0903 - CNS Infections Flashcards
What are the components of the BBB? - How is it important in CNS infections?
Formed by endothelium of capillaries - so continuous barrier restricting transport of ions, molecules, cells and pathogens.
Consists of tight junctions (limit paracellular flux), pinocytotic activity (limiting transcellular flux), and additional components of basement membrane and astrocyte foot processes. Also has specific carrier and transport systems.
Important because it keeps pathogens out, but also barrier to antibiotics.
What are the components of the BCB? - How is it important in CNS infections?
Basically the choroid plexuses. Richly vascularised and separated from ventricles by choroid epithelial cells, which are continuous with ependymal cells lining ventricles. Tight junctions between choroid epithelium prevent most substances from crossing into CSF. They (or arachnoid granulations?) also remove metabolic waste, foreign substances and excess neurotransmitters from CSF.
Important because it keeps pathogens out, but also barrier to antibiotics. CSF is a good place for bacteria to grow.
What are the common sites of CNS infection?
Meningitis - Meninges
Encephalitis - Brain (diffuse)
Myelitis - Spinal Cord
Focal lesions in meninges or in the tissue
What are the common microorganisms associated with CNS infection?
Meningitis - Bacterial (fatal without treatment), viral (more common, self-limiting, enterovirus, HSV1, VCV), Fungal (cryptococcus), Parasites
Encephalitis/Myelitis - Acute Viral infection of neurons (HSV, enterovirus, VCV, CMV, Arbovirus); or post-infectious, immune mediated peri-venular inflammation and demyelination. Can also be bacterial (listeria, mycoplasma) or parasitic.
Abscess/Focal - Contiguous (polymicrobial - strep, anaerobes, Gram negs); Haematogenous (lung abscess); Direct trauma (staph aureus/normal flora).
What are the common causes of bacterial meningitis? What are the host risk factors and associated conditions with each?
Strep Pneumoniae - Hyposplenism, under 2, elderly, HIV infection, hypogammaglobulinaemia (e.g. myeloma). Associated with any respiratory or ear infection, including sinusitis.
Neisseria Meningitidis - Adolescents, young kids. Complement deficiency. Associated with bacteraemia with rash.
Listeria monocytogenes - Pregnant women, T-cell deficiency, Chemo patients. Dietary exposures
Group B Strep - Neonates - infects at birth. Associated with maternal colonisation.
Mycobacterium TB - Risk if past TB exposure. Subacute/chronic presentation.
How can microorganisms disseminate to the CNS? Example of each?
Physically - Congenital defect, trauma, extension from infection in adjacent bone (sinuses/oral cavity/middle ear/bertebrodiscitis). Often caused by strep milleri (causing abscesses) and anaerobes.
Haematogically - Primary Bacteraemia (Neisseria meiningitis/Step. Pneumoniae); or secondary from other primary (pneuomonia - step. pneumoniae; endocarditis - staph aureus).
Perineurally - latent HSV1 (cold sore) encephalitis or rabies/lyssavirus.
Olfactory Nerve - Naegleria fowleri - amoebic meningitis.
How do you Interpret CSF analysis to find the cause of infection?
1 - Glucose, if less than ⅔ of serum level, it is bacterial or fungal/mycobacterial. If more than that, it is viral.
2 - WCC - if over 1000 it is bacterial, if less it is fungal/mycobacterial or viral.
3 - WCC differential - if Polymorphs predominate, it is bacterial, if monocytes predominate it is viral/fungal/mycobacterial. Normal is all monocytes.
