Metastatic brain abscesses arise by hematogenous dissemination from primary site remote from brain. They are increasing in frequency and may be more common now than direct extension abscesses. Basal skull fracture with CSF leak. Post op craniotomy. Formation of a collagen capsule is the single most important response in limiting the spread of infection in the brain.
Brain abscess is the only condtion in which fibroblasts proliferate within the brain parenchyma.
Initially, there is hemorrhagic softening apparent microscopically as acutely inflamed tissue dotted by necrotic blood vessels and extravasated erythrocytes. Seen on CT as an irregular area of low density with little or no enhancement. Cerebritis stimulates the proliferation of fibroblasts from blood vessel walls which eventually knit a collagenous collar around the suppurative center to wall it off from the brain. At this stage, the capsule is evident on CT as a brightly enhancing rim. When the abscess is advanced, the capsule is firm and fibrous, and the contents include lipid laden macrophages, capillary proliferation, and chronic inflammatory cells.