Neurology · Topic 16 of 22
Bacterial Meningitis
Neurology · CNS infection
Bacterial Meningitis
A neurologic emergency: culture blood, then give dexamethasone and empiric antibiotics at once — never wait for the CT or the lumbar puncture.
Key numbers
Start antibiotics before CT or LP≤ 30 min
Ceftriaxone plus vancomycin for adults 18–50ceftriaxone 2 g q12h
Dexamethasone 10 mg IV with first antibiotic10 mg q6h × 4 days
Bacterial CSF: neutrophils, high protein, low glucoseCSF:blood glucose < 0.5
Add ampicillin for Listeria after age 50age > 50
01 · FirstStart antibiotics before CT or LP
Suspected bacterial meningitis is an emergency: draw blood cultures, then give antibiotics (with dexamethasone) within 30 minutes of presentation — before neuroimaging or lumbar puncture; confirmation must never delay treatment.
02 · GiveCeftriaxone plus vancomycin for adults 18–50
Empiric therapy for community-acquired meningitis in adults 18–50: ceftriaxone 2 g IV every 12 hours (or cefotaxime) plus vancomycin 30–45 mg/kg/day IV divided every 8–12 hours, covering S. pneumoniae (including resistant strains) and N. meningitidis.
03 · GiveDexamethasone 10 mg IV with first antibiotic
Give dexamethasone 10 mg IV (0.15 mg/kg) 10–20 minutes before or with the first antibiotic dose, then every 6 hours for 4 days; do not start it once antibiotics have already been given.
01
Causes & risk factors
Why it happensS. pneumoniae most common in adultsS. pneumoniae is the leading adult cause; N. meningitidis affects adolescents and young adults; Listeria rises in older adults, pregnancy and impaired cell-mediated immunity; S. aureus follows penetrating head wounds.
02
Presentation
What you'll seeFever, stiff neck, altered mental statusClassic findings are fever, headache, nuchal rigidity and altered mental status; older, immunocompromised and alcohol-dependent patients may show only new confusion, so keep a low threshold for lumbar puncture.
Petechial or purpuric rash: meningococcemiaA petechial or purpuric rash suggests meningococcemia, which can kill within hours and cause DIC and bilateral adrenal hemorrhage (Waterhouse-Friderichsen syndrome, treated with hydrocortisone).
03
Diagnosis & monitoring
Work-upBacterial CSF: neutrophils, high protein, low glucose must knowBacterial CSF shows high opening pressure, turbid fluid, a neutrophil-predominant pleocytosis, protein usually 100–500 mg/dL and glucose under 50% of blood glucose; CSF glucose ≤ 18 mg/dL or a ratio under 0.23 strongly suggests bacteria.
04
Do right now
In this orderFirst moves
1
→Start antibiotics before CT or LP≤ 30 min
2
→Dexamethasone 10 mg IV with first antibiotic10 mg q6h × 4 days
3
→Ceftriaxone plus vancomycin for adults 18–50ceftriaxone 2 g q12h
4
CT before LP: focal signs, papilledema, immunocompromise
Why each step
Start antibiotics before CT or LPSuspected bacterial meningitis is an emergency: draw blood cultures, then give antibiotics (with dexamethasone) within 30 minutes of presentation — before neuroimaging or lumbar puncture; confirmation must never delay treatment.
Dexamethasone 10 mg IV with first antibioticGive dexamethasone 10 mg IV (0.15 mg/kg) 10–20 minutes before or with the first antibiotic dose, then every 6 hours for 4 days; do not start it once antibiotics have already been given.
Ceftriaxone plus vancomycin for adults 18–50Empiric therapy for community-acquired meningitis in adults 18–50: ceftriaxone 2 g IV every 12 hours (or cefotaxime) plus vancomycin 30–45 mg/kg/day IV divided every 8–12 hours, covering S. pneumoniae (including resistant strains) and N. meningitidis.
CT before LP: focal signs, papilledema, immunocompromiseImage before lumbar puncture for immunocompromise, known CNS disease (mass, stroke, focal infection), seizure within 1 week, papilledema, abnormal consciousness or focal deficit — but draw cultures and give dexamethasone and antibiotics first.
05
Management
OrdersAdd ampicillin for Listeria after age 50Add ampicillin (2–3 g IV every 4 hours) to ceftriaxone plus vancomycin when Listeria is possible: age over 50, impaired cell-mediated immunity (advanced HIV, transplant, immunosuppressants) or pregnancy.
Post-neurosurgery: vancomycin plus antipseudomonal beta-lactamMeningitis after neurosurgery, a shunt or a penetrating head wound needs vancomycin plus an antipseudomonal beta-lactam (cefepime, ceftazidime or meropenem) chosen by local susceptibility.
Add acyclovir until HSV is excludedHerpes simplex encephalitis can mimic early bacterial meningitis, so add empiric IV acyclovir to the antibiotics until CSF HSV PCR excludes it.
Droplet precautions for the first 24 hoursKeep the patient on droplet precautions (mask, gown, gloves) for the first 24 hours of effective antibiotic therapy.
Prophylaxis for close meningococcal contactsGive close contacts of meningococcal meningitis (household, day care, staff exposed to oral secretions) rifampin 600 mg every 12 hours for 4 doses, ceftriaxone 250 mg IM once, or ciprofloxacin 500 mg orally once.
06
Red flags & don'ts
Avoid harmWatch for herniation, seizures and SIADH complicationComplications include raised ICP and herniation (the main cause of early death), seizures, hydrocephalus, infarcts, hearing loss, subdural empyema, septic shock, DIC and hyponatremia from SIADH.
07
The rules
Know theseDexamethasone benefit proven for pneumococcal meningitisAdjunctive dexamethasone is recommended for suspected or proven pneumococcal meningitis in adults, even with a resistant isolate; evidence of benefit for other organisms in adults is insufficient.
Treat 7 days meningococcus, 10–14 pneumococcusUsual IV duration: N. meningitidis or H. influenzae 7 days, S. pneumoniae 10–14 days, aerobic gram-negative bacilli 21 days, Listeria 21 days or longer.
08
Bacterial or viral meningitis CSF?
Commonly confused| Predominant cells | Neutrophils, often greatly increased |
|---|---|
| Protein | Elevated, usually 100–500 mg/dL |
| Glucose | Low — under 50% of blood glucose |
| Key tests | Gram stain, culture, multiplex PCR |
09
More detail
ReferenceShow 7 reference facts
CSF obtained within about 4 hours of the first antibiotic dose still usually grows the organism; draw a simultaneous blood glucose for the CSF:blood ratio and get the cell count within 1 hour.
CSF Gram stain identifies the organism in 60–90% of cases; multiplex PCR panels add speed, especially after antibiotics, but supplement rather than replace culture and susceptibilities.
For raised ICP or impending herniation: head of bed at 30°, IV mannitol, and brief hyperventilation to PCO₂ 27–30 mmHg for no more than 24 hours, weaned gradually.
Repeat CSF analysis when a patient has not improved after 48 hours of appropriate therapy, particularly with resistant pneumococcus or after adjunctive dexamethasone.
Narrow therapy to culture results: penicillin G or ampicillin for susceptible strains; vancomycin with or without rifampin for S. aureus (nafcillin or oxacillin when methicillin-susceptible); TMP-SMX for Listeria with penicillin allergy.
Adult mortality is about 21% despite treatment; poor prognosis follows age over 60, low GCS, focal deficits, seizures, high CSF pressure, low CSF cell count and a low CSF:blood glucose ratio.
Routine Hib, pneumococcal and meningococcal vaccination prevents meningitis; MenACWY is given at age 11–12 with a booster at 16, and MenB is added for high-risk groups.
Cultures, steroid, antibiotics — then the scan and the tap. Imaging never comes before the first antibiotic dose.
Sources · Merck Manual Professional — Acute Bacterial Meningitis (full review Sept 2026) · Smith L. Management of Bacterial Meningitis: New Guidelines from the IDSA (summary of Tunkel AR et al. IDSA practice guidelines, Clin Infect Dis 2004;39:1267–84). Am Fam Physician 2005;71:2003–8 · Tunkel AR et al. 2017 IDSA Clinical Practice Guidelines for Healthcare-Associated Ventriculitis and Meningitis. Clin Infect Dis 2017;64:e34–e65 (as cited by Merck) · Tunkel AR et al. Practice guidelines for the management of bacterial meningitis (IDSA). Clin Infect Dis 2004;39:1267–84 · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).