Most Common Neurological Sites of Infection in Herpetic Encephalitis
In HSV-1 encephalitis—the most common cause of sporadic encephalitis—the temporal and inferior frontal lobes are characteristically affected, with bilateral temporal lobe involvement being nearly pathognomonic for the disease. 1
Primary Sites of HSV-1 Infection
The typical neuroanatomical distribution in HSV-1 encephalitis includes:
- Temporal lobes: The medial temporal structures are the hallmark location, with MRI revealing edema and high signal intensity on FLAIR and T2-weighted images 1, 2
- Inferior frontal lobes: These are commonly involved alongside temporal structures 1
- Cingulate gyrus: Early MRI changes frequently occur in this region, visible within 48 hours of hospital admission 1, 2
- Bilateral involvement: When both temporal lobes are affected, this pattern is considered nearly pathognomonic for HSV-1 encephalitis 1, 2
Less Common HSV Presentations
While temporal-frontal involvement dominates, HSV can occasionally present differently:
- Brainstem encephalitis: This is an uncommon but recognized variant, occurring in approximately 29% of cases as isolated brainstem involvement, or 71% as multifocal disease including the brainstem 3, 4
- Brainstem manifestations: When present, patients develop neuro-ophthalmologic findings (81%), cranial nerve deficits (69%), and carry a mortality rate of 41% 3
Clinical Imaging Correlation
MRI findings within 48 hours show approximately 90% sensitivity for detecting these characteristic temporal and frontal lobe abnormalities, compared to only 25% sensitivity with CT scanning. 2
Specific early MRI features include:
- Gyral edema on T1-weighted images in the cingulate gyrus and medial temporal lobes 2
- High signal intensity on T2-weighted and FLAIR sequences in temporal and inferior frontal regions 1, 2
- Diffusion-weighted imaging (DWI) demonstrates the highest sensitivity for early changes 2
Critical Pitfall to Avoid
Do not delay acyclovir treatment while waiting for imaging confirmation of temporal lobe involvement—early treatment is essential regardless of initial imaging results, as delays significantly worsen mortality and morbidity. 2 The characteristic temporal-frontal pattern may not be evident on initial imaging, particularly if CT is used or if MRI is performed very early in the disease course 1, 2.