Bacterial Meningitis
Infectious Disease / Neurology
Illness script · Infectious Disease / Neurology
Bacterial Meningitis
Acute bacterial infection of the subarachnoid space causing meningeal inflammation, often rapidly fatal without treatment.
This illness script for Bacterial Meningitis covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.
01
Predisposing factors
- Neonates: Group B Strep, E. coli, Listeria
- Children/adults: S. pneumoniae (most common overall), N. meningitidis
- Elderly/immunocompromised: add Listeria monocytogenes
- Asplenia, complement deficiency → high risk for encapsulated organisms (N. meningitidis)
- Crowded settings (dorms, barracks) → N. meningitidis outbreaks
- Recent neurosurgery/skull fracture → gram-negatives, S. aureus
02
Presentation
- Classic triad: fever, nuchal rigidity, altered mental status (present together in only ~44%)
- Headache is severe and rapid-onset ('worst of life')
- Kernig's sign: pain/resistance with knee extension from hip-flexed position
- Brudzinski's sign: passive neck flexion causes involuntary hip/knee flexion
- Petechial/purpuric rash → strongly suggests N. meningitidis (meningococcemia)
- Photophobia and phonophobia common; seizures in ~30%
03
Pathophysiology
- Hematogenous seeding or direct spread → bacterial invasion of subarachnoid space
- Bacterial components trigger cytokine cascade → neutrophilic pleocytosis, BBB breakdown
- Increased ICP from cerebral edema → herniation risk
- Exudate can cause vasculitis, cranial nerve palsy, hydrocephalus
04
Diagnostics
- LP is gold standard: opening pressure ↑, cloudy CSF, WBC >1000 (neutrophilic), protein ↑, glucose ↓
- CSF glucose <40 or CSF:serum glucose ratio <0.4 is key finding
- Blood cultures × 2 before LP but do NOT delay antibiotics for imaging
- CT head before LP only if: papilledema, focal neuro deficits, new seizure, immunocompromised, or altered MS
- Gram stain of CSF is rapid and specific; latex agglutination/PCR for adjuncts
05
Management
- Start empiric IV antibiotics IMMEDIATELY — do not delay for imaging or LP
- Empiric therapy: ceftriaxone + vancomycin (add ampicillin for neonates/elderly for Listeria)
- Dexamethasone 0.15 mg/kg q6h × 4 days given before or with first antibiotic dose → reduces neurologic sequelae (especially in S. pneumoniae)
- Prophylaxis: rifampin or ciprofloxacin for close contacts of N. meningitidis cases
- ICU admission, airway protection if GCS ≤8; treat raised ICP aggressively
06
Clinical pivots
How to separate this script from the look-alikes that show up on exams and on the wards.
Viral (Aseptic) Meningitis
CSF shows lymphocytic pleocytosis, normal glucose, lower WBC (<500); patient less toxic-appearing
Subarachnoid Hemorrhage
Thunderclap headache, bloody or xanthochromic CSF; no fever, normal CSF glucose/WBC
Herpes Simplex Encephalitis
Encephalitis pattern (personality change, temporal lobe seizures); CSF RBCs, temporal lobe changes on MRI; treat empirically with acyclovir
Brain Abscess
Focal neurologic deficits, ring-enhancing lesion on CT/MRI; LP contraindicated; no classic meningismus
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Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.