Herpes simplex encephalitis (HSE) preferentially involves which brain regions, and what is the cornerstone of diagnosis?

• A Basal ganglia and thalami; diagnosed by CT showing hypodense lesions
• B Cerebellum and brainstem; diagnosed by serum anti-HSV IgM titres
• C Parieto-occipital lobes; diagnosed by EEG showing triphasic waves
• D Bilateral temporal lobes and orbitofrontal cortex; diagnosed by CSF HSV PCR ✓

Explanation

HSV-1 encephalitis (the most common cause of sporadic viral encephalitis) characteristically involves the temporal lobes (especially mesial structures: hippocampus and amygdala) and orbitofrontal cortex, reflecting HSV neurotropism via the olfactory nerve route. MRI (FLAIR/DWI) shows asymmetric temporal lobe signal changes. CSF HSV PCR is the gold standard diagnostic test with >95% sensitivity and specificity. IV acyclovir must be started empirically without waiting for PCR results. EEG may show temporal periodic lateralized epileptiform discharges (PLEDs) but is not diagnostic.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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