Recognition & the Two Syndromes
Acute bacterial meningitis is one of the few infections in which an hour's delay measurably changes the outcome. The classic triad of fever, neck stiffness and altered mental status is present in a minority — but nearly every patient has at least one of fever, headache, neck stiffness or altered consciousness. When the picture is dominated by altered behaviour, seizures or focal deficits rather than meningism, think encephalitis, and cover herpes simplex.
Summarised from the Washington Manual of Critical Care — CNS Infections.Meningitis (meninges)
- Fever, headache, neck stiffness/photophobia, altered consciousness
- Kernig / Brudzinski signs (insensitive)
- ± petechial/purpuric rash (meningococcus)
- Bacterial = emergency; viral usually self-limiting
Encephalitis (brain parenchyma)
- Altered behaviour/personality, confusion, ↓GCS
- Seizures & focal deficits (esp. temporal → HSV)
- Meningism may be mild/absent
- HSV is the treatable must-not-miss cause
Commonest pathogens
- Bacterial meningitis (adults): Streptococcus pneumoniae (commonest), Neisseria meningitidis; add Listeria monocytogenes in the elderly, pregnant, neonates and immunocompromised.
- Encephalitis: Herpes simplex virus (HSV-1) is the key treatable cause; also VZV, enteroviruses, and (regionally) arboviruses.
- Consider tuberculous and cryptococcal meningitis (subacute, immunocompromised/HIV).
Do not let investigations delay antibiotics. If bacterial meningitis is suspected, give dexamethasone + empirical antibiotics now — ideally within the first hour and, if a CT is needed before LP, before going to the scanner. Blood cultures and (increasingly) CSF/blood PCR remain useful even after the first dose.
Pathophysiology & Why It Kills
Bacteria reaching the subarachnoid space meet almost no local immune defence, so they multiply freely until the host response arrives. It is that inflammatory response — cytokine release, a breached blood–brain barrier, vasculitis and cerebral oedema — that raises intracranial pressure, chokes cerebral perfusion and drives the neuronal injury. This is why suppressing inflammation with dexamethasone, given with or just before the first antibiotic, improves outcome.
Summarised from Marino PL. The ICU Book, 5th Ed — Infections of the Central Nervous System.- Time to effective antibiotic — every hour of delay increases death and disability. Give the first dose empirically, before culture results.
- Control of the inflammatory response & raised ICP — dexamethasone (pneumococcal disease especially), plus general neuroprotection: treat seizures, manage ICP, maintain cerebral perfusion, and support the airway in the obtunded patient.
Untreated herpes simplex encephalitis has a mortality above 70% and leaves survivors devastated; treated early with aciclovir it is far better. Because you cannot exclude HSV clinically or on early imaging, start IV aciclovir empirically in anyone with suspected encephalitis (fever + altered behaviour/seizures/focal signs) and continue until HSV is excluded by CSF PCR — remembering that a very early PCR can be falsely negative and may need repeating.
Diagnosis & Evidence-Based Care
Viral: clear; lymphocytic, mildly raised protein, normal glucose; HSV/enterovirus PCR.
TB / fungal: lymphocytic, very high protein, low glucose (subacute); India ink/CrAg for cryptococcus, TB PCR/culture.
Always send paired serum glucose, and a CSF sample for PCR (meningococcal/pneumococcal/HSV/enterovirus).
Tuberculous meningitis is common and easily missed — a subacute lymphocytic CSF with very high protein and low glucose, often with basal meningeal enhancement/hydrocephalus on imaging. Start ATT plus adjunctive corticosteroids early when TBM is suspected; do not wait for culture. Cryptococcal meningitis (test CrAg/India ink) and HIV should be considered in the immunocompromised. Regionally, Japanese encephalitis and other arboviral encephalitides are important causes of encephalitis — but still cover HSV with aciclovir until excluded. Meningococcal and pneumococcal vaccination and prompt contact prophylaxis matter for outbreak control.
Empirical & Targeted Therapy
| Drug | Indication | Dose (normal renal function) | Notes |
|---|---|---|---|
| Ceftriaxone | Empirical bacterial meningitis | 2 g IV every 12h | Covers pneumococcus & meningococcus; backbone of empirical therapy |
| Amoxicillin / Ampicillin | Add for Listeria cover | 2 g IV every 4h | Add if >50y, pregnant, neonate, or immunocompromised |
| Vancomycin | Add where resistant pneumococcus likely | Loading + level-guided | For high local penicillin/cephalosporin-resistant S. pneumoniae |
| Dexamethasone | Adjunct (bacterial) | 10 mg IV QDS × 4 days | With/just before first antibiotic; best benefit in pneumococcal disease |
| Aciclovir | Suspected HSV encephalitis | 10 mg/kg IV every 8h (IBW) | Start empirically; ensure hydration (nephrotoxic); continue until HSV excluded |
| Ceftriaxone (contacts) / Ciprofloxacin | Meningococcal prophylaxis | Ciprofloxacin 500 mg PO single dose | Close contacts; alternatives rifampicin/ceftriaxone |
| Anti-tubercular therapy + steroid | Suspected TB meningitis | Standard 4-drug ATT + tapering steroid | Start early on clinical/CSF suspicion in endemic settings |
| Liposomal amphotericin B + flucytosine | Cryptococcal meningitis | Induction per guideline | Immunocompromised/HIV; manage raised ICP with repeated LPs |
Management Algorithm
Suspect & resuscitate
- Fever + headache/neck stiffness/↓consciousness → meningitis; altered behaviour/seizures/focal signs → encephalitis
- ABC, treat shock/seizures; take blood cultures immediately
Give first-hour therapy — do not wait
- Dexamethasone + empirical antibiotics (ceftriaxone ± amoxicillin ± vancomycin) within the first hour
- Add IV aciclovir if encephalitis is possible
- If CT is indicated before LP, give the drugs first
Decide on CT, then LP
- CT first only for red flags (focal deficit, ↓GCS, seizures, immunocompromise, papilloedema)
- LP: cell count/differential, protein, glucose (+ paired serum), Gram/culture, PCR, opening pressure
Refine therapy on results
- Narrow/adjust antibiotics to organism & sensitivities; stop steroid/aciclovir if cause excluded
- Consider TB/cryptococcal/arboviral if lymphocytic CSF or subacute course
ICU support & complications
- Airway/ventilation for ↓GCS; manage raised ICP & seizures; watch for SIADH/hyponatraemia
- Look for complications: hydrocephalus, infarction, abscess, deafness; treat the source (ENT/CSF leak)
- Notify public health; give contact prophylaxis
Common Mistakes in CNS Infection
The commonest fatal error. Give dexamethasone + antibiotics within the hour, before imaging/LP if needed. The CSF and blood PCR still help after the first dose.
Ceftriaxone does not cover Listeria. Add amoxicillin/ampicillin in those >50, pregnant, neonatal or immunocompromised.
Any patient with fever plus altered behaviour, seizures or focal signs should get empirical IV aciclovir until HSV PCR is negative — untreated HSV encephalitis is devastating.
The benefit depends on giving it with or just before the first antibiotic. Starting it hours later adds little; the window is at presentation, not the ward round.
CSF HSV PCR can be falsely negative in the first 24–72h. If clinical suspicion is high, continue aciclovir and repeat the PCR.
A subacute lymphocytic CSF with very high protein and low glucose is TBM until proven otherwise in endemic areas — start ATT + steroid early rather than waiting for culture.
Hunt for a parameningeal focus (otitis, sinusitis, CSF leak, endocarditis) and give meningococcal chemoprophylaxis to close contacts — the case is a public-health event.
Exam Pearls
Q: When must you CT before LP?
Focal neurological deficit, reduced/altered consciousness, new seizures, immunocompromise, papilloedema, or known CNS disease. Otherwise LP without prior CT — and never delay antibiotics for imaging.
Q: Classic bacterial CSF?
Cloudy, neutrophilic pleocytosis, high protein, low CSF:serum glucose (<0.4), high opening pressure, positive Gram/culture/PCR.
Q: Empirical antibiotics for adult bacterial meningitis?
Ceftriaxone 2 g BD; add amoxicillin for Listeria (>50/pregnant/immunocompromised); add vancomycin where resistant pneumococcus is likely.
Q: Dexamethasone — dose, timing and who benefits most?
10 mg IV QDS for 4 days, given with or just before the first antibiotic; greatest benefit (mortality, hearing) in pneumococcal meningitis.
Q: Which encephalitis must you never miss, and how do you treat it?
HSV encephalitis — empirical IV aciclovir 10 mg/kg every 8h; temporal-lobe changes on MRI/EEG support it; confirm with CSF HSV PCR (may need repeating).
Q: CSF pattern of tuberculous meningitis?
Lymphocytic, very high protein, low glucose, subacute onset, often basal enhancement/hydrocephalus — treat with ATT + adjunctive steroid early.
Q: Meningococcal contact prophylaxis?
Ciprofloxacin single dose (or rifampicin/ceftriaxone) to close contacts, plus notification to public health.
Q: Which electrolyte problem complicates meningitis?
Hyponatraemia (SIADH, or cerebral salt wasting) — monitor sodium and manage the cause; avoid rapid correction.
References
- van de Beek D, Cabellos C, Dzupova O, et al. (ESCMID). ESCMID guideline: diagnosis and treatment of acute bacterial meningitis. Clin Microbiol Infect. 2016;22(Suppl 3):S37–S62.
- Tunkel AR, Hartman BJ, Kaplan SL, et al. (IDSA). Practice Guidelines for the Management of Bacterial Meningitis. Clin Infect Dis. 2004;39:1267–1284.
- de Gans J, van de Beek D (European Dexamethasone Study). Dexamethasone in adults with bacterial meningitis. N Engl J Med. 2002;347:1549–1556.
- Brouwer MC, McIntyre P, Prasad K, van de Beek D. Corticosteroids for acute bacterial meningitis. Cochrane Database Syst Rev. 2015;(9):CD004405.
- Tunkel AR, Glaser CA, Bloch KC, et al. (IDSA). The Management of Encephalitis. Clin Infect Dis. 2008;47:303–327.
- Solomon T, Michael BD, Smith PE, et al. Management of suspected viral encephalitis in adults — Joint Specialist Societies guideline. J Infect. 2012;64:347–373.
- Marino PL. The ICU Book, 5th Edition. Infections of the Central Nervous System. Wolters Kluwer; 2025.
- Washington Manual of Critical Care, 4th Edition. Kollef MH, Witt CA (eds). CNS Infections. Wolters Kluwer; 2023.