A 25-year-old immunocompetent man develops progressive encephalopathy with personality change over weeks. CSF shows mild lymphocytosis. Brain biopsy shows widespread neuronal vacuolation with intranuclear eosinophilic Cowdry type A inclusions in cortical neurons. Electron microscopy reveals enveloped icosahedral virions. PCR of CSF is positive for herpes simplex virus type 1. The characteristic pattern of herpes encephalitis involving which specific brain regions differentiates it from other encephalitides?
- A Periventricular white matter (periventricular distribution)
- B Temporal lobes and inferior frontal lobes (limbic structures) ✓
- C Cerebellum and brainstem (rhombencephalitis pattern)
- D Basal ganglia and thalami (deep gray matter)
Explanation
Herpes simplex encephalitis (HSE type 1) characteristically involves the temporal lobes (particularly medial temporal lobe including the hippocampus, amygdala, parahippocampal gyrus) and inferior frontal lobes (orbitofrontal cortex), which together constitute limbic structures. This distribution reflects retrograde viral spread from the trigeminal ganglion via the olfactory tracts or along the trigeminal nerve branches to the base of the temporal lobe. MRI shows asymmetric T2/FLAIR signal in these regions. Periventricular distribution is classic for CMV or PML (JC virus); rhombencephalitis is caused by Listeria monocytogenes or HSV2; basal ganglia involvement characterizes Japanese encephalitis virus.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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