Acute meningitis: pyogenic vs aseptic
- Lesion
Meningitis is infection of the brain's coverings; bacteria give pus and an emergency, viruses usually a milder illness.
What it is
Acute meningitis is inflammation of the leptomeninges and CSF. Bacterial (pyogenic) meningitis fills the subarachnoid space with neutrophils and fibrin and can kill within hours through raised pressure, vasculitis and sepsis. Aseptic meningitis, usually viral, gives a lymphocytic CSF and is generally self-limited. The organism depends on age and setting, and the CSF profile separates the main causes.
Structure
- Pyogenic: creamy purulent exudate over the convexities (pneumococcus) or base (Haemophilus), with congested vessels.
- Micro: neutrophils fill the subarachnoid space and spread along vessels; phlebitis and arteritis can cause venous thrombosis and infarcts.
- Late: leptomeningeal fibrosis can block CSF flow and cause hydrocephalus.
Step by step
- Bacteria colonise the nasopharynx or reach the blood from another focus.
- They cross the blood-brain barrier at the choroid plexus or meningeal vessels.
- Bacterial products trigger cytokines; neutrophils pour into the CSF.
- Inflammation raises pressure, damages vessels and injures cranial nerves.
- Without treatment: cerebral edema, infarction, herniation and death; survivors may be deaf.
CSF in the main types of meningitis
| Type | Cells | Protein | Glucose | Other clue |
|---|---|---|---|---|
| Bacterial (pyogenic) | Many neutrophils | High | Low | Gram stain and culture positive |
| Viral (aseptic) | Lymphocytes | Normal or mildly high | Normal | PCR for enterovirus, herpesviruses |
| Tuberculous | Lymphocytes (neutrophils early) | Very high | Low | Acid-fast stain, molecular tests; basal exudate |
| Cryptococcal | Few to moderate lymphocytes | High | Low or normal | Cryptococcal antigen, India ink; high opening pressure |
Clinical notes
- Fever, headache, neck stiffness and altered mental state; a petechial rash suggests meningococcus.
- Sensorineural hearing loss is a common sequel, especially after pneumococcal meningitis in children.
Test yourself
Think of the answer, then tap the question to see it.
Which three bacteria cause most neonatal meningitis?
Group B streptococcus, Escherichia coli and Listeria monocytogenes.
How does the CSF glucose differ between bacterial and viral meningitis?
Low in bacterial meningitis because bacteria and neutrophils consume it; normal in viral meningitis.
What is Waterhouse-Friderichsen syndrome?
Bilateral adrenal hemorrhage with shock in fulminant meningococcal septicemia.
Elsewhere in DurgiPedia
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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Bacterial Meningitis (StatPearls (NCBI Bookshelf), 2026).
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Representative model, for educational purposes only.