Bacterial meningitis and CSF interpretation

Paediatric Bacterial Meningitis

Bacterial meningitis represents acute, life-threatening purulent infection of the arachnoid membrane, pia mater, and intervening cerebrospinal fluid space.

Clinical Presentation

• Older Children: Classic triad of high fever, severe headache, and neck stiffness (nuchal rigidity), accompanied by photophobia, vomiting, positive Kernig sign (pain and resistance when extending the knee with the hip flexed at 90 degrees), and positive Brudzinski sign (passive neck flexion induces spontaneous flexion of hips and knees).
• Infants: Meningeal signs are notoriously absent in infants under 12 to 18 months. Cardinal signs include:
  • High-pitched, irritable cry and extreme lethargy.
  • Paradoxical irritability: The infant cries inconsolably when held and rocked by parents, but calms when left motionless in the cot (holding stretches inflamed spinal meninges).
  • Full or tense, bulging anterior fontanelle (evaluated while the infant is upright and calm).
  • Poor feeding, vomiting, vacant stare, or new-onset unprovoked seizures.

Absolute Contraindications to Immediate Lumbar Puncture

Lumbar puncture must be performed to obtain CSF for cytology, biochemistry, and microbiology, EXCEPT when any of the following contraindications exist:

1. Signs of Raised Intracranial Pressure (ICP):
   • Altered mental status (Glasgow Coma Scale score $< 13$ or a dynamic drop of $\ge 2$).
   • Focal neurological signs (cranial nerve palsies, unilateral pupillary dilatation, hemiparesis).
   • Abnormal motor posturing (decorticate or decerebrate posturing).
   • Cushing Triad: Systemic hypertension, bradycardia, and irregular/abnormal respirations.
2. Cardiorespiratory Instability: Shock, severe hypoxaemia, or active respiratory compromise where positioning the child causes acute collapse.
3. Bleeding risk: Significant coagulopathy or thrombocytopenia can make LP unsafe; use the local procedural criteria. Petechiae alone is not an absolute contraindication. Shock, cardiorespiratory instability, impaired consciousness or raised-ICP concerns need assessment; a normal CT does not prove LP is safe.
4. Local Cutaneous Infection over the planned lumbar needle insertion site.

Critical Practice Rule: When lumbar puncture is delayed or contraindicated, never delay antibiotic administration. Obtain blood cultures immediately and administer intravenous empirical antibiotics and corticosteroids without waiting for cranial CT imaging.

Cerebrospinal Fluid (CSF) Analysis Interpretation

• Opening Pressure: Normal Child: $10\text{ to }20\text{ cmH}_2\text{O}$; Acute Bacterial Meningitis: Markedly elevated ($> 25\text{ to }30\text{ cmH}_2\text{O}$); Viral Meningitis / Encephalitis: Normal or mildly elevated; Tuberculous / Fungal Meningitis: Moderately to markedly elevated
• Visual Appearance: Normal Child: Crystal clear; Acute Bacterial Meningitis: Turbid, cloudy, or purulent; Viral Meningitis / Encephalitis: Clear or opalescent; Tuberculous / Fungal Meningitis: Clear, xanthochromic, or "cobweb" clot
• White Cell Count: Normal Child: $< 5 \times 10^6/\text{L}$; Acute Bacterial Meningitis: Markedly elevated: $> 1,000 \times 10^6/\text{L}$; Viral Meningitis / Encephalitis: Moderately elevated: $10\text{ to }500 \times 10^6/\text{L}$; Tuberculous / Fungal Meningitis: Moderately elevated: $100\text{ to }500 \times 10^6/\text{L}$
• Predominant Cell: Normal Child: Mononuclear ($> 95\%$); Acute Bacterial Meningitis: Polymorphs (Neutrophils $> 80\%$); Viral Meningitis / Encephalitis: Mononuclear (Lymphocytes $> 80\%$); Tuberculous / Fungal Meningitis: Lymphocytes ($> 80\%$)
• Protein Level: Normal Child: $0.15\text{ to }0.45\text{ g/L}$; Acute Bacterial Meningitis: Markedly elevated: $> 1.0\text{ to }2.0\text{ g/L}$; Viral Meningitis / Encephalitis: Normal or mildly elevated ($< 1.0\text{ g/L}$); Tuberculous / Fungal Meningitis: Markedly elevated: $> 1.0\text{ to }5.0\text{ g/L}$
• CSF:Blood Glucose: Normal Child: $> 0.6$ (or $> 2.5\text{ mmol/L}$); Acute Bacterial Meningitis: Markedly reduced: $< 0.4$ (often $< 1.0\text{ mmol/L}$); Viral Meningitis / Encephalitis: Normal: $> 0.6$; Tuberculous / Fungal Meningitis: Markedly reduced: $< 0.3$

Empirical Treatment Protocol

• Intravenous Ceftriaxone: $100\text{ mg/kg/day}$ IV (given once daily or divided 12-hourly, max $4\text{ g/day}$) to cover Neisseria meningitidis and cephalosporin-susceptible Streptococcus pneumoniae.
• Paediatric meningitis: Use the local age-specific empirical regimen. RCH recommends adding vancomycin when Gram-positive cocci are seen on CSF Gram stain; resistance risks and clinical circumstances may also change cover. It is not obligatory for every child with suspected bacterial meningitis.
• Adjunctive Corticosteroid Therapy (Dexamethasone):
  • Dose: Intravenous dexamethasone $0.15\text{ mg/kg}$ 6-hourly for 4 days.
  • Administration Rule: Administer immediately prior to or concurrent with the first dose of antibiotics. Dexamethasone attenuates the inflammatory cascade triggered by bacterial lysis (release of endotoxin, peptidoglycan, and TNF-alpha).
  • Proven Clinical Benefit: Significantly reduces the incidence of severe sensorineural hearing loss and long-term neurological sequelae in meningitis caused by Haemophilus influenzae type b and Streptococcus pneumoniae. Corticosteroids provide minimal benefit once antibiotics have already been circulating for several hours.

Interpreting CSF in context

A partly treated bacterial infection can have less typical CSF findings, and early infection may not show a textbook pattern. Interpret cells, glucose relative to blood, protein, Gram stain, culture and molecular results with age and clinical course. An initial negative test does not justify stopping treatment in a deteriorating child without senior review. LP is deferred when unsafe, with treatment begun on clinical concern. Arrange hearing and other follow-up after bacterial meningitis when appropriate; apparent recovery from fever does not prove there are no later complications.

Primary references (checked 7 October 2026): RCH Kawasaki disease; RCH meningitis/encephalitis.