Microbiology
Central Nervous System Infections
CNS infections for MBBS: bacterial, viral, tuberculous and fungal meningitis, encephalitis and brain abscess with CSF interpretation, mapped to NMC codes MI5.1 to MI5.3.
MedNext Academy | 3 min read
Central Nervous System Infections
CNS infections for MBBS: bacterial, viral, tuberculous and fungal meningitis, encephalitis and brain abscess with CSF interpretation, mapped to NMC codes MI5.1 to MI5.3.
This chapter covers infections of the brain and its coverings. It deals with bacterial, viral, tuberculous and fungal meningitis, viral encephalitis and brain abscess, and it emphasises the interpretation of cerebrospinal fluid, which is central to distinguishing these conditions.
High-yield: Central Nervous System Infections
- Streptococcus pneumoniae is the commonest cause of bacterial meningitis in adults and the elderly.
- Neisseria meningitidis causes epidemic meningococcal meningitis and can produce a purpuric rash and Waterhouse-Friderichsen syndrome.
- Group B Streptococcus and Escherichia coli are leading causes of neonatal meningitis.
- Haemophilus influenzae type b meningitis in children has declined sharply with conjugate vaccination.
- Bacterial meningitis shows a turbid cerebrospinal fluid with high neutrophils, high protein and low glucose.
- Viral (aseptic) meningitis shows clear fluid with lymphocytes, mildly raised protein and normal glucose.
- Tuberculous meningitis gives a lymphocytic fluid with very high protein, low glucose and a cobweb clot.
- Cryptococcal meningitis is diagnosed by an India ink preparation showing budding yeast with a wide capsule.
- The cryptococcal antigen latex agglutination test on cerebrospinal fluid is sensitive for Cryptococcus neoformans.
- Herpes simplex virus is the leading cause of sporadic fatal viral encephalitis and preferentially affects the temporal lobe.
- Rabies is almost invariably fatal once symptomatic and shows Negri bodies in neurons on histology.
- Japanese encephalitis is a mosquito-borne arbovirus and an important cause of encephalitis in India, preventable by vaccine.
- A lumbar puncture is the key investigation for meningitis and should follow imaging if raised intracranial pressure is suspected.
- Empirical treatment of bacterial meningitis is started immediately after cultures because delay increases mortality.
- Brain abscess is often polymicrobial with streptococci and anaerobes and presents with focal signs and raised intracranial pressure.
Cerebrospinal fluid patterns
- **Bacterial (pyogenic):** Turbid, neutrophils high, protein high, glucose low.
- **Viral (aseptic):** Clear, lymphocytes, protein mildly high, glucose normal.
- **Tuberculous:** Cobweb clot, lymphocytes, protein very high, glucose low.
- **Fungal (cryptococcal):** Lymphocytes, India ink positive, capsular antigen detected.
NMC competencies in this chapter
- **MI5.1:** Aetiology and pathogenesis of meningitis, encephalitis and brain abscess
- **MI5.2:** Cerebrospinal fluid analysis and laboratory diagnosis of CNS infections
- **MI5.3:** Treatment, prevention and vaccination for CNS infections
Frequently Asked Questions
How does cerebrospinal fluid help distinguish types of meningitis?
Bacterial meningitis gives turbid fluid with neutrophils, high protein and low glucose, viral meningitis gives clear fluid with lymphocytes and normal glucose, and tuberculous meningitis gives lymphocytes with very high protein and low glucose.
Which virus causes fatal sporadic encephalitis?
Herpes simplex virus is the commonest cause of sporadic fatal encephalitis and characteristically involves the temporal lobe, so early aciclovir is critical.
How is cryptococcal meningitis diagnosed?
An India ink preparation of cerebrospinal fluid shows budding yeast with a wide clear capsule, and the cryptococcal antigen test provides a sensitive confirmation, especially in HIV infection.
Why is empirical antibiotic therapy started quickly in meningitis?
Bacterial meningitis is rapidly fatal, so empirical antibiotics are started immediately after taking cultures, since any delay increases mortality and neurological damage.
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