What is the sensitivity of Magnetic Resonance Imaging (MRI) for Herpes Simplex Virus (HSV) encephalitis?

MRI Sensitivity for HSV Encephalitis

MRI has a sensitivity of over 90% for detecting HSV encephalitis, making it the preferred imaging modality for diagnosis and should be performed as soon as possible in all patients with suspected encephalitis. 1

Diagnostic Value of MRI in HSV Encephalitis

Sensitivity and Timing

• MRI is significantly more sensitive than CT for detecting cerebral changes in viral encephalitis, with abnormalities found in approximately 90% of HSV encephalitis cases documented by CSF PCR 1
• MRI abnormalities can be detected within 48 hours of hospital admission in most cases
• The Infectious Diseases Society of America recommends performing MRI, including diffusion sequences, within 24 hours of hospital admission in all patients with suspected encephalitis 1

MRI Sequence Considerations

• Diffusion-weighted imaging (DWI) is superior to conventional MRI for detecting early signal abnormalities in HSV encephalitis 1
• T2-weighted and FLAIR sequences are sensitive for detecting vasogenic edema
• Post-contrast T1 sequences can identify areas of blood-brain barrier breakdown
• The combination of these sequences improves diagnostic accuracy and helps exclude mimics 1

Characteristic MRI Findings in HSV Encephalitis

Typical Patterns

• Significant edema and hemorrhage in the temporal lobes 1
• Hypodense areas on T1-weighted images
• Nonhomogeneous contrast enhancement
• Bilateral temporal lobe involvement is nearly pathognomonic for HSV encephalitis 1
• Involvement of limbic system, cingulate gyrus, and insular region with gyral edema 2

Evolution of MRI Findings

• Early: Gyral swelling and high signal intensity on T2-weighted images in subfrontal region, temporal lobe, and insula 3
• Progressive: Regional extension with enhancement and possible hemorrhage despite appropriate acyclovir therapy 3
• Late: Encephalomalacia and brain atrophy 3

Important Clinical Considerations

Limitations and Pitfalls

• MRI findings may initially be normal or remain normal during the course of illness in up to 10% of cases 1, 4
• Atypical presentations are increasingly recognized, including extensive global brain swelling and severe brainstem involvement 4
• Initial CT scans are normal in approximately 69% of patients, highlighting the superior sensitivity of MRI 4

Differential Diagnosis

• MRI can help distinguish HSV encephalitis from other viral encephalitides:
  • CMV encephalitis: Characterized by periventricular involvement 2
  • VZV encephalitis: Presents with large vessel vasculopathy and ischemic/hemorrhagic infarcts 2
  • ADEM: MRI sensitivity is much lower (approximately 20%) compared to HSV encephalitis 5

Diagnostic Algorithm for Suspected HSV Encephalitis

1. Perform MRI with DWI, T2, FLAIR, and contrast-enhanced T1 sequences as soon as possible (ideally within 24 hours)
2. If MRI is unavailable or contraindicated, perform CT with and without contrast (recognizing its lower sensitivity of approximately 30%) 4, 5
3. Combine imaging findings with CSF analysis including PCR for HSV (noting that 22% of patients may have normocellular CSF) 4
4. Consider empiric acyclovir treatment while awaiting confirmatory test results, as early treatment significantly improves outcomes 4

MRI remains the gold standard imaging modality for HSV encephalitis with high sensitivity, but clinicians should maintain a high index of suspicion and not exclude the diagnosis based on normal initial imaging, particularly when clinical presentation is suggestive.