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Neurology · Neuro

CNS Infections: Meningitis & Encephalitis

A boards-focused walkthrough of CNS infection: distinguishing meningitis from encephalitis, reading the CSF, and driving the emergency next-best-step decisions — when to CT before LP, age-based empiric antibiotics plus dexamethasone, and empiric acyclovir for HSV encephalitis — anchored to classic vignette buzzwords (Waterhouse–Friderichsen, temporal-lobe HSV, Listeria, Cryptococcus).

10 min readHigh yield

Meninges vs Parenchyma

CNS infection is classified by the tissue inflamed. Meningitis = inflammation of the leptomeninges and CSF → the meningeal triad of fever, nuchal rigidity, and headache with photophobia; higher cortical function is largely preserved. Encephalitis = inflammation of the brain parenchyma → altered mental status, focal deficits, seizures, and personality change. Overlap is meningoencephalitis.

Pathogens reach the CNS by:

  • Hematogenous spread — the usual route: nasopharyngeal colonization → bacteremia → seeding of the choroid plexus.
  • Direct extension — otitis media, sinusitis, mastoiditis.
  • Retrograde axonal transport — HSV, rabies.

Board framing:

  • Bacterial meningitis — an emergency; acute, toxic, neutrophil-predominant CSF.
  • Viral ('aseptic') meningitis — usually benign/self-limited; enteroviruses are #1.
  • Encephalitis — in the US most often HSV-1, which is treatable, so empiric acyclovir covers anyone with encephalitis until HSV is excluded.

The full triad (fever + stiff neck + AMS) appears in a minority (~44%), but ~95% have at least two of headache, fever, neck stiffness, or AMS.

Must-Know Facts
  • Antibiotics can't wait — give within ~1 hour of suspicion; never delay abx for CT or LP
  • Add dexamethasone just before/with the first antibiotic dose (↓ mortality in pneumococcal adults, ↓ hearing loss in H. influenzae children)
  • Empiric backbone (1 mo–50 y): vancomycin + ceftriaxone; add ampicillin (for Listeria) if >50 y, pregnant, or immunocompromised
  • Neonates (<1 mo): ampicillin + cefotaxime — avoid ceftriaxone (kernicterus)
  • CT head before LP only if: immunocompromised, known CNS disease, new-onset seizure, papilledema, focal deficit, or ↓consciousness — otherwise LP first
  • S. pneumoniae = most common bacterial cause in adults; N. meningitidis → petechial/purpuric rash, dorms/military recruits
  • Terminal complement (C5–C9 / MAC) deficiency → recurrent Neisseria
  • HSV-1 encephalitis → temporal lobe; start IV acyclovir empirically, confirm with CSF HSV PCR
  • Cryptococcus (HIV, CD4 <100) → CrAg (most sensitive) / India ink, high opening pressure needing serial LPs

CSF Profiles by Cause

CSF parameterBacterialViral (aseptic)Fungal / TB
Opening pressure↑↑Normal / mild ↑↑↑
Predominant cellNeutrophils (PMNs), 100s–1000sLymphocytesLymphocytes
Protein↑↑Normal / mild ↑↑↑
Glucose↓↓ (CSF:serum <0.4)Normal↓
Classic clueGram stain +, turbid fluidEnteroviruses; clearIndia ink / AFB; TB "cobweb" clot
Gram stain of cerebrospinal fluid at 1000x showing gram-negative diplococci of Neisseria meningitidis among neutrophils.
CSF Gram stain in meningococcal meningitis — gram-negative diplococci, often intracellular within neutrophils; a positive Gram stain supports a presumptive diagnosis. · Wikimedia Commons — Microman12345 — CC BY-SA 4.0, via Wikimedia Commons
Vignette → Dx → Next Step
  • 19-y-old dorm resident: fever, headache, stiff neck, petechiae → purpura, hypotension. → Meningococcemia ± Waterhouse–Friderichsen (adrenal hemorrhage). Next step: blood cultures + dexamethasone + vancomycin/ceftriaxone now, LP after; droplet precautions; close contacts get rifampin / ciprofloxacin / ceftriaxone prophylaxis.
  • 68-y-old: fever, confusion, aphasia, focal seizure; MRI temporal-lobe edema, CSF lymphocytes with RBCs. → HSV-1 encephalitis → IV acyclovir immediately; confirm with CSF HSV PCR (don't wait for it).
  • Neonate: poor feeding, lethargy, bulging fontanelle; mother ate unpasteurized cheese. → GBS / E. coli / Listeria → ampicillin + cefotaxime (avoid ceftriaxone → kernicterus).
  • AIDS, CD4 40: subacute headache, very high opening pressure, India-ink yeast / CrAg+. → Cryptococcus → amphotericin B + flucytosine (induction); serial LPs to control pressure.

Management & Next-Best-Step

Suspected bacterial meningitis is a 'treat-first' emergency — the highest-yield rule is that neither LP nor CT may delay antibiotics.

  1. Draw blood cultures immediately.
  2. Does the patient need CT before LP? Yes if immunocompromised, known CNS disease, new-onset seizure, papilledema, focal deficit, or reduced consciousness (herniation risk).
  3. If CT is indicated: give dexamethasone + empiric antibiotics first, then CT, then LP. If not: LP first, then dexamethasone + antibiotics.
  4. Dose empiric antibiotics by age/host (next table); give dexamethasone 10 mg IV with or just before the first dose.
  5. De-escalate on Gram stain/culture/PCR; in adults continue dexamethasone only if S. pneumoniae is confirmed.

For encephalitis, start IV acyclovir the moment HSV is on the differential — untreated HSV encephalitis kills ~70%, and acyclovir works best before coma. CSF HSV PCR is the diagnostic test; MRI shows temporal-lobe involvement; EEG may show temporal periodic discharges (PLEDs/LPDs).

Coronal T2-weighted brain MRI showing high signal in the medial temporal lobes and hippocampi in herpes simplex encephalitis.
HSV-1 encephalitis: T2 hyperintensity of the medial temporal lobes/hippocampi — the classic MRI pattern that should trigger empiric IV acyclovir. · Wikimedia Commons — dr Laughlin Dawes — CC BY 3.0, via Wikimedia Commons

Empiric Antibiotics by Age / Host

PopulationLikely organismsEmpiric therapy
Neonate (<1 mo)GBS (S. agalactiae), E. coli, ListeriaAmpicillin + cefotaxime (avoid ceftriaxone)
1 mo–50 yS. pneumoniae, N. meningitidis (Hib if unvaccinated)Vancomycin + ceftriaxone
>50 y / pregnant / immunocompromisedAbove + ListeriaVancomycin + ceftriaxone + ampicillin
Post-neurosurgery / trauma / shuntStaph (incl. MRSA), gram-negatives (Pseudomonas)Vancomycin + cefepime (or meropenem)
Please SHiNE my SKiS — Encapsulated Bugs

"Please SHiNE my SKiS" — the encapsulated bacteria: the classic meningitis pathogens and the organisms that overwhelm asplenic patients.

  • Please — Pseudomonas aeruginosa
  • S — *Streptococcus pneumoniae* (#1 adult meningitis)
  • H — Haemophilus influenzae type b
  • N — *Neisseria meningitidis*
  • E — Escherichia coli (neonatal)
  • S — Salmonella
  • K — Klebsiella pneumoniae
  • S — group B *Streptococcus* (neonatal)

The polysaccharide capsule resists phagocytosis; the spleen clears these organisms, so asplenia / sickle-cell → fulminant infection. Their conjugate vaccines (pneumococcal, Hib, meningococcal) link the capsular polysaccharide to a protein carrier to recruit T-cell help and protect infants.

Classic Vignette Buzzwords
  • Petechial/purpuric rash + shock + adrenal hemorrhage → N. meningitidis (Waterhouse–Friderichsen)
  • Recurrent Neisseria → terminal complement (C5–C9 / MAC) deficiency
  • After otitis media/sinusitis/pneumonia, or in asplenics → S. pneumoniae
  • Deli meats/unpasteurized dairy; pregnant, neonate, or elderly → Listeria (tumbling motility; ampicillin)
  • Temporal lobe, personality change, aphasia, RBCs in CSF → HSV-1 encephalitis
  • Young woman + psychosis + orofacial dyskinesias + ovarian teratoma → anti-NMDA-receptor encephalitis
  • Warm freshwater swimming, rapidly fatal → Naegleria fowleri
  • Summer flaccid paralysis + encephalitis; birds/mosquitoes → West Nile virus
  • Hydrophobia, agitation, Negri bodies; animal bite → rabies
  • Recurrent 'aseptic' (Mollaret) meningitis → HSV-2

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