Viral Encephalitis

Overview

Viral encephalitis is an acute, life-threatening inflammatory condition of the brain parenchyma caused by direct viral invasion or secondary immune-mediated mechanisms. It represents a neurological emergency associated with high rates of mortality and long-term neurodevelopmental or cognitive sequelae if treatment is delayed.

Infectious encephalitis is distinct from aseptic viral meningitis; while meningitis is confined to the leptomeninges (presenting with headache, fever, and nuchal rigidity), encephalitis involves parenchymal dysfunction, manifesting as altered mental status, focal neurological deficits, motor/sensory deficits, personality changes, or seizures. Herpes Simplex Virus Type 1 (HSV-1) is the single most common cause of non-epidemic, sporadic viral encephalitis in developed nations, accounting for 50–75% of identified cases.

Definition

Encephalopathy: Disruption of brain function characterized by altered state of consciousness, lethargy, confusion, behavioral changes, or personality changes lasting longer than 24 hours.

Meningoencephalitis: Co-existing inflammatory involvement of both the leptomeninges and the underlying brain parenchyma.

Temporal Lobe Tropism: The selective affinity of HSV-1 for limbic structures, specifically causing localized necrotizing hemorrhagic inflammation of the temporal lobes, orbital frontal cortex, and insula.

Periodic Lateralized Epileptiform Discharges (PLEDs): Characteristic electroencephalographic (EEG) waveform pattern over the temporal lobes strongly associated with acute HSV encephalitis.

Klüver-Bucy Syndrome: A rare neurobehavioral syndrome resulting from bilateral temporal lobe damage (such as post-HSV encephalitis), characterized by hyperorality, hypersexuality, visual agnosia, and emotional blunting.

HSV-1 accounts for over 90% of adult herpes encephalitis cases, whereas HSV-2 typically causes neonatal encephalitis (acquired during passage through an infected birth canal) or benign recurrent lymphocytic meningitis (Mollaret meningitis) in adults.

Aetiology & Risk Factors

Common Viral Pathogens

Viral Category	Specific Agents	Clinical Context / Clues
Herpesviruses	HSV-1 (most common overall) VZV (second most common) HSV-2, CMV, EBV, HHV-6	HSV-1: Temporal lobe predilection. VZV: Co-existing zoster rash or vasculopathy. CMV/HHV-6: Immunocompromised / transplant patients.
Enteroviruses	Coxsackievirus A & B, Echovirus, Enterovirus A71 / D68	Common in pediatric populations; hand-foot-and-mouth lesions; summer/autumn peak.
Arboviruses	West Nile, Japanese Encephalitis, Dengue, Murray Valley Encephalitis	Mosquito/tick vectors; seasonal exposure; flaccid paralysis (West Nile) or parkinsonian tremors (JE).
Paramyxoviruses & Rabies	Measles, Mumps, Rabies Virus	Rabies: Hydrophobia, aerophobia, agitation; 100% fatal once symptoms manifest.

Risk Factors

Age Extremes: Infants and elderly adults exhibit higher susceptibility and worse outcomes.
Immunosuppression: Advanced HIV/AIDS, organ transplantation, chemotherapy, or biological agents (e.g., natalizumab) increase risk for CMV, VZV, and HHV-6.
Geographic & Vector Exposure: Outdoor travel to endemic regions (e.g., Southeast Asia for Japanese Encephalitis) during peak vector seasons without vaccination or prophylaxis.

Pathophysiology

CNS Neurotropism & Access:
- HSV-1: Enters via the olfactory mucosa or reactivates from latency within the trigeminal ganglion ($CN\ V_1$), travelling retrogradely along nerve fibers to the basal frontal and medial temporal lobes.
- Arboviruses / Enteroviruses: Replicate peripherally, enter the circulation (viremia), and breach the blood-brain barrier (BBB).
Neuronal Destruction: Viral replication leads to host cell lysis, triggering a robust inflammatory response with perivascular lymphocytic cuffing, microglial nodules, and tissue necrosis.
Hemorrhage & Edema: In HSV encephalitis, intense necrotizing vasculitis causes petechial hemorrhages within the temporal cortex, producing vasogenic and cytotoxic edema that rapidly elevates Intracranial Pressure (ICP).

HSV-1 encephalitis selectively destroys the medial temporal lobe and orbital frontal cortex, leading to its classic presentation of memory impairment, aphasia, and focal temporal seizures.

Clinical Manifestations

Triad of Encephalitis

Fever (high-grade, abrupt or insidious onset)
Headache (diffuse, progressive)
Altered Mental Status (confusion, disorientation, lethargy, or coma)

Behavioral & Focal Neurological Features

Temporal Lobe / Limbic Signs (Classic HSV): Olfactory or gustatory hallucinations, sudden personality changes, bizarre/psychotic behavior, vivid deja vu, and memory loss (anterograde amnesia).
Seizures: Present in up to 50% of patients; can be focal temporal lobe seizures (lip-smacking, chewing, automatisms) or generalized tonic-clonic status epilepticus.
Focal Deficits: Hemiparesis, dysphasia/aphasia, ataxia, and cranial nerve palsies (CN III, VI, VII).
Autonomic & Movement Abnormalities: Tremors, myoclonus, or chorea (prominent in Japanese Encephalitis and West Nile Virus).

Diagnosis

Diagnostic evaluation must occur urgently. Empiric treatment must never be delayed while awaiting diagnostic results.

1. Lumbar Puncture & Cerebrospinal Fluid (CSF) Analysis

Mandatory unless contraindicated by signs of severely raised ICP or impending herniation (e.g., papilledema, focal neurological deficits, obtundation).

Opening Pressure: Elevated (> 200 mmH2O).
Cell Count: Lymphocytic pleocytosis (typically 10–500 cells)
Glucose: Normal (CSF-to-serum glucose ratio > 0.6). Note: Low glucose can occasionally occur in severe HSV or CMV.
Protein: Mild-to-moderately elevated (50–150 mg/dL).
Red Blood Cells (RBCs): Elevated RBCs in the absence of a traumatic tap strongly suggest necrotizing hemorrhagic encephalitis caused by HSV-1.

2. CSF Molecular Diagnostics (Gold Standard)

Multiplex CSF Polymerase Chain Reaction (PCR): Gold standard diagnostic method with sensitivity and specificity exceeding 95% for HSV-1, HSV-2, VZV, and Enteroviruses.
Diagnostic Pitfall: A false-negative HSV PCR can occur if performed within the first 24–48 hours of symptom onset. If clinical suspicion remains high, empiric antiviral therapy must be continued and a repeat LP performed in 3–7 days.

3. Neuroimaging

Magnetic Resonance Imaging (MRI – Gold Standard Imaging): Far more sensitive than CT.
- HSV Findings: T2-weighted and FLAIR hyperintensity with restricted diffusion in the medial temporal lobes, inferior frontal lobes, and insular cortex (frequently asymmetric).
Computed Tomography (CT) Brain: Performed prior to LP to rule out mass effect; may be normal in early disease or show subtle hypodensities in the temporal regions.

4. Electroencephalogram (EEG)

Demonstrates diffuse background slowing and localized temporal lobe spikes or Periodic Lateralized Epileptiform Discharges (PLEDs) in up to 80% of HSV cases.

A normal head CT does NOT rule out viral encephalitis. MRI with FLAIR sequences is the imaging modality of choice and detects temporal lobe changes within 24 hours of symptom onset.

*Brain MRI demonstrating temporal lobe hyperintensity in Herpes Simplex Encephalitis. Source: ResearchGate / A-F): Brain MRI images at first presentation with HSV encephalitis*

Treatment

Targeted Antiviral Therapy

Empiric First-Line Therapy:Intravenous Acyclovir (10 mg/kg IV every 8 hours) initiated immediately upon clinical suspicion.
- Dosing Note: Dosed based on ideal body weight; adjust for renal impairment.
- Toxicity Prevention: Acyclovir can cause crystalline nephropathy; ensure vigorous IV hydration to maintain high urine output.
Duration of Therapy: 14 to 21 days for confirmed HSV or VZV encephalitis. Repeat LP and CSF PCR near completion to confirm viral clearance before stopping therapy.
Cytomegalovirus (CMV) Encephalitis: Dual therapy with IV Ganciclovir + IV Foscarnet.

Critical Care & Supportive Management

Seizure Control: Intravenous Levetiracetam, Fosphenytoin, or Lorazepam for acute seizures or status epilepticus.
Management of Elevated ICP: Head elevation (30 degrees), hypertonic saline (3%), or Mannitol (0.5–1.0 g/kg IV) for intracranial hypertension.
Airway Protection

The most critical determinant of survival and functional recovery in HSV encephalitis is the time from presentation to IV Acyclovir administration. Never delay IV Acyclovir while awaiting imaging or lumbar puncture!

Complications & Prognosis

Complications

Post-Encephalitic Epilepsy: Secondary unprovoked recurrent seizures resulting from focal temporal cortical scarring.
Neuropsychiatric Sequelae: Severe anterograde amnesia, dysphasia, executive dysfunction, depression, psychosis, and Klüver-Bucy syndrome.
Secondary Autoimmune Encephalitis: Development of Anti-NMDAR Encephalitis 2 to 6 weeks following resolved HSV infection (manifests as choreoathetosis, behavioral relapse, or autonomic instability).
Acyclovir Nephrotoxicity: Acute tubular necrosis and intrarenal crystal precipitation.

Prognosis

Untreated HSV Encephalitis: Mortality approaches 70%, with virtually all survivors suffering severe, permanent neurological damage.
Treated HSV Encephalitis: Prompt IV Acyclovir reduces overall mortality to 15–20%; however, up to 50% of survivors suffer long-term cognitive or behavioral deficits.

References

Steiner I, Budka H, Chaudhuri A, et al. Viral encephalitis: a review of diagnostic methods and guidelines for management. Eur J Neurol. 2010;17(8):999-e57. doi:10.1111/j.1468-1331.2010.02970.x

Solomon T, Michael BD, Smith PE, et al. Management of suspected viral encephalitis in adults: Association of British Neurologists and British Infection Association national guidelines. J Infect. 2012;64(4):347-373. doi:10.1016/j.jinf.2011.11.014

Venkatesan A, Tunkel AR, Bloch KC, et al. Case definitions, diagnostic algorithms, and ward priorities in encephalitis: consensus statement of the International Encephalitis Consortium. Clin Infect Dis. 2013;57(8):1114-1128. doi:10.1093/cid/cit458