A 65-year-old immunocompetent patient presents with acute bacterial meningitis. CSF shows: glucose 28 mg/dL (serum glucose 90), protein 380 mg/dL, cells 4200 (90% polymorphs). Gram stain shows Gram-positive diplococci. What is the correct empirical antibiotic regimen AND the rationale for adding dexamethasone?
- A Penicillin G + dexamethasone; vancomycin not needed as S. pneumoniae is always penicillin-susceptible
- B Ceftriaxone + vancomycin + dexamethasone 0.15 mg/kg IV every 6 hours; dexamethasone reduces CSF inflammation and decreases TNF/IL-1, lowering risk of hearing loss and neurological sequelae ✓
- C Meropenem alone; broadest coverage without need for steroids
- D Ceftriaxone alone; dexamethasone contraindicated as it reduces antibiotic CSF penetration
Explanation
Gram-positive diplococci in adult bacterial meningitis indicate S. pneumoniae. Empirical treatment must cover penicillin-resistant S. pneumoniae (PRSP): ceftriaxone 2 g every 12 hours + vancomycin 15–20 mg/kg every 8–12 hours until susceptibility is confirmed. Dexamethasone 0.15 mg/kg every 6 hours for 4 days, started 15–30 minutes BEFORE first antibiotic dose, is recommended based on evidence (de Gans & van de Beek, NEJM 2002) — it reduces TNF-alpha and IL-1 mediated BBB inflammation, significantly reducing hearing loss and neurological sequelae in S. pneumoniae meningitis. Dexamethasone should be stopped if the organism is NOT S. pneumoniae or H. influenzae.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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