What are the typical CT findings in herpes simplex virus encephalitis?

CT Findings in HSV Encephalitis

CT scanning in HSV encephalitis typically shows low-density (hypodense) lesions in the temporal lobes, though CT is significantly less sensitive than MRI and should only be used when MRI is unavailable.

Primary Imaging Findings on CT

The characteristic CT findings in herpes simplex encephalitis include 1, 2:

• Temporal lobe hypodensity: Low-absorption abnormalities affecting primarily the medial temporal lobes, often extending to the insular cortex 3, 4
• Unilateral initially, then bilateral: The lesion typically begins unilaterally in one temporal lobe (days 1-4), then spreads to the contralateral temporal lobe as a separate new lesion (days 6-8) 5
• Mass effect: Present in approximately 50% of cases, indicating significant edema 3
• Hemorrhagic changes: May show hemorrhagic foci within the temporal lobes in some cases 6, 7

Temporal Evolution of CT Findings

Understanding the timing is critical 5, 8:

• Days 0-4: CT may be completely normal early in the disease course
• Days 3-11: Hypodense temporal lobe lesions become visible
• Days 6-8: Contralateral temporal involvement typically appears
• Later stages: May extend to basal frontal lobes, and occasionally involve basal ganglia or thalamus

Contrast Enhancement Patterns

When contrast is administered 3, 9:

• Streaked enhancement: Affects primarily the sylvian fissure and opercular areas
• Present in 50% of enhanced scans within 7-8 days of symptom onset
• Enhancement pattern is nonhomogeneous

Critical Limitations of CT

MRI is vastly superior and should be the primary imaging modality 1, 2:

• CT is only recommended when MRI is unavailable, impractical, or cannot be performed 1
• More than 90% of PCR-confirmed HSV encephalitis cases show abnormalities on MRI 1
• CT's main utility is excluding alternative diagnoses (subarachnoid hemorrhage, mass lesions) and assessing safety for lumbar puncture 2

Clinical Pitfalls

• A normal CT scan does not exclude HSV encephalitis, especially in the first 2-4 days 5, 8
• Bilateral temporal lobe involvement is nearly pathognomonic for HSV encephalitis when present 1
• The temporal lobe predilection reflects the virus's tropism for these areas 7
• CT findings, while characteristic in the proper clinical setting, should prompt immediate antiviral therapy without waiting for confirmatory testing 1

Practical Algorithm

When CT is the only available modality 7:

1. Look specifically for hypodensity in the mesial temporal lobes
2. Check for hemorrhagic foci in frontotemporal regions
3. Assess for mass effect or edema
4. Remember that negative CT does not rule out HSV encephalitis
5. Initiate acyclovir empirically based on clinical suspicion regardless of imaging findings