A 35-year-old previously healthy man presents to the emergency department with a 3-day history of fever, severe headache, and progressive confusion. On examination, he has a temperature of 39.2°C, is disoriented to time and place, and shows focal neurologic deficits with receptive language difficulty. An EEG is performed and shows the pattern marked **B** in the diagram. Which of the following is the most likely diagnosis?
A. Herpes simplex virus-1 encephalitis with predilection for medial temporal lobe involvement
B. Bacterial meningitis with diffuse cortical involvement
C. Hepatic encephalopathy with metabolic background slowing
D. Generalized absence seizure disorder with secondary EEG changes
Explanation
Why Herpes simplex virus-1 encephalitis with predilection for medial temporal lobe involvement is right
The EEG pattern marked B — diffuse slowing with left temporal periodic lateralized epileptiform discharges (PLEDs) — is an early and highly sensitive finding in HSV-1 encephalitis. According to the IDSA Encephalitis Guidelines 2024 and Harrison 21e, HSV-1 is the most common cause of sporadic fatal viral encephalitis in immunocompetent adults, with characteristic predilection for the medial temporal and orbitofrontal lobes. The clinical presentation (fever >90%, altered mental status, focal neurologic deficits including aphasia suggesting dominant temporal involvement, and subacute progression over days) combined with the pathognomonic EEG finding of focal temporal PLEDs occurring every 1–3 seconds makes HSV-1 encephalitis the diagnosis. This patient requires immediate empirical IV acyclovir 10 mg/kg q8h without waiting for CSF PCR or MRI confirmation.
Why each distractor is wrong
Bacterial meningitis with diffuse cortical involvement: While bacterial meningitis presents with fever and altered mental status, the EEG pattern of focal temporal PLEDs is not characteristic. Bacterial meningitis typically produces more diffuse changes without the lateralized periodic discharges seen in HSV encephalitis. Additionally, the focal language deficit and temporal lobe predilection point to viral rather than bacterial etiology.
Generalized absence seizure disorder with secondary EEG changes: Absence seizures present with 3 Hz spike-wave discharges (pattern C in the diagram), not PLEDs. The acute fever, headache, and altered mental status are inconsistent with primary seizure disorder. PLEDs are acquired pathologic findings associated with acute brain injury, not a primary epilepsy syndrome.
Hepatic encephalopathy with metabolic background slowing: Hepatic encephalopathy causes diffuse background slowing but does not produce focal temporal PLEDs or the focal neurologic deficits (aphasia, memory impairment) characteristic of temporal lobe involvement. The acute fever and subacute progression over days are not typical of hepatic encephalopathy, which usually develops in a patient with known liver disease.
High-YieldNEET PG
HSV-1 encephalitis = fever + altered mental status + focal temporal signs + temporal PLEDs on EEG → start acyclovir empirically; mortality untreated ~70%, reduced to <30% with timely treatment.
IDSA Encephalitis Guidelines 2024; Harrison 21e
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