NeurologyTier 1Disease (DEADMAN)

Encephalitis

Description

Inflammation of brain parenchyma presenting with altered mental state, seizures, and/or focal neurological signs, +/- fever
Distinguished from meningitis by parenchymal dysfunction (confusion, seizures, focal deficit) rather than meningism alone - overlap ("meningoencephalitis") is common

HSV-1 is the commonest identified sporadic cause in immunocompetent adults
A specific cause is identified in only ~40-50% of cases despite thorough work-up
Autoimmune encephalitis (e.g. anti-NMDAR) increasingly recognised, particularly in younger patients and post-HSV encephalitis relapse

Infective

Viral: HSV-1/2, VZV, enterovirus, arboviruses (Japanese encephalitis, Murray Valley encephalitis - relevant in Australia), CMV/EBV (immunosuppressed), HIV seroconversion
Bacterial: Listeria (rhombencephalitis), TB, syphilis, Mycoplasma
Other: fungal (immunosuppressed), parasitic

Autoimmune/paraneoplastic

Anti-NMDA receptor encephalitis - young patients, often female, associated with ovarian teratoma; psychiatric symptoms, dyskinesias, autonomic instability, seizures
Anti-LGI1, anti-GABA-B, anti-CASPR2 and other antibody syndromes - varying clinical patterns (faciobrachial dystonic seizures in LGI1)
HSV encephalitis can trigger secondary anti-NMDAR encephalitis weeks later - a recognised relapse pattern distinct from true viral relapse
Paraneoplastic (limbic encephalitis) - screen for underlying malignancy

Clinical

Fever, headache, altered consciousness/behaviour, seizures, focal neurological signs
Temporal lobe features (memory disturbance, olfactory/gustatory hallucinations, behavioural change) suggest HSV

Investigations

MRI brain - HSV classically shows temporal lobe/inferomedial frontal T2/FLAIR hyperintensity, +/- haemorrhage; autoimmune encephalitis may show mesial temporal changes or be normal
LP (after imaging if raised ICP risk/focal signs) - lymphocytic pleocytosis, elevated protein; HSV PCR on CSF (may be falsely negative in first 24-72h - repeat if high suspicion)
EEG - may show periodic lateralised epileptiform discharges (PLEDs) in HSV, supports non-convulsive status exclusion
Autoimmune antibody panel (serum and CSF) - anti-NMDAR, anti-LGI1, anti-GABA-B, paraneoplastic panel - send when infective work-up is unrevealing or clinical picture suggests autoimmune cause (subacute psychiatric change, dyskinesias, faciobrachial seizures)
Tumour screen (e.g. pelvic imaging for ovarian teratoma) if anti-NMDAR suspected/confirmed

A. Empirical treatment - start immediately, do not wait for confirmation

IV aciclovir empirically in every suspected encephalitis case while awaiting HSV PCR - delay materially worsens outcome
Continue empirical aciclovir even if initial CSF HSV PCR negative but clinical suspicion remains high - repeat PCR at 72 hours
Add empirical antibacterial cover (as for bacterial meningitis) +/- listeria cover (amoxicillin/ampicillin) if bacterial cause cannot be excluded, particularly in the immunosuppressed/elderly

B. Confirmed viral encephalitis

HSV: IV aciclovir 14-21 days (21 days in immunosuppressed or if repeat CSF PCR still positive at 14 days)
VZV encephalitis: IV aciclovir similarly
CMV (immunosuppressed): ganciclovir +/- foscarnet

C. Autoimmune encephalitis - do not wait for antibody confirmation if clinical suspicion is high and infection excluded

First-line immunotherapy: high-dose corticosteroids, IVIG, or plasma exchange (often combined)
Second-line (if inadequate response): rituximab, cyclophosphamide
Anti-NMDAR encephalitis: identify and remove any underlying tumour (e.g. ovarian teratoma) - tumour removal itself is therapeutic
Manage seizures and autonomic instability supportively; ICU-level care often required for severe anti-NMDAR disease (dyskinesias, ventilatory failure)

D. Supportive

Seizure management (see Seizures notes) - treat clinical/EEG-confirmed seizures, no role for universal prophylaxis
ICU care for reduced consciousness, raised ICP management if present

Untreated HSV encephalitis mortality ~70%; treated mortality reduced to ~20%, but significant long-term morbidity persists in survivors (memory impairment, epilepsy, behavioural change) - earlier aciclovir strongly predicts better outcome
Autoimmune encephalitis - anti-NMDAR has a relatively good prognosis with early aggressive immunotherapy and tumour removal, though relapse can occur
Long-term sequelae common across causes - cognitive impairment, epilepsy, personality/behavioural change
Ongoing rehabilitation and neuropsychological support often required

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