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Suspected herpes simplex encephalitis with early negative CSF PCR — MRCEM SBA MCQ

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HardNeurological emergenciesSuspected herpes simplex encephalitis with early negative CSF PCRMRCEM SBA

A 44-year-old man is brought to the emergency department after 24 hours of fever, headache and increasingly disinhibited behaviour. He has a focal seizure that stops spontaneously, but remains confused. Capillary glucose is 5.7 mmol/L. MRI shows left mesial temporal lobe signal change. Lumbar puncture, performed 18 hours after symptom onset, shows 8 white cells/µL, predominantly lymphocytes, with a normal CSF:serum glucose ratio. CSF herpes simplex virus (HSV) PCR is negative. Intravenous aciclovir was started on arrival; he is haemodynamically stable and has had no further seizures. What is the most appropriate next management plan?

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Correct answer: D — Continue intravenous aciclovir; repeat lumbar puncture for HSV PCR after 48 hours as an inpatient.

Fever, altered behaviour and a focal seizure indicate encephalitis rather than an isolated seizure disorder. Mesial temporal MRI changes make HSV encephalitis a particular concern. The lumbar puncture was performed only 18 hours after symptom onset: an early negative HSV PCR does not reliably exclude the diagnosis, even when the CSF white-cell count is low. He therefore needs continued intravenous aciclovir, inpatient assessment and a repeat lumbar puncture for HSV PCR after approximately 48 hours. Specialist infection and neurology input should guide subsequent treatment. ([dbth.nhs.uk](https://www.dbth.nhs.uk/wp-content/uploads/2025/11/CNS-infections-2025_FINAL.pdf)) A delays reassessment despite an early potentially false-negative result; end-of-treatment CSF testing serves a different purpose in confirmed disease. B obtains the appropriate repeat test but leaves suspected HSV encephalitis untreated meanwhile. C retests material collected at the same early point and does not address the timing-related limitation. E retains the repeat-testing plan but substitutes oral treatment for the recommended intravenous aciclovir in suspected encephalitis. ([dbth.nhs.uk](https://www.dbth.nhs.uk/wp-content/uploads/2025/11/CNS-infections-2025_FINAL.pdf))

Reference: Policy for Treatment of Central Nervous System Infections in Adults — Encephalitis (October 2025) — https://www.dbth.nhs.uk/wp-content/uploads/2025/11/CNS-infections-2025_FINAL.pdf Suspected encephalitis in adults (29 April 2026) — https://pn.bmj.com/content/early/2026/04/29/pn-2024-004299