Viral Encephalitis in Infants

Clinical Presentation and Diagnosis

The cessation of smiling in a 6-month-old following viral fever represents a significant behavioral regression that warrants aggressive evaluation for post-infectious encephalitis, even with normal initial investigations 1, 2, 3
MRI can be normal in up to 10% of viral encephalitis cases within the first 48 hours of presentation, particularly in HSV encephalitis where only 25% show abnormalities on initial CT scanning 1, 2, 4
Normal blood counts do not exclude CNS infection, as viral encephalitis frequently presents with normal peripheral laboratory values 5
The behavioral change (loss of smiling) represents altered mental status and personality change, which are hallmark features of encephalitis 1, 2

Perform LP immediately unless contraindications exist (GCS <9, signs of raised intracranial pressure, coagulopathy with platelets <50-100 × 10⁹/L, or clinical instability) 5
CSF analysis is essential as it provides diagnostic information that imaging cannot, particularly for viral encephalitis where MRI may be normal 5, 1
Send CSF for: cell count, protein, glucose (with paired serum glucose), HSV PCR, enterovirus PCR, VZV PCR, and bacterial/viral cultures 5
Approximately 50% of HSV encephalitis cases show elevated CSF red cell count, though acellular CSF can occur with VZV, EBV, and CMV 5

Start intravenous acyclovir immediately for suspected viral encephalitis, even before LP results return 4
This is standard of care for any child with suspected encephalitis given the devastating consequences of untreated HSV encephalitis 4

Post-infectious encephalopathy from various viral pathogens (enterovirus, VZV, influenza) should be considered 1, 2, 3
Mycoplasma pneumoniae CNS involvement (can show focal cortical lesions, white matter lesions, or demyelination on MRI) should be considered 1, 2, 3
HIV-related CNS infection if risk factors present (though less likely at 6 months without other symptoms) should be considered 2, 3

Early aggressive treatment with acyclovir significantly impacts morbidity and mortality outcomes 4
The extent of early MRI abnormality in HSV encephalitis does not correlate well with clinical evolution, though correlation exists between number of seizures and subsequent brain atrophy 1, 2, 3