered mental status or behaviour change + fever, seizures, focal deficits, or CSF pleocytosis. Start IV acyclovir 10 mg/kg q8h immediately — HSV-1 is the commonest treatable cause and delay worsens outcome. MRI (temporal lobe involvement), CSF HSV/VZV/enterovirus PCR, EEG. Repeat HSV PCR at 3–7 days if the first is negative early and suspicion persists. If infection is excluded, think autoimmune — anti-NMDA receptor encephalitis in young women with psychiatric onset, dyskinesias, and autonomic instability.
do not miss
- HSV encephalitis — acyclovir before imaging or LP results
- bacterial meningitis and cerebral abscess mimic encephalitis — cover empirically per bacterial meningitis until excluded
- non-convulsive status epilepticus — EEG in any encephalopathic patient
meningitis vs encephalitis
| meningitis | encephalitis | |
|---|---|---|
| brain function | preserved early | abnormal early — confusion, personality change, reduced consciousness |
| seizures / focal signs | uncommon early | common |
| key treatment | antibiotics + dexamethasone | acyclovir (+ antibiotics until bacterial excluded) |
Meningoencephalitis overlaps both — treat both.
workup
- MRI brain with DWI/FLAIR: HSV → asymmetric mesial temporal/insular, often haemorrhagic; West Nile → thalami/basal ganglia; autoimmune (limbic) → bilateral mesial temporal
- LP: opening pressure, cells (lymphocytic), protein, glucose (usually normal in viral), Gram stain/culture, HSV-1/2, VZV, enterovirus PCR; arboviral IgM (West Nile) seasonally; cryptococcal antigen, VDRL, AFB if immunocompromised
- EEG: periodic lateralised discharges (HSV); non-convulsive seizures; extreme delta brush (anti-NMDAR)
- HIV test; blood cultures; serum and CSF autoimmune panel (NMDAR, LGI1, CASPR2, GABA-B, AMPA) when infection not found
- exposure history: mosquitoes, ticks (Powassan, Lyme), animals/bats (rabies), travel, immunosuppression
causes and treatment
| cause | clues | treatment |
|---|---|---|
| HSV-1 | any age, any season; temporal lobe syndrome, aphasia, seizures | acyclovir 10 mg/kg IV q8h × 14–21 days (renal dosing; hydrate — crystalluria/AKI) |
| VZV | often with zoster rash (may be absent); vasculopathy/stroke in immunocompromised | acyclovir 10–15 mg/kg IV q8h ± steroids for vasculopathy |
| enterovirus | summer–autumn; mostly meningitis | supportive |
| West Nile virus | late summer; older/immunocompromised; flaccid paralysis, tremor, movement disorder | supportive |
| Powassan virus | Ixodes tick exposure (Ontario, Quebec, Maritimes) | supportive |
| rabies | bat/animal exposure, hydrophobia, agitation | palliative; prevention is everything — see infection control and post-exposure prophylaxis |
| CMV, JC virus, toxoplasma, Cryptococcus | advanced HIV/transplant | see HIV opportunistic infections |
| Listeria (rhombencephalitis) | brainstem signs, older/immunocompromised/pregnant | ampicillin |
| anti-NMDAR encephalitis | young women; psychiatric onset → seizures, orofacial dyskinesia, autonomic instability, hypoventilation; ovarian teratoma | methylprednisolone + IVIG or plasma exchange; remove teratoma; rituximab/cyclophosphamide second-line |
| LGI1 encephalitis | older men; faciobrachial dystonic seizures, hyponatraemia | immunotherapy |
| paraneoplastic (Hu, Ma2) | cancer (SCLC, testicular) | treat tumour; immunotherapy less effective |
- post-HSV anti-NMDAR encephalitis: relapse of neuropsychiatric symptoms weeks after treated HSV encephalitis with negative repeat HSV PCR → autoimmune, not viral relapse
- Graus criteria allow possible autoimmune encephalitis to be treated before antibody results (subacute onset <3 months + memory/mental status change + CSF/MRI/EEG support + alternative causes excluded)
traps
- waiting for MRI or LP to start acyclovir
- stopping acyclovir on a negative HSV PCR taken in the first 72 h when the picture fits
- acyclovir-induced AKI — hydrate, adjust dose to renal function
- labelling a young woman’s first psychosis as primary psychiatric illness without considering anti-NMDAR encephalitis
- missing non-convulsive status epilepticus without EEG