intro to CNS infections

• Central Nervous System (CNS) includes the brain and spinal cord
• Covered by three layers of meninges: Dura mater (outermost), Arachnoid (middle layer), Pia mater (inner layer)
• Bacteria can cause meningitis, brain abscess or shunt infections, and neurotoxin-mediated diseases like tetanus and botulism
• Infective syndromes of the CNS include meningitis, encephalitis, space-occupying lesions, and meningoencephalitis
• Meningitis is the inflammation of the meninges surrounding the brain and spinal cord, with involvement of the subarachnoid space
• Encephalitis is an acute inflammation of the brain caused by invasion of infectious agents, often viruses
• CNS infections can spread hematogenously, directly, through anatomical defects in the CNS, or via direct intraneural spread along nerves
• Hematogenous spread is a common route for pathogens to enter the subarachnoid space through blood vessels of the brain
• Direct spread can occur from an infected site close to the meninges, such as otitis media or sinusitis
• Anatomical defects in the CNS, like lumbar punctures or congenital defects, can allow easy access for organisms to infect the CNS
• Direct intraneural spread along nerves is another route for CNS infections
• Classification of meningitis can be based on the time course (acute, subacute, chronic) and the causative agent (bacteria, virus, fungi, parasite, amebic, non-infectious)
• Acute meningitis has a short episode duration, progresses rapidly, and can be bacterial or viral, while chronic meningitis worsens over weeks and can be caused by various agents
• Septic meningitis is always caused by bacteria
• Septic meningitis:

  • pyogenic/polymorphonuclear meningitis
  • Always caused by bacterial infection
  • CSF: Culture positive / PCR positive
• Aseptic meningitis:

  • lymphocytic meningitis
  • Infectious (mainly virus)/ Non-infectious
  • Most common cause: Viruses
  • CSF culture and stained negative/sterile
• Cerebrospinal Fluid (CSF) Analysis:

  • An ultrafiltrate of plasma contained within the ventricles of the brain and the subarachnoid spaces of the cranium and spine
  • Made by tissue called the choroid plexus in the ventricles in the brain
  • Examination of CSF is critical in making the diagnosis of CNS infections
  • CSF is obtained by performing a lumbar puncture at the L3 – L4 interspace
  • CSF pressure is measured, and fluid is obtained for analysis of cells, protein, and glucose
• CSF Changes in Septic VS Aseptic Meningitis:

  • Normal appearance: Clear and colourless
  • Septic (purulent) appearance: Turbid
  • Aseptic appearance: Usually clear
  • Total protein:
  • Normal (15 - 45) in normal
  • Highly increased (>100) in septic
  • Normal or slightly increased (50 - 100) in aseptic
  • Glucose:
  • Normal (45 - 85) in normal
  • Greatly decreased or absent (<45) in septic
  • Normal but low <45 in the case of tuberculosis, fungi & leptospira in aseptic
• Lactate:

  • Normal in normal
  • Increased in septic
  • Normal in aseptic
  • Cell count:
  • Lymphocytes (0 - 5) in normal
  • Greatly increased; polymorphs (mainly neutrophils) in septic
  • Increased in aseptic
• Septic Meningitis:

  • Characterized by elevated polymorphonuclear cells (i.e. neutrophils) in CSF
  • commonly cause: streptococcus pneumoniae
• Acute Viral Meningitis:

  • Caused by a number of viruses
  • Majority of cases: enteroviruses (>85%)
  • CSF is predominantly lymphocytic
  • Develop into meningitis a few days after the infection; many of these viruses progress slower and can also occasionally cause chronic meningitis
• Chronic Meningitis:

  • Persistence of meningitis exists for >4 weeks
  • Associated with a persistent inflammatory response in CSF (white blood cell count >5/μL)
• Neonatal Meningitis:

  • Acute bacterial meningitis
  • Most commonly occurs in the first 3 months of life
  • Classified as early-onset (EOM) or late-onset meningitis (LOM)
  • Common agents: Group B streptococci (GBS)(Streptococcus agalactiae) - 50%, Gram-negative bacilli (Escherichia coli - 20%, Klebsiella, Listeria monocytogenes)
• Tuberculous Meningitis:

  • Chronic meningitis
  • Seen most commonly in young children
  • Causative agent: Mycobacterium tuberculosis