CNS infections - microbiology Flashcards
What are the main routes if invasion of pathogens to cause a CNS infection?
blood vessels and nerves that traverse the walls of the skull
What barriers in the CNS protect against pathogen invasion?
blood-brain barrier
blood-CSF barrier
Gross examination of the CSF
normal CSF is clear and colourless
viscosity equal to water
clot seen in traumatic tap, not SAH
viscous CSF seen with increased protein content
When is CSF turbid?
WBC>200 cells/microlitre
RBC>400 cells/microlitre
How is CSF analysed?
appearance - clarity, colour
microscopy - cell count, differential white cell count, gram stain, other stains
culture
PCR
serology
How does meningococcal meningitis spread?
droplet infection
facilitated by other respiratory infections that cause increased respiratory secretions
Clinical features of meningococcal meningitis
haemorrhagic skin rash
sore throat
headache
drowsiness
fever
irritability
neck stiffness
photophobia
How can bacterial infection in the CSF be shown in a CSF sample?
low CSF sugar compared with serum glucose indicates a bacterial infection in the CNS
because bacteria break down glucose
Why is serology not helpful in the diagnosis of meningococcal meningitis?
infection is too acute for an antibody response to be detectable
What is the most common type of meningitis?
viral meningitis
Describe the CSF in viral meningitis
clear
cells are mainly lymphocytes (polymorphonuclear leukocytes may be dominant in early phases)
What is encephalitis usually caused by?
viruses
Causes of recurrent bacterial meningitis
basal skull defects
recurrent aseptic meningitis (Mollaret’s)
Congenital dermal sinus
What can cause a brain abscess?
bacterial spread via nearby tissue infections:
- persistent otitis media
- sinusitis
- mastoiditis
Main microorganisms responsible for brain abscesses
streptococcus pneumoniae, H.Influenzae or Candida from nearby ENT infection
Staphylococcus species from iatrogenic infection
any other microorganism by haematogenous spread
