What is the approach to a case with an MRI brain suggestive of early features of encephalitis?

Approach to MRI Brain Suggestive of Early Features of Encephalitis

Immediate intravenous aciclovir (10 mg/kg three times daily) should be started when MRI findings suggest viral encephalitis, ideally within 6 hours of admission, without waiting for confirmation of HSV by PCR. 1

Initial Diagnostic Approach

• MRI is the imaging modality of choice for suspected encephalitis, with approximately 90% sensitivity when performed within 48 hours of admission, compared to CT which has only about 25% sensitivity for initial diagnosis 2, 3
• Characteristic early MRI findings in HSV encephalitis include:
  • Gyral edema on T1-weighted images 1
  • High signal intensity on T2-weighted and FLAIR images in the cingulate gyrus and medial temporal lobes 2
  • Diffusion-weighted imaging (DWI) shows higher sensitivity for early changes and should be included in the protocol 2, 3
• If MRI is unavailable or impractical (e.g., in acutely ill, comatose patients), CT scanning may be performed to exclude structural causes of raised intracranial pressure, but should not be relied upon to rule out encephalitis 1
• Lumbar puncture should be performed for CSF analysis, including:
  • PCR for HSV DNA (the diagnostic gold standard) 3
  • Cell count, protein, and glucose levels 1

Treatment Protocol

• For adults and adolescents (≥12 years):
  • Administer intravenous aciclovir 10 mg/kg every 8 hours 4
  • Continue for 14-21 days in confirmed HSE cases 3
• For children (3 months to 12 years):
  • Administer intravenous aciclovir 20 mg/kg every 8 hours 4
  • For children 3 months-12 years: 500mg/m² 8 hourly 1
• For neonates (birth to 3 months):
  • Administer intravenous aciclovir 10 mg/kg every 8 hours 4
• Dose adjustment is required for patients with renal impairment 4, 1:
  • CrCl >50 mL/min: 100% dose every 8 hours
  • CrCl 25-50 mL/min: 100% dose every 12 hours
  • CrCl 10-25 mL/min: 100% dose every 24 hours
  • CrCl 0-10 mL/min: 50% dose every 24 hours

Additional Management Considerations

• EEG should be performed if subtle motor or non-convulsive seizures are suspected, or to differentiate between psychiatric and organic causes in patients with mildly altered behavior 1
• Brain biopsy is not indicated in the initial assessment but may be considered in patients with suspected encephalitis in whom no diagnosis has been made after the first week, especially if there are focal abnormalities on imaging 1
• Supportive care is essential, including:
  • Seizure management 5
  • Management of increased intracranial pressure 5
  • Access to intensive care units for close monitoring 5

Specific MRI Patterns by Etiology

• HSV encephalitis: Bilateral temporal lobe involvement is nearly pathognomonic 1, 2
• VZV CNS disease: Ischemic or hemorrhagic infarcts and intracranial arterial abnormalities 1, 2
• Japanese B encephalitis: Typically involves the thalamus and basal ganglia with T2 hyperintensity 1
• Enterovirus: May result in generalized parenchymal destruction or predominantly affect the brainstem 1
• M. pneumoniae: May show focal cortical lesions, deep white matter lesions, and large areas of demyelination 1

Common Pitfalls to Avoid

• Delaying aciclovir treatment while waiting for imaging or laboratory confirmation increases mortality and morbidity 3
• Relying solely on a single negative CSF PCR to rule out HSE, as false negatives can occur, especially early in the disease course 3
• Using CT as the only imaging modality due to its poor sensitivity for early encephalitis changes 3
• Discontinuing aciclovir prematurely if initial CSF or imaging findings are normal but clinical suspicion remains high 1

Follow-up Imaging

• Follow-up MRI is recommended to evaluate evolving necrosis or demyelination and to assess treatment response 2, 6
• Serial MRI changes after aciclovir therapy may show regional extension with enhancement and hemorrhage despite appropriate therapy, followed by encephalomalacia and brain atrophy 6