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Acute Bacterial Meningitis

CNS

Source: IDMP View on spectrum chart →
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Annotations only — chart still shows the full chemistry of each drug.

1. Clinical Syndrome

Acute (<24–48 h) bacterial infection of the meninges in adults: fever + headache + neck stiffness + altered mental status (classic triad ~50% sensitivity). CSF shows neutrophilic pleocytosis (>500–1000 WBC, >80% PMN), low glucose (<40 mg/dL or CSF:serum ratio <0.4), elevated protein (>100 mg/dL), ± positive Gram stain.

Excludes: viral / aseptic meningitis (no empiric abx beyond rule-out), chronic meningitis (TB, fungal — separate workup), nosocomial post-neurosurgical meningitis (different empirics), brain abscess (separate, longer course).

2. Pathogens

Consider the patient: Age >50 (Listeria coverage required), immunocompromise (Listeria, Cryptococcus), pregnancy (Listeria, GBS), unvaccinated (H. influenzae, S. pneumoniae, N. meningitidis), recent neurosurgery / CSF leak (S. aureus, GNR), splenectomy / complement deficiency (encapsulated organisms — N. meningitidis especially).

Consider the case: Classic triad sensitivity poor — have low threshold for LP. Petechial / purpuric rash → think N. meningitidis. Don't delay antibiotics for LP if delay >30 min — give empiric + dexamethasone, then LP, then adjust per Gram stain / culture.

Common

Less common

3. Empiric Therapy

TierFirst choiceAlternativesDurationComments
Admitted to ward
  • 2 g · q12h · IV · 7 days N. meningitidis; 10–14 days S. pneumo
  • 15–20 mg/kg · q8–12h (target trough 15–20 / AUC 400–600 for CNS) · IV · until PCN-resistant pneumo excluded; otherwise 10–14 days
  • Dexamethasone 0.15 mg/kg (~10 mg) · q6h × 4 days · IV · 4 days

    Start with or just before 1st antibiotic dose. De Gans NEJM 2002 — mortality benefit in pneumococcal meningitis. Discontinue if not pneumococcus.

—S. pneumo 10–14 d; N. meningitidis 7 d; Listeria ≥21 dEmpiric for community-acquired adult ≤50 y, no immunocompromise, no Listeria risk. Add ampicillin if any Listeria risk factor.
Admitted to ICU
  • 2 g · q12h · IV
  • 15–20 mg/kg · AUC-guided q8–12h · IV
  • 2 g · q4h · IV

    Add for Listeria coverage if age >50, immunocompromised, alcohol use, or pregnancy.

  • Dexamethasone 0.15 mg/kg · q6h × 4 days · IV
—Listeria ≥21 d; S. pneumo 10–14 d; N. meningitidis 7 dSeverely ill OR Listeria risk. Don't delay antibiotics for imaging/LP. Imaging before LP only if focal deficits, AMS, papilledema, immunocompromise, history of CNS disease, or seizure.

Add coverage if:

Anaerobe coverage
  • Age >50
  • Immunocompromise (HIV, transplant, chemotherapy, steroids)
  • Pregnancy
  • Alcohol use disorder

Add:

  • 2 g · q4h · IV

    Listeria coverage. NOT covered by ceftriaxone (intrinsic ceph resistance).

Anaerobe label is the schema's category but the actual organism is Listeria monocytogenes (gram-positive rod). Ampicillin is mandatory in this risk group.

4. Directed Therapy

Once organism + susceptibilities back:

  • Pneumococcus PCN-S: penicillin G 4 MU IV q4h or continue ceftriaxone
  • Pneumococcus PCN-resistant: ceftriaxone + vancomycin until clinical response
  • N. meningitidis: ceftriaxone monotherapy 7 days; chemoprophylaxis for close contacts (ceftriaxone 250 mg IM × 1, ciprofloxacin 500 mg PO × 1, or rifampin 600 mg PO BID × 2 days)
  • Listeria: ampicillin ± gentamicin (synergy debated) ≥21 days; TMP-SMX if true PCN allergy
  • MRSA / GNR (post-neurosurgical): per susceptibilities; ID consult

5. Monitoring

Resolution: repeat LP only if no clinical improvement at 48 h, resistant pneumococcus, or Listeria. Watch for SIADH, seizures, raised ICP, hearing loss (audiology consult on discharge for pneumococcal meningitis).

Toxicity: vancomycin AUC for CNS target; dexamethasone-related (hyperglycemia, GI bleed prophylaxis).

Pearls

Don't delay antibiotics for LP — give empiric within 30 minutes of suspicion + dexamethasone (must precede or accompany first abx dose for steroid benefit). Imaging before LP only with: focal deficit, AMS, papilledema, immunocompromise, prior CNS disease, seizure. Ceftriaxone alone is inadequate for Listeria (intrinsic resistance). Audiology consult for pneumococcal meningitis (sensorineural hearing loss is the most common neurologic sequela).

References