How does MRI imaging differentiate between Herpes Simplex Virus (HSV) encephalitis and autoimmune encephalitis?

MRI Differentiation Between HSV Encephalitis and Autoimmune Encephalitis

MRI is the preferred imaging modality for differentiating HSV encephalitis from autoimmune encephalitis, with distinct patterns: HSV typically shows unilateral or bilateral involvement of the medial temporal lobes and cingulate gyrus with T2/FLAIR hyperintensities, while autoimmune encephalitis patterns vary by antibody type but often show more subtle or normal findings.

Key MRI Characteristics of HSV Encephalitis

• Early MRI changes occur in the cingulate gyrus and medial temporal lobe, showing gyral edema on T1-weighted images and high signal intensity on T2-weighted and T2 fluid attenuated inversion recovery (FLAIR) images 1
• Diffusion-weighted MRI (DWI) is especially sensitive for detecting early changes in HSV encephalitis and should be included in the protocol 1, 2
• MRI obtained within 48 hours of hospital admission is abnormal in approximately 90% of patients with HSV encephalitis 1
• Later in the disease course, hemorrhagic changes may develop in affected areas 1
• The changes seen on MRI are reported to be highly specific (87.5%) for PCR-confirmed HSV encephalitis 1
• Limbic encephalitis pattern with unilateral or bilateral involvement of the medial temporal lobes is highly characteristic, with bilateral temporal lobe involvement being nearly pathognomonic for HSV 2
• Insular and diffuse temporal lobe involvement is more frequent in HSV encephalitis 3

Key MRI Characteristics of Autoimmune Encephalitis

• MRI findings in autoimmune encephalitis vary significantly depending on the specific antibody type 1, 4
• In NMDA receptor encephalitis, most patients have a normal MRI in the acute stage, and when lesions are present, they are typically subtle and non-specific white matter lesions 4
• LGI1, CASPR2, and GAD antibody-associated encephalitis typically present as limbic encephalitis with unilateral or bilateral T2/FLAIR hyperintensities in the medial temporal lobe, but these changes are often more subtle than in HSV encephalitis 4
• GABA-A receptor encephalitis features multifocal, confluent lesions in cortical and subcortical areas 4
• Mesial temporal involvement is more characteristic of autoimmune encephalitis 3
• Basal ganglia involvement is more common in autoimmune encephalitis and its absence strongly supports HSV encephalitis (sensitivity 0.82, specificity 1) 3
• Brain FDG-PET may be more sensitive than MRI in autoimmune encephalitis, showing bilateral temporal hypermetabolism in limbic encephalitis and bilateral occipito-parietal hypometabolism in NMDAR-antibody encephalitis 1

Differential Diagnostic Features

• Acute onset of symptoms and fever strongly favor HSV encephalitis over autoimmune encephalitis (sensitivity 0.92, specificity 1) 3
• Psychiatric symptoms are more characteristic of autoimmune encephalitis 3
• Autoimmune encephalitis may have normal MRI findings in up to 40% of cases, while HSV encephalitis almost always shows abnormalities 3
• The combination of clinical and radiological features provides the highest diagnostic accuracy:
  • HSV: acute onset, fever, absence of basal ganglia involvement, diffuse temporal lobe involvement 3
  • Autoimmune: subacute onset, psychiatric symptoms, normal MRI or mesial temporal/basal ganglia involvement 3

Important Considerations

• MRI should be performed as soon as possible on all patients with suspected encephalitis, ideally within 24 hours of hospital admission, but certainly within 48 hours 1, 2
• If MRI is not immediately available, CT scanning may exclude structural causes of raised intracranial pressure but has limited sensitivity (approximately 25%) for HSV encephalitis 5
• Treatment decisions should not be delayed while waiting for imaging results, especially for HSV encephalitis where early acyclovir administration is critical 5
• Follow-up MRI may be useful to assess treatment response and disease progression 2
• In rare cases, HSV encephalitis and autoimmune encephalitis may occur concurrently, complicating the radiological picture 6

Clinical Correlation

• Imaging findings should always be interpreted in conjunction with clinical presentation, cerebrospinal fluid analysis (including HSV PCR and autoimmune antibody testing), and other laboratory tests 2
• The gold standard for HSV encephalitis diagnosis remains CSF PCR for HSV DNA, though MRI findings can strongly support the diagnosis 5
• For autoimmune encephalitis, specific antibody testing is essential, as MRI findings alone may be insufficient for diagnosis 1