1. Introduction and Definition

• Definition: Meningitis is the inflammation of the leptomeninges (pia and arachnoid mater) with variable involvement of the encephalon.
• Classification:** Acute meningitis is classified by etiology into bacterial (pyogenic), viral (aseptic), fungal, or parasitic. Bacterial meningitis is a medical emergency with high mortality if untreated.

2. Epidemiology and Etiology

The etiological agents vary significantly by age and immune status.

A. Common Etiological Agents

B. Predisposing Factors

• Hematogenous spread: From distant foci (lungs, skin, bones).
• Contiguous spread: Otitis media, mastoiditis, sinusitis.
• Anatomical defects: Neural tube defects, fracture base of skull (CSF rhinorrhea), pilonidal sinus.
• Immunocompromised states: HIV, asplenia (Sickle cell disease), hypogammaglobulinemia, complement deficiency.

3. Pathogenesis

The bacteria typically colonize the nasopharynx and invade the bloodstream.

Sequence of Events:

1. Colonization: Nasopharyngeal colonization by organisms.
2. Invasion: Cleavage of cell junctions and IgA protease activity allows entry into the bloodstream.
3. Bacteremia: Survival in blood (evasion of complement/phagocytosis via capsule).
4. CNS Entry: Lodging in the choroid plexus and crossing the blood-brain barrier.
5. Inflammation: Bacterial lysis releases cell wall components $\rightarrow$ production of cytokines (IL-1, IL-6, TNF).
6. Pathology:
   • Cerebral Edema: Vasogenic, cytotoxic, and interstitial.
   • Raised ICP: Due to edema and obstructed CSF flow (hydrocephalus).
   • Neuronal Damage: Cortical injury from inflammation and ischemia.

4. Clinical Features

Presentation depends on the age of the child and duration of illness.

A. Symptoms

• Constitutional: High-grade fever ($\ge$ 80%), lethargy, irritability, anorexia.
• Raised ICP: Headache (worse in mornings), vomiting (projectile), bulging fontanelle (in infants).
• Neurological: Seizures (1/3rd of patients), altered sensorium (drowsy, stupor, coma), photophobia.

B. Signs of Meningeal Irritation

These may be absent in very young infants or deeply comatose children.

1. Neck Rigidity: Resistance to passive neck flexion.
2. Kernig’s Sign: Pain/resistance on extending the knee beyond 135° when the hip is flexed to 90°.
3. Brudzinski’s Sign: Spontaneous flexion of knees/hips upon passive flexion of the neck.

C. Other Signs

• Skin: Petechiae/purpura (Meningococcemia).
• Neurological Deficits: Focal deficits (10-20%), VI nerve palsy (false localizing sign).
• Fundus: Papilledema is uncommon in acute meningitis; if present, suggests complications like abscess or venous sinus thrombosis.

5. Investigations

Lumbar Puncture (LP) is the gold standard diagnostic test

A. Lumbar Puncture

• Timing: Perform as soon as possible unless contraindicated. Do not delay antibiotics for LP if the patient is unstable.
• Contraindications: Papilledema (raised ICP), infection at LP site, bleeding diathesis, cardio-respiratory instability.
• Procedure: Lateral recumbent position, L3-L4 or L4-L5 space.

B. CSF Analysis (Comparison Table)

C. Other Investigations

• Blood: CBC (Leukocytosis), Blood Culture (Positive in 50%), CRP, Electrolytes (SIADH).
• Latex Agglutination: For partially treated cases (detects antigens of Hib, Pneumococcus, Meningococcus).
• Neuroimaging (CT/MRI): Not routine. Indicated for focal signs, persistent fever, suspected abscess, or raised ICP.

6. Management of Acute Bacterial Meningitis

Goal: Rapid sterilization of CSF and control of intracranial pressure.

A. Empiric Antibiotic Therapy

Start immediately after samples are drawn.

B. Steroid Therapy (Dexamethasone)

• Rationale: Reduces cytokine-mediated inflammation, hearing loss, and neurological sequelae.
• Dose: 0.15 mg/kg/dose IV every 6 hours for 4 days.
• Timing: Must be given 15 minutes before or with the first dose of antibiotics.
• Indication: Strongly recommended for H. influenzae and Pneumococcal meningitis.

C. Supportive Care

1. Fluids: Maintenance fluids recommended. Restrict to 2/3rd maintenance only if SIADH is confirmed (Na <120 mEq/L). Avoid hypotonic fluids.
2. Raised ICP: Elevate head to 30°. Mannitol (0.25-0.5 g/kg) if signs of herniation. Hyperventilation (maintain pCO2 25-30 mmHg) in ventilated patients.
3. Seizures: Treat with IV Benzodiazepines and Phenytoin.
4. Vitals: Maintain normothermia, normoglycemia, and adequate perfusion.

7. Complications

Acute (First few days)

• Seizures (early onset <4 days).
• Raised Intracranial Tension / Herniation.
• Shock / DIC (Waterhouse-Friderichsen syndrome).
• Hyponatremia (SIADH).

Subacute/Chronic

• Subdural Effusion: Suspect if persistent fever, increasing head size. Treat only if symptomatic (taps).
• Subdural Empyema: Requires surgical drainage.
• Hydrocephalus: Communicating or obstructive.
• Brain Abscess: Focal deficits, fever.

Sequelae

• Sensorineural Hearing Loss (10-25%).
• Mental Retardation / Developmental Delay.
• Epilepsy.
• Motor deficits (Hemiparesis).

8. Prevention

A. Vaccination

1. Hib Vaccine: Dramatically reduced H. flu meningitis (>95% reduction).
2. Pneumococcal Conjugate Vaccine (PCV): Prevents invasive pneumococcal disease.
3. Meningococcal Vaccine: For epidemics and high-risk groups (>2 yrs).

B. Chemoprophylaxis for Contacts

• Meningococcus: Rifampicin (20 mg/kg/day BD for 2 days) OR Ciprofloxacin (single dose).
• H. influenzae: Rifampicin (20 mg/kg/day OD for 4 days).