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 timing 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].