Overview of Acute Meningitis

Acute Meningitis Overview

• Meningitis is inflammation of the meninges (membranes surrounding the brain and spinal cord) and cerebrospinal fluid (CSF) in the subarachnoid space.
• Meningoencephalitis involves inflammation of both meninges and brain tissue.
• Causes include infections (most common), non-bacterial irritants (chemical meningitis), and autoimmune diseases.
• Types include acute pyogenic (bacterial), aseptic (viral), and chronic (tuberculous, spirochetal, or cryptococcal). Each type has distinct CSF characteristics.

Acute Pyogenic (Bacterial) Meningitis

• Common causes vary by age group:
  • Neonates: Escherichia coli, group B streptococci
  • Elderly: Streptococcus pneumoniae, Listeria monocytogenes
  • Adolescents/young adults: Neisseria meningitidis
• Haemophilus influenzae infection incidence has decreased in developed countries due to immunization, especially in infants.
• Symptoms include systemic infection signs plus headache, photophobia, irritability, altered consciousness, and neck stiffness.
• CSF findings include cloudy/purulent CSF, high neutrophil count (up to 90,000/mm³), increased CSF pressure and protein, and decreased glucose.
• Untreated can be fatal; antibiotics dramatically reduce mortality.
• A serious complication is Waterhouse-Friderichsen syndrome (adrenal gland hemorrhages) often associated with meningococcal and pneumococcal meningitis.
• Immunosuppressed individuals can develop purulent meningitis caused by different infectious agents (e.g., Klebsiella, anaerobic organisms); this can be harder to diagnose.

Morphology of Acute Pyogenic Meningitis

• Exudate (pus) is present on the brain surface, thickening the leptomeninges.
• Blood vessels in the meninges are engorged.
• Exudate distribution varies depending on the causative organism (e.g., H. influenzae typically basal, S. pneumoniae often over convolutions).
• Pus may follow blood vessels to other brain areas.
• Fulminant meningitis can cause inflammation in the ventricles (ventriculitis).
• Ventricles are potential access points for blood-borne infections due to lack of a blood-brain barrier in the choroid plexus.
• Microscopic examination shows neutrophils in the subarachnoid space; often around blood vessels. Gram stain can reveal bacteria.
• Inflammation might extend into brain tissue, causing vasculitis and potential hemorrhagic infarcts.
• Leptomeningeal fibrosis can lead to hydrocephalus.
• Chronic adhesive arachnoiditis, especially in pneumococcal meningitis, results from large capsular polysaccharide quantities producing gelatinous exudate that promotes arachnoid fibrosis.

Acute Aseptic (Viral) Meningitis

• Characterized by absence of bacteria in CSF while presenting typical meningitis symptoms (meningeal irritation, fever, altered consciousness).
• Primarily viral, but may have bacterial, rickettsial, or autoimmune origins.
• Less severe than pyogenic meningitis.
• CSF findings: lymphocytic pleocytosis (increased lymphocytes), moderate protein elevation, and usually normal glucose content.
• Usually self-limiting and treated symptomatically.
• Etiologic agent identified in a small percentage of cases; new detection techniques (e.g., next-generation sequencing) are being developed.
• Enteroviruses are common causative agents (accounting for ~80% of cases), with seasonal and geographic variations.
• Chemical meningitis can mimic aseptic meningitis (caused by substance introduced into the subarachnoid space).
• CSF is sterile, neutrophil pleocytosis, increased protein, and normal glucose content.

This quiz provides an overview of acute meningitis, covering its causes, symptoms, and types, including acute pyogenic and aseptic meningitis. You'll learn about the specific pathogens responsible for infections in different age groups, as well as the implications of immunization on incidence rates. Test your understanding of this critical health issue.