AIM Step 10
Student Memory Support
CNS Infections and Rabies: Meningitis, Encephalitis, Brain Abscess, Toxoplasmosis and Post-Exposure Prophylaxis
4th Year MBBS • Neuroscience • High-yield memory reinforcement and last-minute KMU revision
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1. High-Yield Flashcards
Tap each question to reveal the answer.
Q: What CSF pattern is typical of acute pyogenic meningitis?
A: Neutrophilic pleocytosis, raised protein and reduced glucose.
Q: Why can acute bacterial meningitis cause cerebral infarction?
A: Inflammatory vasculitis and thrombosis can reduce cerebral blood flow.
Q: Which pathological feature explains hydrocephalus and cranial nerve palsies in tuberculous meningitis?
A: Thick inflammatory exudate around the base of the brain.
Q: What CSF pattern supports tuberculous meningitis?
A: Predominantly lymphocytes, raised protein and reduced glucose.
Q: Which clinical feature most strongly distinguishes encephalitis from uncomplicated meningitis?
A: Cerebral dysfunction such as altered behavior, confusion, memory impairment or seizures.
Q: Which brain region is an important imaging clue in herpes simplex encephalitis?
A: The temporal lobe.
Q: What imaging appearance is typical of a mature brain abscess?
A: A ring-enhancing lesion with surrounding cerebral edema.
Q: Why is lumbar puncture generally avoided when brain abscess with mass effect is suspected?
A: Sudden pressure change may precipitate brain herniation.
Q: In which setting should CNS toxoplasmosis be strongly considered?
A: Severe cellular immunosuppression with focal or multiple ring-enhancing brain lesions.
Q: How does rabies virus reach the central nervous system after inoculation?
A: By retrograde transport along peripheral nerves.
Q: What is the difference between hydrophobia and aerophobia in rabies?
A: Hydrophobia is triggered by attempts to drink; aerophobia is triggered by air movement.
Q: Which rabies exposure category includes transdermal bites or saliva on a mucous membrane?
A: Category III exposure.
Q: What is the role of rabies vaccine in post-exposure prophylaxis?
A: It induces active immunity and production of virus-neutralizing antibodies.
Q: What is the role of rabies immunoglobulin in post-exposure prophylaxis?
A: It provides immediate passive neutralizing antibodies at the exposure site.
Q: Which findings may suggest meningitis in a neonate even when neck stiffness is absent?
A: Poor feeding, lethargy, temperature instability, apnea or seizures.
2. Mnemonics
Mnemonic Title: Pyogenic CSF Pattern
NPL
Meaning: Neutrophils ↑, Protein ↑, gLucose ↓.
Mnemonic Title: TB Meningitis Basal Complications
CVH
Meaning: Cranial nerve palsies, Vasculitis/infarction, Hydrocephalus.
Mnemonic Title: Rabies PEP Core Sequence
WVI
Meaning: Wound cleansing → Vaccine → Immunoglobulin when indicated.
3. Memory Tables
Meningitis and Encephalitis — Rapid Differentiation
| Feature | Pyogenic Meningitis | TB Meningitis | Viral Encephalitis |
|---|---|---|---|
| Course | Acute | Subacute | Acute |
| Main clue | Meningism | Basal complications | Cerebral dysfunction |
| CSF cells | Neutrophils | Lymphocytes | Lymphocytes |
| CSF glucose | Low | Low | Relatively preserved |
Rabies Exposure Categories and PEP
| Category | Exposure | Vaccine | RIG |
|---|---|---|---|
| I | Touching/feeding; lick on intact skin | Not required | Not required |
| II | Minor scratch/abrasion without bleeding | Required | Generally not required |
| III | Transdermal bite/scratch or mucosal exposure | Required | Required if previously unvaccinated |
4. Rapid Revision Points — Last-Minute Revision
Must Remember:
- Pyogenic meningitis produces neutrophilic CSF with raised protein and low glucose.
- TB meningitis is subacute and classically causes basal meningeal inflammation.
- Basal TB exudate can produce cranial nerve palsies, vasculitis and hydrocephalus.
- Encephalitis is suggested by confusion, behavioral change, memory impairment or seizures.
- A brain abscess is a focal lesion with edema, mass effect and ring enhancement.
- Suspected brain abscess with mass effect is a setting in which lumbar puncture can be dangerous.
- Multiple focal brain lesions in severe cellular immunosuppression suggest CNS toxoplasmosis.
- Rabies travels from the exposure site to the CNS through peripheral nerves.
- Rabies vaccine gives active immunity; rabies immunoglobulin gives immediate passive immunity.
- Neonatal meningitis may present with poor feeding, lethargy, apnea, temperature instability or seizures rather than neck stiffness.
KMU Trap: A ring-enhancing lesion is not specific for one diagnosis. Clinical context helps separate bacterial brain abscess from CNS toxoplasmosis.
5. Clinical Memory Hooks
Fever + neck stiffness + neutrophilic low-glucose CSF → Acute pyogenic meningitis
Subacute meningitis + cranial nerve palsy + hydrocephalus → Basal tuberculous meningitis
Fever + behavioral change + seizures + temporal-lobe involvement → Viral encephalitis, especially HSV pattern
Rabid animal bite + transdermal wound → Immediate cleansing + vaccine + immunoglobulin if previously unvaccinated
6. Starred High-Yield Exam Points
- ⭐ Neutrophils + raised CSF protein + low CSF glucose strongly support acute pyogenic meningitis.
- ⭐ Basal exudate in TB meningitis explains cranial nerve palsies, cerebral infarction and hydrocephalus.
- ⭐ Temporal-lobe involvement with encephalopathic features is an important clue to herpes simplex encephalitis.
- ⭐ Ring enhancement with surrounding edema and focal neurological deficit is a key brain abscess pattern.
- ⭐ Category III rabies exposure in a previously unvaccinated patient requires vaccine plus rabies immunoglobulin.
- ⭐ Poor feeding, lethargy or temperature instability may be the presenting clues to neonatal bacterial meningitis even without neck stiffness.