From the Guidelines
MRI findings in herpes simplex encephalitis (HSE) typically show asymmetric involvement of the temporal lobes, insular cortex, and inferior frontal regions, with early changes including gyral edema on T1-weighted images and high signal intensity on T2-weighted and FLAIR images, as reported in the 2012 study by the Association of British Neurologists and British Infection Association 1. The temporal lobe involvement is nearly pathognomonic for HSE, with more than 90% of patients with HSE documented by CSF PCR having abnormalities seen on MRI, according to the 2008 guidelines by the Infectious Diseases Society of America 1. Key characteristics of HSE on MRI include:
- Asymmetric involvement of the temporal lobes, insular cortex, and inferior frontal regions
- Early changes: gyral edema on T1-weighted images and high signal intensity on T2-weighted and FLAIR images
- Diffusion-weighted imaging (DWI) may show restricted diffusion, indicating cytotoxic edema
- Contrast enhancement may be present but is variable
- Hemorrhagic changes can develop as the disease progresses, appearing as hypointense areas on T2* or susceptibility-weighted sequences
- Medial temporal lobe involvement, often affecting the limbic system structures including the hippocampus and amygdala, is particularly characteristic of HSE It is essential to perform an MRI urgently in suspected cases of HSE, as it can help identify alternative diagnoses and guide treatment, as emphasized in the 2012 guidelines by the Association of British Neurologists and British Infection Association 1. In cases where MRI is not feasible, a CT scan may be performed, but it is crucial to note that CT scans may be normal in patients with HSE, especially early in the disease, as reported in the 2012 study by the Association of British Neurologists and British Paediatric Allergy, Immunology and Infection Group 1.
From the Research
MRI Findings in Herpetic Encephalitis
- High signal intensity in the temporal lobe on T2-weighted images (T2WIs) and diffusion-weighted images (DWIs) 2, 3
- Gyral swelling and high signal intensity on T2-weighted images in the subfrontal region, temporal lobe, and insula in the initial stage 3
- Regional extension with enhancement and hemorrhage despite appropriate acyclovir therapy 3
- Encephalomalacia and brain atrophy in the later stages 3
- High signal intensity lesions on T1-weighted MRI after clinical improvement, indicating hemorrhagic inflammatory lesions 4
- Abnormalities on T1-weighted MRI may not be detectable by CT 4
Recommended MRI Examination Protocol
- Coronal T1-weighted MR imaging before and after administration of gadopentate dimeglumine 5
- Coronal FLAIR sequence 5
- Axial T2-weighted imaging 5