MRI Differentiation Between HSV Encephalitis and Autoimmune Encephalitis

Key MRI Characteristics

Early MRI changes in HSV encephalitis 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, with diffusion-weighted MRI (DWI) being especially sensitive for detecting early changes 1, 2
MRI obtained within 48 hours of hospital admission is abnormal in approximately 90% of patients with HSV encephalitis, with later hemorrhagic changes developing in affected areas 1, 2
The changes seen on MRI are reported to be highly specific (87.5%) for PCR-confirmed HSV encephalitis, with limbic encephalitis pattern being highly characteristic 2
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 3

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, and treatment decisions should not be delayed while waiting for imaging results 2, 4, 5
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
Follow-up MRI may be useful to assess treatment response and disease progression, and imaging findings should always be interpreted in conjunction with clinical presentation, cerebrospinal fluid analysis, and other laboratory tests 4, 5

The gold standard for HSV encephalitis diagnosis remains CSF PCR for HSV DNA, though MRI findings can strongly support the diagnosis, and for autoimmune encephalitis, specific antibody testing is essential, as MRI findings alone may be insufficient for diagnosis 4, 3, 5

A normal MRI does not exclude encephalitis and should never delay treatment, as approximately 10% of HSV encephalitis cases can have normal initial imaging, and untreated HSV encephalitis carries significant mortality and severe neurological morbidity, according to the Journal of Infection guidelines 6
Initial CT scans are even less sensitive, showing abnormalities in only 25-80% of HSV encephalitis cases, highlighting the importance of prompt treatment regardless of imaging findings 6, 7

The American Academy of Neurology recommends immediate empiric treatment with acyclovir when encephalitis is suspected, regardless of imaging findings, to prevent poor outcomes 6
The treatment protocol involves starting IV aciclovir 10 mg/kg every 8 hours immediately for adults and adolescents, with adjustments for renal impairment and continuation for 14-21 days for confirmed HSV encephalitis, as per standard medical guidelines 6

The American College of Physicians and Infectious Diseases Society recommends MRI as the preferred first imaging test for suspected encephalitis, to be performed as soon as possible, ideally within 24 hours of hospital admission but certainly within 48 hours 8, 9
MRI with diffusion-weighted imaging (DWI) is significantly superior to CT scanning for detecting early cerebral changes in viral encephalitis, showing abnormalities in approximately 90% of patients with HSV encephalitis within 48 hours of admission, compared to only 25% sensitivity with initial CT scans 9, 10

CT scanning is appropriate as the initial imaging modality in specific urgent situations, such as when MRI is not immediately available and imaging cannot be delayed, or before lumbar puncture in patients with contraindications, including focal neurological signs, papilledema, severe immunocompromise, depressed level of consciousness, or new-onset seizures 9, 11
The Infectious Diseases Society recommends CT scanning to exclude structural causes of raised intracranial pressure that would contraindicate lumbar puncture, but notes that CT's sensitivity for diagnosing encephalitis is limited, ranging from 25-80% 9, 11

The Infectious Diseases Society advises that empiric acyclovir treatment should never be delayed while waiting for imaging results, and should be started immediately if encephalitis is suspected clinically, as untreated HSV encephalitis carries significant mortality and severe neurological morbidity 12
A normal MRI does not exclude encephalitis, and treatment decisions should never be delayed based on negative imaging, with approximately 10% of HSV encephalitis cases having normal initial imaging 11