Acute Meningitis¶
Chapter 143 | Part 5: Infectious Diseases
KEY CLINICAL POINTS¶
- Bacterial meningitis is an acute purulent infection of the subarachnoid space (SAS) with CNS inflammation, leading to decreased consciousness, seizures, and raised ICP.
- Common pathogens include S. pneumoniae (~50%), N. meningitidis (~25%), L. monocytogenes (~10%), and H. influenzae (~15%) in adults.
- CSF findings include lymphocytic pleocytosis, low glucose (<2.2 mmol/L), elevated protein (>0.45 g/L), and positive Gram stain in >60% of cases.
- Dexamethasone (10 mg IV) reduces mortality and hearing loss in bacterial meningitis when administered 20 min before antibiotics.
- Viral meningitis is typically self-limiting with a lymphocytic CSF profile, while bacterial meningitis requires urgent antibiotic therapy.
1. DEFINITION & OVERVIEW¶
Bacterial meningitis is an acute purulent infection within the subarachnoid space (SAS) associated with CNS inflammation. It may result in decreased consciousness, seizures, raised intracranial pressure (ICP), and stroke. The meninges, SAS, and brain parenchyma are frequently involved in the inflammatory reaction (meningoencephalitis).
1.1 Subtopic¶
Subacute Sclerosing Panencephalitis (SSPE) is a rare, progressive neurologic disorder caused by measles virus, characterized by progressive intellectual deterioration, seizures, and myoclonus. No definitive therapy exists, but isopropine and interferon- α may prolong survival.
2. EPIDEMIOLOGY¶
Bacterial meningitis is the most common form of suppurative CNS infection, with an annual incidence of ~1.4 cases/100,000 in the US. Common pathogens: S. pneumoniae (~50%), N. meningitidis (~25%), group B strep (~15%), L. monocytogenes (~10%), and H. influenzae type b (<10%). Vaccination has reduced Hib cases, but remains a risk in non-immunized populations.
Table 143-1: Antibiotics for Empirical Therapy¶
| INDICATION | ANTIBIOTIC |
|---|---|
| Preterm infants to infants <1 month | Ampicillin + cefotaxime |
| Infants 1–3 months | Ampicillin + cefotaxime or ceftriaxone |
| Immunocompetent children >3 months and adults <55 | Cefotaxime, ceftriaxone, or cefepime + vancomycin |
| INDICATION | ANTIBIOTIC |
|---|---|
| Adults >55 and any age with alcoholism or other debilitating illnesses | Ampicillin + cefotaxime, ceftriaxone, or cefepime + vancomycin |
| Hospital-acquired meningitis, posttraumatic or postneurosurgery meningitis, neutropenic patients, or patients with impaired cell-mediated immunity | Ampicillin + ceftazidime or meropenem + vancomycin |
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Pathogens enter via bloodstream, evade immune defenses via capsules (e.g., S. pneumoniae, N. meningitidis). Inflammatory response leads to increased ICP, CSF exudate, and complications like hydrocephalus. Bacterial cell-wall components (LPS, teichoic acid) induce cytokine release (IL-1 β , TNF- α ), causing vasogenic edema and BBB disruption.
3.1 Pathogenesis¶
Bacterial invasion of SAS triggers cytokine release (IL-1 β , TNF- α ), leading to BBB disruption, vasogenic edema, and increased ICP. CSF exudate obstructs CSF flow, causing hydrocephalus. Neutrophil degranulation contributes to cytotoxic edema and neuronal injury.
4. CLINICAL FEATURES¶
Classic triad: fever, headache, nuchal rigidity. Kernig's and Brudzinski's signs indicate meningeal irritation. Seizures occur in 15–40% of cases. Altered consciousness, photophobia, and vomiting are common. CSF findings: lymphocytic pleocytosis, low glucose, elevated protein, and positive Gram stain in >60% of cases.
5. DIFFERENTIAL DIAGNOSIS¶
Viral encephalitis (HSV, VZV), fungal meningitis (C. neoformans, H. capsulatum), tuberculous meningitis, neoplastic meningitis, and non-infectious causes (e.g., medication-induced hypersensitivity, sarcoidosis). HSV encephalitis presents with focal deficits and EEG abnormalities. Fungal meningitis shows mononuclear pleocytosis and low glucose.
6. INVESTIGATIONS & DIAGNOSIS¶
CSF analysis: lymphocytic pleocytosis, low glucose (<2.2 mmol/L), elevated protein (>0.45 g/L), and positive Gram stain. PCR detects pathogens (e.g., HSV, VZV). MRI shows meningeal enhancement and cerebral edema. Empirical antibiotics and dexamethasone initiated immediately.
Table 143-2: CSF Abnormalities in Bacterial Meningitis¶
| Parameter | Findings |
|---|---|
| Opening pressure | >180 mmH2O |
| WBC count | 10–10,000/mL; neutrophils predominate |
| Glucose | <2.2 mmol/L (<40 mg/dL); CSF/serum ratio <0.4 |
| Protein | >0.45 g/L (>45 mg/dL) |
| Gram’s stain | Positive in >60% |
7. MANAGEMENT & TREATMENT¶
Empirical antibiotics (Table 143-1) based on pathogen: ceftriaxone/cefotaxime for S. pneumoniae, vancomycin for L. monocytogenes, and ceftriaxone for N. meningitidis. Dexamethasone (10 mg IV) reduces mortality and hearing loss. Supportive care includes ICP management, anticonvulsants, and hydration. Specific therapies for fungal/tuberculous meningitis include amphotericin B and antitubercular drugs.
Table 143-3: Antimicrobial Therapy by Pathogen¶
| Organism | Antibiotic |
|---|---|
| Neisseria meningitidis | Penicillin-sensitive: Penicillin G or ampicillin; Penicillin-resistant: Ceftriaxone or cefotaxime |
| Streptococcus pneumoniae | Penicillin-sensitive: Penicillin G; Penicillin-resistant: Ceftriaxone + vancomycin |
| Listeria monocytogenes | Ampicillin + gentamicin |
| Haemophilus influenzae | Ceftriaxone or cefotaxime (b-lactamase positive); Ampicillin (b-lactamase negative) |
| Staphylococcus aureus | Nafcillin (methicillin-sensitive); Vancomycin (methicillin-resistant) |
7.1 Adjunctive Therapy¶
Dexamethasone inhibits cytokine production (IL-1 β , TNF- α ), reduces BBB permeability, and decreases neurologic sequelae. Administer 20 min before antibiotics for optimal effect.
8. PROGNOSIS & COMPLICATIONS¶
Mortality: 3–7% for H. influenzae/N. meningitidis, 15% for L. monocytogenes, 20% for S. pneumoniae. Complications include hearing loss, intellectual disability, hydrocephalus, and seizures. Delayed treatment and comorbidities (e.g., immunosuppression) worsen outcomes.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: L. monocytogenes risk; avoid certain antibiotics. Pediatrics: Hib vaccine reduced cases, but still a risk in non-immunized. Elderly: higher mortality. Immunocompromised: increased risk of fungal/tuberculous meningitis. Vaccination is critical for prevention.
10. KEY POINTS & CLINICAL PEARLS¶
- CSF findings (lymphocytosis, low glucose, high protein) distinguish bacterial from viral meningitis.
- Dexamethasone reduces mortality in bacterial meningitis.
- Viral meningitis is self-limiting with a lymphocytic CSF profile.
- Empirical antibiotics must be initiated immediately.
- Vaccination prevents many bacterial causes (e.g., Hib, pneumococcus).
- Monitor for hydrocephalus and ICP elevation in all cases.