Encephalitis

• Pathophysiology & Etiology
  • Inflammation of the brain parenchyma, primarily due to infection or an autoimmune process.
  • Infectious (Most Common):
    • Herpes Simplex Virus 1 (HSV-1): The most common cause of sporadic, fatal encephalitis. It has a predilection for the temporal lobes.
    • Arboviruses: Transmitted by mosquitoes or ticks (e.g., West Nile Virus, St. Louis encephalitis, La Crosse virus). Often has a seasonal peak in the summer.
    • Other Viruses: VZV (causes chickenpox/shingles), EBV, CMV, Enteroviruses, Measles, Mumps.
  • Autoimmune:
    • Post-infectious (Acute Disseminated Encephalomyelitis - ADEM): An immune reaction following a viral infection or, rarely, vaccination.
    • Paraneoplastic/Antibody-Mediated: The immune system attacks neuronal proteins (e.g., anti-NMDA receptor encephalitis), sometimes associated with an underlying tumor like an ovarian teratoma.
• Clinical Presentation
  • The hallmark triad includes fever, headache, and altered mental status (AMS).
  • Other key findings include:
    • Focal neurological deficits: Hemiparesis, cranial nerve palsies, or aphasia.
    • Seizures: Common, especially with HSV.
    • Behavioral/Psychiatric changes: Personality changes, psychosis, or bizarre behavior, particularly prominent in HSV (temporal lobe) and autoimmune encephalitis.
• Diagnosis
  • Lumbar Puncture (LP) & CSF Analysis: This is the most crucial diagnostic step.
    • CSF Profile: Shows a lymphocytic pleocytosis (↑ WBCs, primarily lymphocytes), elevated protein, and normal glucose.
    • HSV Encephalitis: May also show ↑ RBCs due to hemorrhagic necrosis.
    • PCR: The most accurate test to identify the specific viral cause from CSF, especially HSV.
  • Neuroimaging:
    • MRI: The preferred imaging modality. It is more sensitive than CT and can show characteristic findings.
    • HSV Encephalitis: Classically shows unilateral or bilateral inflammation/necrosis of the temporal and inferior frontal lobes.
  • EEG: Can show diffuse slowing or focal abnormalities (e.g., periodic lateralizing epileptiform discharges in HSV), and helps detect seizure activity.
• Differential Diagnostics
  • Meningitis: Presents with more prominent nuchal rigidity (stiff neck) and photophobia, with less severe alteration in mental status compared to encephalitis. The presence of focal neurological deficits or seizures strongly suggests parenchymal (encephalitis) involvement.
  • Brain Abscess: Typically presents as a headache with focal neurologic deficits and fever, but imaging will reveal a ring-enhancing lesion.
  • Toxic-Metabolic Encephalopathy: Causes altered mental status but lacks the fever and CSF inflammatory changes seen in infectious encephalitis.
  • Autoimmune Encephalitis: Distinguished by specific antibody testing in the CSF or serum.
• Management
  • IMMEDIATE Empiric Therapy: Do NOT delay treatment while awaiting test results.
    • IV Acyclovir: Must be started immediately in any patient with suspected encephalitis to cover for HSV. Early treatment dramatically reduces the high mortality and morbidity associated with HSV encephalitis.
  • Supportive Care:
    • Manage airway, breathing, and circulation (ABCs).
    • Control fever and pain.
    • Seizure precautions/management: Use antiepileptic drugs as needed.
    • Monitor and manage for increased intracranial pressure (ICP).
  • Specific Therapy: Once a cause is identified, treatment is tailored (e.g., Ganciclovir for CMV; steroids, IVIG, or plasmapheresis for autoimmune causes).
• Key Associations & Complications
  • HSV-1: Associated with hemorrhagic necrosis of the temporal lobes. Untreated, it has a mortality rate of ~70%.
  • West Nile Virus: Can be associated with a polio-like syndrome of flaccid paralysis.
  • Complications: Seizures (including status epilepticus), SIADH, increased ICP, and permanent neurologic sequelae (memory loss, cognitive deficits, motor impairments).