17.3 Bacterial Meningitis & CNS Infections
Key Takeaways
- Empiric antibiotic choice is age-based: ampicillin plus cefotaxime for neonates (Listeria and Group B Streptococcus coverage; ceftriaxone avoided due to bilirubin displacement), transitioning to vancomycin plus a third-generation cephalosporin beyond 3 months.
- Bacterial meningitis CSF classically shows neutrophil predominance, low glucose, and high protein, while viral meningitis shows lymphocyte predominance with normal glucose and protein.
- Classic meningeal signs (neck stiffness, Kernig, Brudzinski) are frequently absent in neonates and young infants, who instead present with poor feeding, lethargy, temperature instability, or a bulging fontanelle.
- Dexamethasone's neuroprotective benefit depends on timing: it must be given with or just before the first antibiotic dose, not afterward, and has the strongest evidence in Haemophilus influenzae type b meningitis.
- Any child with a picture suggestive of HSV encephalitis (fever, focal seizures, personality change) should receive empiric IV acyclovir while awaiting CSF PCR confirmation.
Bacterial Meningitis & CNS Infections
Bacterial meningitis is a high-mortality, high-morbidity pediatric emergency, and the exam tests it from four angles: which organism to suspect by age, how presentation differs in infants versus older children, how to read the lumbar puncture (LP), and which empiric antibiotics, plus adjunctive dexamethasone, to start immediately.
Common Organisms by Age
| Age group | Typical organisms |
|---|---|
| Neonates (birth to 1 month) | Group B Streptococcus (GBS, Streptococcus agalactiae), Escherichia coli and other gram-negative enteric rods, Listeria monocytogenes |
| 1-3 months | Overlap of neonatal organisms (GBS, E. coli, Listeria) with the community organisms below |
| Older than 3 months | Streptococcus pneumoniae, Neisseria meningitidis |
Since routine Haemophilus influenzae type b (Hib) conjugate vaccination became widespread, Hib meningitis has become rare in vaccinated populations, and S. pneumoniae has become the leading cause of bacterial meningitis beyond the neonatal period; the exam expects you to know this shift rather than defaulting to the pre-vaccine epidemiology.
Presentation: Classic vs. Subtle
Older infants and children present with the classic picture: fever, headache, neck stiffness, photophobia, vomiting, altered mental status, and a positive Kernig sign (pain with knee extension while the hip is flexed) or Brudzinski sign (involuntary hip and knee flexion with neck flexion). A petechial or purpuric rash strongly suggests meningococcemia and should prompt immediate empiric therapy without waiting for confirmatory testing.
Neonates and young infants present subtly, and classic meningeal signs are frequently absent. Instead, expect fever or hypothermia, poor feeding, lethargy or irritability (sometimes a paradoxical combination of both), a high-pitched or weak cry, apnea, a bulging fontanelle, and seizures. Because neck stiffness is unreliable in this age group, the exam rewards a low threshold for LP in any young infant with nonspecific, non-localizing findings plus fever, even without classic meningeal signs.
Reading the Lumbar Puncture
| Cerebrospinal fluid (CSF) parameter | Bacterial | Viral | Tuberculous (TB) |
|---|---|---|---|
| Appearance | Turbid/cloudy | Clear | Clear, may form a cobweb pellicle on standing |
| Opening pressure | Elevated | Normal to mildly elevated | Elevated |
| White cell count | High, often 1,000-10,000/microliter | Low to moderate, roughly 10-500/microliter | Moderate, roughly 50-500/microliter |
| Predominant cell type | Neutrophils, greater than 80% | Lymphocytes (neutrophils can predominate very early) | Lymphocytes |
| Glucose | Low (CSF-to-serum ratio less than 0.4) | Normal | Very low |
| Protein | High, often greater than 100 mg/dL | Normal or mildly elevated | Very high, can exceed 200-300 mg/dL |
Exam pattern-recognition shortcut: bacterial meningitis is neutrophils plus low glucose plus high protein; viral meningitis is lymphocytes plus normal glucose plus normal-to-mild protein; TB meningitis looks like an exaggerated, more indolent version of viral meningitis but with markedly low glucose and very high protein, occurring in a child with a subacute course over days to weeks and a TB exposure history. Partially treated bacterial meningitis, from oral antibiotics given before the LP, can blur this picture; a lymphocyte-predominant CSF with low glucose in a child who looks clinically bacterial should raise suspicion for pretreatment rather than reassure you the process is viral.
Empiric Antibiotic Choice by Age
Empiric therapy must start immediately after, or alongside, the LP; antibiotics should not be delayed to obtain imaging in a child without focal neurologic signs, papilledema, or a bulging fontanelle suggesting a contraindication to LP.
| Age | Empiric regimen | Why |
|---|---|---|
| Younger than 1 month (neonates) | Ampicillin plus cefotaxime (or ampicillin plus an aminoglycoside) | Ampicillin covers Listeria and enterococcus, which cephalosporins alone do not cover; ceftriaxone is avoided in neonates because it displaces bilirubin from albumin and can cause biliary sludging and increase kernicterus risk |
| 1-3 months | Ampicillin plus cefotaxime or ceftriaxone | Bridges neonatal organisms (Listeria, GBS) and community organisms (pneumococcus, meningococcus) |
| Older than 3 months | Vancomycin plus ceftriaxone (or cefotaxime) | The third-generation cephalosporin covers N. meningitidis and most S. pneumoniae; vancomycin is added to cover cephalosporin-resistant S. pneumoniae |
Adjunctive Dexamethasone
Dexamethasone is given as adjunctive therapy mainly to reduce the risk of sensorineural hearing loss and other neurologic sequelae, with the strongest evidence in Hib meningitis and reasonable, though less robust, evidence in pneumococcal meningitis. The single most exam-tested detail is timing: dexamethasone must be given with or shortly before the first dose of antibiotics, not after, because giving it late, once the antibiotic-induced bacterial lysis and inflammatory cascade has already started, loses most of its benefit. Typical dosing is 0.15 mg/kg every 6 hours for approximately 4 days. Dexamethasone is generally not used in neonates and is not indicated for meningococcal or viral meningitis.
A Note on Viral and Other CNS Infections
Most viral, or aseptic, meningitis in children is caused by enteroviruses and is self-limited, managed supportively. Herpes simplex virus (HSV) encephalitis is the critical do-not-miss viral CNS infection; it classically presents with fever, focal seizures, and behavioral or personality change from temporal lobe involvement, with temporal lobe changes on magnetic resonance imaging (MRI) and periodic lateralized epileptiform discharges on electroencephalogram (EEG). Any child with a picture suggestive of encephalitis should receive empiric IV acyclovir while awaiting CSF HSV polymerase chain reaction (PCR) testing, because untreated HSV encephalitis carries very high morbidity and mortality and treatment should never be withheld pending confirmatory results.
A 3-week-old infant presents with poor feeding, lethargy, and a low-grade fever, with no neck stiffness on exam. Which empiric antibiotic regimen is most appropriate while cultures are pending?
A 5-year-old with fever and neck stiffness has a lumbar puncture showing a white cell count of 4,500/microliter with 90% neutrophils, a glucose of 22 mg/dL against a serum glucose of 100 mg/dL, and a protein of 220 mg/dL. Which pattern does this represent?
A previously healthy 4-year-old develops fever, personality change, and focal seizures. Cerebrospinal fluid herpes simplex virus polymerase chain reaction is sent. What is the most appropriate immediate management while awaiting results?
When adjunctive dexamethasone is used for suspected bacterial meningitis, what timing maximizes its benefit in reducing neurologic sequelae such as hearing loss?