Why HSV Encephalitis Predominantly Involves the Temporal Lobes
The Anatomical Predilection
HSV encephalitis classically involves the medial temporal lobes, insula, and cingulate gyri because the virus demonstrates neurotropism for these specific limbic structures, with the basal ganglia and thalami nearly always spared. 1 This characteristic pattern is so consistent that it serves as a key diagnostic feature on neuroimaging studies.
Imaging Manifestations of Temporal Lobe Involvement
The temporal lobe predilection manifests distinctly on different imaging modalities:
MRI findings show early changes in the cingulate gyrus and medial temporal lobe, appearing as gyral edema on T1-weighted images and high signal intensity on T2-weighted and FLAIR sequences, with approximately 90% sensitivity within 48 hours of hospital admission 2
FDG-PET demonstrates regions of hypermetabolism most frequently in the medial temporal lobes in HSV encephalitis, which may reflect seizure activity 3
The temporal lobe involvement is so characteristic that it helps distinguish HSV encephalitis from other viral encephalitides and non-infectious conditions 3
Clinical Consequences of Temporal Involvement
The temporal lobe localization produces specific clinical manifestations:
Temporal lobe spike-and-wave activity and periodic lateralized epileptiform discharges (PLEDs) occur in many cases, reflecting the temporal focus of infection 3
Temporal lobe seizures are relatively frequent as a direct consequence of this anatomical distribution 3
Memory deficits and neuropsychiatric sequelae occur in 20-60% of survivors, reflecting damage to hippocampal and limbic structures 3
Pathophysiological Basis
The mechanism underlying this predilection involves:
Autopsy studies reveal hippocampal disease with HHV-6 protein in astrocytes and neurons, suggesting local virus reactivation in these specific anatomical structures 3
The virus demonstrates specific involvement of areas such as the entorhinal cortex and amygdala, which are integral components of the temporal lobe limbic system 3
Important Caveats
While temporal involvement is the hallmark, clinicians should recognize important exceptions:
Atypical presentations can occur: HSV encephalitis may rarely involve the brainstem (29% limited to brainstem, 71% multifocal including brainstem in one review) 4, or even the thalamus, though thalamic involvement is extremely rare 1
The characteristic temporal pattern helps distinguish HSV from other pathogens: Japanese B encephalitis typically involves thalamus and basal ganglia, while enterovirus may predominantly affect the brainstem 3
Early imaging may be normal: CT scanning has only 25% sensitivity for initial HSE diagnosis, and even MRI may be negative at disease onset 3, 2