What are the key imaging features and clinical implications of adult herpes simplex virus (HSV) encephalitis on CT and MRI?

Adult HSV encephalitis may cause encephalitis or cranial neuritis with a 70% mortality rate if untreated. CT may show ill-defined hypodensities in the temporal lobes, often with swelling and mass effect, but findings typically appear after the fifth day of symptoms. MRI is more sensitive, showing increased FLAIR and T2 signal in the temporal and/or frontal lobes with sparing of the putamen. The virus is usually latent in the gasserian ganglion. Clinically, insular cortex involvement is common, but the adjacent putamen is spared, distinguishing it from a middle cerebral artery infarct, which often involves the putamen. Early recognition and treatment with antivirals are crucial to reduce morbidity and mortality.