Temporal lobe abnormalities on MRI are most characteristic of which infection?

• A. HSV encephalitis
• B. West Nile virus encephalitis
• C. Varicella-zoster virus encephalitis
• D. La Crosse encephalitis

Answer: (A)

Temporal lobe involvement on MRI is a hallmark of herpes simplex encephalitis due to HSV-1’s strong affinity for the limbic system. The virus preferentially damages the medial and inferior portions of the temporal lobes, often spreading to adjacent structures, and produces edema, hemorrhagic necrosis, and enhancement that show up as T2/FLAIR hyperintensity with possible blood-related susceptibility on MRI. This distinct pattern helps distinguish HSV encephalitis from other viral encephalitides, which tend to affect different brain regions: West Nile virus more commonly targets the thalamus, brainstem, or basal ganglia; La Crosse encephalitis can involve deep structures with a less specific temporal pattern; Varicella-zoster virus encephalitis often presents with multifocal lesions and vasculopathy rather than a predominant temporal lobe involvement. Clinically, HSV encephalitis presents with fever, altered mental status, and seizures, and diagnosis is confirmed by CSF PCR for HSV DNA. Because of its high mortality, start IV acyclovir promptly when HSV encephalitis is suspected, even before PCR results are back.