Diagnosis: Viral Encephalitis (HSV-1 Most Likely)
The diagnosis is viral encephalitis, specifically herpes simplex virus type 1 (HSV-1) encephalitis, based on the classic triad of fever, new-onset tonic-clonic seizure, and bilateral temporal lobe hyperintensities on MRI following a prodromal upper respiratory infection. 1, 2
Clinical Reasoning
Why HSV-1 Encephalitis is the Answer
HSV-1 accounts for 25-40% of all sporadic encephalitis cases worldwide and is the most common identified cause of nonendemic sporadic encephalitis 1, 3, 4
Bilateral temporal lobe involvement on MRI is nearly pathognomonic for HSV encephalitis, occurring in >90% of laboratory-confirmed cases 1, 2, 3
The classic triad of fever, seizures, and temporal lobe abnormalities strongly suggests HSV-1 encephalitis 2
A preceding upper respiratory infection is frequently reported in patients who later develop HSV-1 encephalitis 2
CSF showing isolated elevated protein with normal glucose is consistent with viral encephalitis, where CSF typically reveals mild-to-moderate protein elevation with mononuclear pleocytosis 1
Why NOT Multiple Sclerosis (Option B)
MS does not present with acute high fever and tonic-clonic seizures—these features are not typical of MS 2
MS typically shows periventricular white matter lesions, not acute bilateral temporal lobe hyperintensities 1
MS has a relapsing-remitting course rather than acute encephalitic presentation with systemic illness 1
Why NOT Primary Temporal Lobe Epilepsy (Option C)
Primary temporal lobe epilepsy does not produce fever or acute systemic illness 2
Epilepsy shows chronic structural lesions (hippocampal sclerosis, gliosis) rather than acute hyperintensities on MRI 2
The acute presentation following URI with high fever indicates an infectious/inflammatory process, not a primary seizure disorder 1, 2
Immediate Management Algorithm
Step 1: Initiate Empiric Antiviral Therapy Immediately
Start intravenous acyclovir 10 mg/kg every 8 hours for adults immediately without awaiting PCR confirmation 1, 2
Treatment should not be delayed by serological confirmation—early acyclovir dramatically decreases morbidity and mortality by 50% 3, 4
Adjust dose for renal impairment; continue for minimum 14-21 days 2, 5
Step 2: Obtain Diagnostic Confirmation
Perform lumbar puncture for CSF analysis including HSV PCR (sensitivity 96-98%, specificity 95-99% in adults) 1, 6
CSF PCR is the diagnostic gold standard and should be obtained between day 2-10 of illness for optimal sensitivity 6
Send CSF for cell count, protein, glucose, and bacterial culture 1
Step 3: Additional Diagnostic Testing
Throat and rectal swabs for enterovirus PCR should be obtained 1
EEG may show periodic lateralizing epileptiform discharges in temporal regions in 80% of HSV encephalitis cases 1
Serum acute and convalescent samples for viral serology 1
Critical Pitfalls to Avoid
Never delay acyclovir while awaiting imaging or laboratory confirmation—this increases mortality and morbidity 2, 4
Do not rely on a single negative CSF PCR to rule out HSE, as false negatives can occur within the first 72 hours of illness 6, 3
Hemoglobin or other PCR inhibitors in CSF can cause false-negative results 6
If initial CSF PCR is negative but clinical suspicion remains high, repeat CSF PCR 3-7 days later 6
Continue acyclovir despite negative PCR when temporal lobe abnormalities on MRI or characteristic clinical presentation strongly suggests HSV encephalitis 2, 6
Prognosis and Follow-up
CSF HSV PCR should become negative after 14 days of acyclovir treatment; persistent positive PCR should prompt consideration of additional or revised antiviral therapy 3
Follow-up MRI is recommended to evaluate evolving necrosis or demyelination and assess treatment response 2
Intracranial hemorrhage is a rare complication (2.7%) but requires close neurological monitoring 4