Brain abscess

The cleverly drawn figure above demonstrates the principle mechanisms by which people develop a brain abscess. The mechanism of acquisition of the brain abscess has direct bearing on the likely organisms. The most common mechanism is by spread from the adjacent sinuses or oral cavity making the most common organisms in the immunocompetent:

• Oral streptococci (viridans group, milleri group)
• Staphylococcus aureus
  
• Oral anaerobes including peptostreptococcus, bacteroides and fusobacterium species

Empiric coverage should, in the absence of a focus outside of contiguous spread, include a third generation cephalosporin at meningitic doses combined with metronidazole.

The "heart" in the diagram includes:

• Hematogenous spread in bacteremia such as seen in the lung/brain or liver/brain axis
• Right to left shunting in HHT or cyanotic heart disease or other AV malformations
• Infective endocarditis
  

A full issue of the journal neurosurgical focus is dedicated to brain abscess. A recent article on using molecular sequencing to identify pathogens in brain absess suggests more pathogens are present in these abscesses than commonly thought -- even though it is unclear how many of them are involved in the actual pathogenesis.