Acute bacterial meningitis is a life-threatening purulent infection of the subarachnoid space (SAS) , leading to inflammation of the meninges and potentially the brain parenchyma ( meningoencephalitis ). It can cause altered mental status, seizures, elevated intracranial pressure (ICP), and stroke .
Etiology
| Pathogen | Key Risk Groups |
|---|---|
| Streptococcus pneumoniae | Most common in adults; risk increased with alcoholism, DM, sinusitis, otitis media, splenectomy, complement deficiency |
| Neisseria meningitidis | Common in children and young adults; risk ↑ with complement deficiencies (esp. properdin, C5–C9) |
| Listeria monocytogenes | Neonates, elderly (>60), pregnant women, immunocompromised |
| Haemophilus influenzae type b (Hib) | Unvaccinated children, older adults |
| Gram-negative bacilli (e.g., E. coli, Klebsiella) | Diabetics, cirrhosis, UTIs, elderly |
| Staphylococcus aureus / coagulase-negative staph | Post-neurosurgical procedures (e.g., shunts, trauma) |
Pathophysiology
- Colonization : Bacteria colonize the nasopharynx and penetrate mucosal barriers.
- Hematogenous Spread : Bacteria evade phagocytosis via polysaccharide capsules and enter bloodstream.
- Invasion of CNS : They infect choroid plexus epithelium , gaining access to CSF.
- Replication in CSF : CSF is immune-privileged (low immunoglobulins, complement), allowing bacterial proliferation.
- Inflammatory Response :
- Bacterial lysis releases cell wall components (e.g., LPS, peptidoglycan, teichoic acid).
- Inflammatory cytokines (TNF-α, IL-1β) disrupt the blood-brain barrier , causing vasogenic edema .
- ↑ CSF proteins, leukocytosis, and ↓ CSF glucose.
- Complications :
- Cerebral edema (cytotoxic, vasogenic, interstitial) → ↑ ICP, herniation .
- Vascular inflammation → vasculitis, thrombosis → ischemia, infarction.
- Hydrocephalus : obstructive & communicating due to exudate and impaired CSF resorption.
Clinical Features
| Symptom/Sign | Description |
|---|---|
| Fever, headache, neck stiffness | Classic triad (but present in ~50%) |
| Altered mental status | Occurs in >75% of patients |
| Photophobia, nausea, vomiting | Common constitutional symptoms |
| Seizures | Focal (due to infarction) or generalized (e.g., hyponatremia) |
| Signs of raised ICP | Papilledema, Cushing reflex, CN VI palsy, decerebrate posturing |
| Kernig & Brudzinski signs | Positive in meningeal irritation |
| Petechial rash | Seen in meningococcemia (Neisseria meningitidis) |
Diagnosis and Investigations
Initial Assessment
- Immediate neuro exam for signs of ↑ICP.
- If no focal signs or papilledema , perform lumbar puncture (LP) immediately.
- If focal deficits, altered consciousness, or immunocompromised , obtain CT/MRI before LP .
CSF Analysis – Classic Findings in Bacterial Meningitis :
| Parameter | Typical Finding |
|---|---|
| Appearance | Turbid |
| Opening Pressure | ↑ (>180 mmH₂O) |
| WBC Count | ↑ (≥1000/μL), PMN predominant |
| Protein | ↑ (>0.45 g/L) |
| Glucose | ↓ (<2.2 mmol/L or <40 mg/dL) |
| CSF/Serum Glucose Ratio | <0.4 (highly suggestive) |
- Gram stain and culture : Most definitive.
- Latex agglutination (LA) : High specificity for S. pneumoniae and N. meningitidis .
- Blood cultures : Always before antibiotics if LP delayed.
- MRI with gadolinium : Detects cerebral edema, infarcts, meningeal enhancement better than CT.
Complications
- Cerebral herniation (life-threatening)
- Hydrocephalus
- Seizures
- Cranial nerve palsies
- Sensorineural hearing loss
- Vascular infarcts
Management Overview
Empirical Antibiotic Therapy (Immediately after cultures):
- Adults <50 years : Ceftriaxone + Vancomycin
- Adults >50 or immunocompromised : Add Ampicillin (for Listeria)
- Consider Dexamethasone : Reduces neurological sequelae, especially in pneumococcal meningitis
Supportive Measures :
- Monitor for ICP (head elevation, hypertonic saline, mannitol).
- Treat seizures , maintain normovolemia and oxygenation .
- Admit to ICU if unstable.
Key High-Yield Points
- CSF glucose <2.2 mmol/L and CSF/serum glucose ratio <0.4 = strong indicator.
- Do NOT delay antibiotics for neuroimaging in unstable patients.
- Most common cause in adults : Streptococcus pneumoniae .
- Meningococcal rash = petechiae, purpura, can lead to DIC.
- Immunocompromised or elderly : Always consider Listeria .
- Dexamethasone should be given before or with first dose of antibiotics.
