Microbiology Cheat Sheet
Encephalitis & Focal Cns Infection
Encephalitis and focal CNS infection for NEET-PG: HSV temporal lobe encephalitis, Japanese encephalitis, rabies Negri bodies, Nipah, and brain abscess imaging.
MedNext Academy | 3 min read
Encephalitis & Focal Cns Infection
Encephalitis and focal CNS infection for NEET-PG: HSV temporal lobe encephalitis, Japanese encephalitis, rabies Negri bodies, Nipah, and brain abscess imaging.
This topic covers the etiopathogenesis, clinical course, and laboratory diagnosis of viral encephalitis and focal CNS infections including brain abscess.
High-yield lines
- Herpes simplex virus type 1 is the most common cause of sporadic fatal encephalitis worldwide and has a characteristic predilection for the temporal lobe.
- CSF PCR for HSV DNA is the gold standard for herpes simplex encephalitis with over 95 percent sensitivity, and may be negative in the first 24 to 48 hours.
- Intravenous aciclovir must be started empirically in any suspected encephalitis without waiting for CSF, MRI, or PCR results.
- Japanese encephalitis virus is the most important cause of epidemic viral encephalitis in Asia, transmitted by Culex tritaeniorhynchus mosquitoes with pigs as the amplifying host.
- IgM capture ELISA on CSF is the recommended diagnostic test for Japanese encephalitis, as intrathecal IgM does not cross the intact blood-brain barrier.
- The SA 14-14-2 live attenuated Japanese encephalitis vaccine is included in India's UIP in endemic districts.
- Rabies virus travels by retrograde axonal transport at about 12 to 24 millimetres per day after binding the nicotinic acetylcholine receptor.
- Negri bodies are eosinophilic intracytoplasmic inclusions in hippocampal and Purkinje neurons, pathognomonic of rabies but absent in 20 to 30 percent of cases.
- Rabies is almost invariably fatal once symptoms appear, with hydrophobia and aerophobia characterising the furious form.
- Nipah virus causes encephalitis with respiratory involvement, is a BSL-4 pathogen with fruit bat reservoirs, and has caused outbreaks in Kerala.
- Brain abscess classically shows a ring-enhancing lesion with restricted diffusion on MRI, distinguishing it from necrotic tumour.
- Contiguous brain abscess from sinusitis or dental infection is often polymicrobial with streptococci and anaerobes, favouring the frontal lobe.
- Toxoplasma gondii causes multiple ring-enhancing lesions in the basal ganglia in AIDS patients with CD4 below 100.
- Lumbar puncture is contraindicated in suspected brain abscess due to the risk of transtentorial herniation.
Mapped competency codes
- MI5.1
- MI5.2
Continue into the full chapter
This summary maps to MI5-central-nervous-system-infections.
Frequently Asked Questions
What is the gold-standard test for herpes simplex encephalitis?
CSF PCR for HSV DNA, with over 95 percent sensitivity, though it may be negative in the first 24 to 48 hours of illness.
Which region does HSV-1 encephalitis characteristically involve?
The temporal lobe, causing behavioural change, temporal lobe seizures, and haemorrhagic necrosis.
What is the diagnostic test of choice for Japanese encephalitis?
IgM capture ELISA on CSF, because intrathecal IgM synthesis is specific since immunoglobulins do not cross the intact blood-brain barrier.
What histological finding is pathognomonic of rabies?
Negri bodies, eosinophilic intracytoplasmic inclusions in hippocampal and cerebellar Purkinje neurons, though absent in 20 to 30 percent of cases.
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