Viral Encephalitis

1. Definition

Acute Encephalitis: Inflammation of the brain parenchyma with acute onset (duration <4 weeks).
Meningoencephalitis: Presence of associated meningeal inflammation.
Differentiation: Distinguished from acute febrile encephalopathy (AFE) by evidence of brain inflammation (CSF pleocytosis, imaging changes).

Primary Encephalitis: Direct viral invasion and replication leading to neuronal tissue necrosis (e.g., HSV, Japanese Encephalitis).
Parainfectious (ADEM): Immune-mediated injury (demyelination) following viral infection or vaccination; usually preserves neurons/axons.

Viral agents are classified into sporadic and epidemic forms.

Category	Common Agents	Remarks
Sporadic	Herpes Simplex (HSV-1, HSV-2)	Most common cause of sporadic encephalitis in West; HSV-1 (93-96%), HSV-2 (4-7%).
	Mumps, Measles, Varicella
	Enteroviruses (ECHO, Coxsackie)
	Rabies	History of animal bite.
Epidemic	Japanese Encephalitis (JE)	Most common epidemic cause in India; Zoonotic (Pig host, Culex vector).
	Chandipura Virus	Rapid progression, high mortality.
	Arboviruses	Eastern/Western Equine, Dengue.

Presentation is highly variable.

Prodrome: Fever, malaise, headache (1–7 days).
Neurological Phase:
- Altered Sensorium: Ranges from irritability/lethargy to deep coma.
- Seizures: Generalized or focal (seen in HSV/JE).
- Focal Deficits: Hemiparesis, cranial nerve palsies, ataxia, aphasia.
- Raised ICP: Vomiting, headache, bulging fontanelle.
Specific Clinical Clues:
- Parotitis: Mumps.
- Hydrophobia/Aerophobia: Rabies.
- Vesicular Rash: Varicella/Herpes Zoster.
- Extrapyramidal Signs: Japanese Encephalitis (Parkinsonian features).

A. Lumbar Puncture (Crucial Investigation)

Contraindications: Signs of raised ICP (papilledema), focal deficits, herniation, shock (perform CT first).
CSF Findings:
- Appearance: Clear or slightly turbid.
- Cells: Pleocytosis (10–500 cells/mm³). Early: Polymorphonuclear; Late: Lymphocytic predominance.
- Biochemistry: Protein mild-moderate elevation; Sugar usually normal (unlike bacterial).
- Hemorrhagic CSF: Suggestive of HSV (necrotizing encephalitis).

B. Etiological Diagnosis

PCR (Gold Standard): Highly sensitive/specific for HSV (DNA), Enteroviruses, JE.
Serology:
- IgM Capture ELISA (CSF/Serum) for JE (MAC-ELISA).
- Rising IgG titers in paired sera (Acute and Convalescent).
Viral Isolation: From CSF, stool, nasopharyngeal swabs (low yield, slow).

C. Neuroimaging

MRI (Preferred): Higher sensitivity than CT.
- HSV: Hyperintensities in temporal lobes, orbital frontal regions, insular cortex, cingulate gyrus.
- Japanese Encephalitis: T2 hyperintensities in thalamus and basal ganglia; "Panda Sign" in midbrain.
- ADEM: Bilateral, asymmetrical white matter hyperintensities.
CT Scan: Often normal in early stages; hypodensities appearing later.

D. Electroencephalogram (EEG)

Diffuse slowing (non-specific).
PLEDs (Periodic Lateralized Epileptiform Discharges): Highly suggestive of Herpes Simplex Encephalitis.

A. Supportive Care (Mainstay of Treatment)

Airway/Breathing: Protect airway in comatose child; oxygenation; ventilation if GCS <8 or respiratory failure.
Circulation: Treat shock with IV fluids/inotropes.
Raised ICP Management:
- Head elevation 30°.
- Mannitol (0.25–0.5 g/kg/dose) or Hypertonic Saline (3%).
- Controlled hyperventilation (transient use).
Seizure Control: Benzodiazepines (Lorazepam/Midazolam) followed by Phenytoin/Fosphenytoin or Levetiracetam.
Fluid/Electrolytes: Monitor for SIADH (restrict fluids if present) or Cerebral Salt Wasting. Maintenance of normoglycemia.

B. Specific Antiviral Therapy
Empiric Acyclovir should be started immediately if HSV is suspected until proven otherwise.

Etiology	Drug	Dosage & Duration
Herpes Simplex	Acyclovir (IV)	10–20 mg/kg/dose q8h for 14–21 days.
Varicella	Acyclovir (IV)	10–20 mg/kg/dose q8h for 7–10 days.
CMV	Ganciclovir	Induction and maintenance doses.
Influenza	Oseltamivir	If suspected.
Note: No specific antiviral exists for Japanese Encephalitis (supportive care only).

Epidemiology: Endemic in parts of India (UP, Assam, West Bengal, Karnataka).
Transmission: Pig (Amplifying host) $\to$ Culex Mosquito $\to$ Human (Dead-end host).
Prevention (Vaccines):
- Live Attenuated (SA 14-14-2): Single dose (0.5ml SC), highly effective (current National Immunization Schedule).
- Inactivated (Mouse brain/Hamster kidney): Older vaccines, require boosters.
Vector Control: Fogging, bed nets, pig control.

Mortality: High in untreated HSV (70%) and JE (20–30%).
Sequelae:
- Cognitive: Mental retardation, learning disabilities.
- Motor: Spasticity, extrapyramidal movement disorders (common in JE).
- Epilepsy: Post-encephalitic seizures.
- Psychiatric: Behavioral changes.