CNS Infections — Written Clinical Reasoning

Model answer — Part A: suspected bacterial meningitis

Sequence is the answer. State it, then justify it: blood cultures, immediate IV ceftriaxone plus vancomycin (per local protocol), dexamethasone 10 mg with the first dose, then lumbar puncture — no CT, because none of the Hasbun criteria apply (no immunocompromise, no recent seizure, no papilloedema, normal consciousness, no focal deficit) [1] [2] [3].

Justify each step in one line: cultures before antibiotics so the organism survives to be named; antibiotics immediately because every hour of delay worsens outcome and imaging must never precede them [1]; dexamethasone with or before the first dose because that is the timing the trial evidence supports, with benefit concentrated in pneumococcal disease — stopped later if the diagnosis is not bacterial [2]; LP without CT because Hasbun showed that patients with none of the six risk features can be punctured safely, and a normal CT would not have excluded raised pressure anyway [3].

Then the CSF reasoning: expect a bacterial pattern (neutrophils, high protein, low glucose) and send cell count with differential, protein, paired glucose, Gram stain and culture, plus meningococcal/pneumococcal PCR given he is a student — and state that meningococcal disease is notifiable with chemoprophylaxis for close contacts if confirmed [1].

Close with monitoring: watch for deterioration and cerebral oedema, review antibiotics against cultures at 48 hours, and arrange audiology follow-up for post-meningitic hearing loss [1].

Model answer — Part B: suspected HSV encephalitis

Name the syndrome first: fever with personality change and seizure is encephalitis, not meningitis — the early encephalopathic features define it. The treatable cause is HSV, so aciclovir 10 mg/kg IV three times daily starts now, before any result returns; the vidarabine-versus-aciclovir trial established it as life-saving and delay costs cortex [4].

Diagnostics in parallel: MRI brain (temporal FLAIR hyperintensity supports HSV; may be normal in the first 48 hours), EEG if seizures are subtle, and LP for cell count, protein, glucose and HSV PCR — sent together with autoimmune encephalitis antibodies, because the subacute psychiatric phenotype overlaps with anti-NMDAR encephalitis [4].

Management detail that scores: renal-adjust the aciclovir, continue for 14–21 days, and if the early PCR is negative but suspicion remains high, repeat the PCR rather than stopping — premature cessation relapses. Supportive care includes seizure control and cerebral-oedema vigilance; steroids have no role in HSV encephalitis itself [4].

Finish with the differential statement: if the course is subacute (weeks) with movement disorder or autonomic features, autoimmune encephalitis — classically anti-NMDAR — becomes the lead diagnosis, and tumour screening (ovarian teratoma) is part of treatment, not just diagnosis [5].