Meningitis Lecture Notes

Definition

• Meningitis: Inflammation of the meninges (layers protecting the brain).

Focus on Bacterial Meningitis

• Main causative organisms:
  • Group B Streptococcus
  • Streptococcus pneumoniae
  • Neisseria meningitidis
  • Gram-negative bacilli
  • Haemophilus influenzae
• Vaccinations (Haemophilus influenzae and pneumococcal) have decreased infant mortality.

Pathways to Meningitis

• Contact with Meninges:
  • Causative agents need to interact with meninges.
  • Can occur through:
    • Primary infections (e.g., otitis media, sinusitis, pneumonia)
    • Abnormalities (e.g., cribriform plate defect, basal skull fracture)

Pathophysiology of Bacterial Meningitis

1. Virulence Factors of Bacteria:
   • Colonization:
     • Bacteria can evade antibodies using IgA proteases (e.g., Strep pneumoniae, Neisseria meningitidis).
     • Pili and fimbriae help latch onto host epithelium.
   • Invasion:
     • Bacteria can invade body leading to primary infection.
   • Immune Evasion:
     • Capsules in bacteria (e.g., Group B Streptococcus) help evade macrophages and immune responses.
   • Meningeal Invasion:
     • Spread through hematogenous route or cerebrospinal fluid (CSF).

Anatomy and Physiology of the Brain

• Cerebrospinal Fluid (CSF):
  • Produced by brain ventricles and nourishes brain tissue.
  • Circulates through:
    • Lateral ventricles -> Third ventricle -> Fourth ventricle -> Subarachnoid space -> Venous sinus.
• Blood-Brain Barrier:
  • Semi-permeable barrier protecting the brain; allows only selective substances (e.g., glucose, oxygen).
• Blood-Cerebrospinal Fluid Barrier:
  • Capillaries in the ventricles lack astrocytes, allowing more substances to enter.

Mechanism of Bacterial Invasion

• CSF Sterility:
  • Normally sterile with low immunoglobulins and complement proteins.
• Bacterial Entry:
  • From bloodstream or directly through the CSF environment.
  • Can invade the meninges via the ventricles and subarachnoid space.

Symptoms and Clinical Signs of Meningitis

• Classical signs include:
  • Fever
  • Headache
  • Photophobia
  • Nausea and vomiting
  • Neck stiffness
• Changes in CSF during lumbar puncture:
  • Presence of bacteria
  • Increased protein levels
  • Increased neutrophils
  • Decreased glucose levels.

Local Invasion of Meninges

• Bacteria can also invade locally through sinuses or normal circulation directly into the subarachnoid space.