4.3 Central Nervous System Infections

Acute CNS infections, rapidly spreading soft-tissue emergencies, and joint/bone infections require immediate diagnostic precision and rapid initiation of targeted antimicrobial therapy. This section details age-based meningitis regimens, lumbar puncture safety, necrotizing fasciitis management, and musculoskeletal infection workups.

1. Acute Bacterial Meningitis

Acute bacterial meningitis is a medical emergency characterized by bacterial replication and neutrophilic inflammation within the subarachnoid space and leptomeninges.

Etiology by Patient Age and Host Factors

• Adults < 50 Years: Streptococcus pneumoniae (~50–60%), Neisseria meningitidis (~20–30%), Haemophilus influenzae (<5%).
• Adults >= 50 Years or Immunocompromised (Cirrhosis, Alcoholism, Malignancy, Corticosteroid Therapy): Streptococcus pneumoniae, Neisseria meningitidis, PLUS Listeria monocytogenes (~15–20%) and aerobic Gram-negative bacilli (E. coli, Klebsiella spp.).
• Post-Neurosurgery, Penetrating Head Trauma, or CSF Shunts: Staphylococcus aureus (including MRSA), Staphylococcus epidermidis, Pseudomonas aeruginosa, and enteric Gram-negative bacilli.

Clinical Presentation

• Classic Triad: Fever, altered mental status, and nuchal rigidity (present in >95% of patients with at least 2 of 4 symptoms: headache, fever, neck stiffness, altered sensorium).
• Physical Signs: Kernig sign (resistance or pain upon passive knee extension with hip flexed 90°) and Brudzinski sign (involuntary flexion of hips/knees upon passive neck flexion). Both have high specificity (>95%) but low sensitivity (<15%).
• Petechial/Purpuric Rash: Strongly suggests Neisseria meningitidis bacteremia with meningococcemia and disseminated intravascular coagulation (Waterhouse-Friderichsen syndrome).

CSF Analysis Profiles Across CNS Infections

Safe Approach to Lumbar Puncture & Indications for Prior Head CT

• Critical Rule: If bacterial meningitis is suspected, draw blood cultures and immediately administer empiric IV antibiotics and dexamethasone. Never delay antimicrobial administration to perform neuroimaging or lumbar puncture.
• Indications for Head CT Prior to Lumbar Puncture:
  1. Focal neurological deficits (e.g., hemiparesis, facial droop, visual field defect, unequal pupils).
  2. New-onset seizure within the preceding 1 week.
  3. Papilledema on funduscopic examination.
  4. Severely depressed level of consciousness (Glasgow Coma Scale <10, stupor, or coma).
  5. Immunocompromised state (HIV infection, solid organ or stem cell transplant, active immunosuppressive therapy).

Empiric Antimicrobial Regimens & Role of Dexamethasone

• Age < 50 Years: Vancomycin (15–20 mg/kg IV q8–12h targeting trough 15–20 mcg/mL) PLUS Ceftriaxone (2 g IV q12h) OR Cefotaxime (2 g IV q4h).
• Age >= 50 Years or Immunocompromised: Vancomycin (15–20 mg/kg IV q8–12h) PLUS Ceftriaxone (2 g IV q12h) PLUS Ampicillin (2 g IV q4h for Listeria monocytogenes coverage).
  • Note: Ceftriaxone and all other cephalosporins lack intrinsic activity against Listeria monocytogenes.
• Post-Neurosurgical / CSF Shunt: Vancomycin (15–20 mg/kg IV q8–12h) PLUS Cefepime (2 g IV q8h) or Meropenem (2 g IV q8h).
• Severe Beta-Lactam Anaphylaxis: Vancomycin + Moxifloxacin (or Levofloxacin) + Trimethoprim-sulfamethoxazole (for Listeria).
• Adjunctive Dexamethasone:
  • Dose: 10 mg IV every 6 hours for 4 days.
  • Timing: Must be administered prior to or concurrently with the first dose of antimicrobial therapy. If antibiotics have already been administered hours earlier, dexamethasone should not be initiated.
  • Benefit: Significantly reduces mortality and long-term sensorineural hearing loss in adults with Streptococcus pneumoniae meningitis. Discontinue if culture identifies N. meningitidis, Listeria, or other organisms.

2. Viral Meningitis & Encephalitis

• Aseptic (Viral) Meningitis: Most commonly caused by Enteroviruses (Coxsackievirus, Echovirus); self-limiting with headache, fever, photophobia, normal glucose, and lymphocytic pleocytosis.
• Herpes Simplex Virus-1 (HSV-1) Encephalitis:
  • Pathophysiology: Retrograde transport of HSV-1 along olfactory or trigeminal nerves into the temporal and inferior frontal lobes, causing hemorrhagic necrotizing encephalitis.
  • Clinical Manifestations: Acute fever, severe headache, confusion, personality/behavioral changes, olfactory/gustatory hallucinations, and focal temporal lobe seizures (lip smacking, automatisms).
  • Diagnostics:
    • Lumbar Puncture: Lymphocytic pleocytosis, elevated protein, and characteristically elevated red blood cells (RBCs) in CSF from hemorrhagic cortical necrosis.
    • CSF HSV-1 PCR: Diagnostic test of choice (sensitivity >95%, specificity >98%).
    • MRI Brain: Asymmetric T2/FLAIR hyperintensity and restricted diffusion in the temporal lobes, insular cortex, and inferior frontal lobes.
    • EEG: Periodic Lateralized Epileptiform Discharges (PLEDs) over the affected temporal lobe.
  • Treatment: Initiate IV Acyclovir 10 mg/kg IV every 8 hours for 14–21 days immediately upon clinical suspicion. Maintain vigorous IV crystalloid hydration to avoid acyclovir crystal precipitation and acute crystalline nephropathy.
• West Nile Virus Encephalitis: Transmitted by Culex mosquitoes. Manifests as fever, encephalitis, flaccid asymmetric paralysis (anterior horn cell loss resembling poliomyelitis), and coarse Parkinsonian tremors. Diagnosed via CSF West Nile Virus IgM antibody capture ELISA.