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V. Brain Abscesses

A. Etiologic Agents:

a. Usually pyogenic bacteria; streptococci and staphylococci are the most common.

b. Fungi may produce localized areas of intracerebral inflammation called granulomas; these occur most often in immunosuppressed patients. The difference between an abscess and granuloma is determined by pathological examination.

B. Pathogenesis:

Secondary to hematogenous spread, direct extension from nearby suppurative focus or direct implantation of foreign bodies (surgery, trauma). Predisposing conditions include acute bacterial endocarditis; cyanotic congenital heart disease, in which there is a right-to left shunt and loss of pulmonary filtration of organisms; and chronic pulmonary sepsis.

C. Pathological Changes

Discrete lesions with central liquefactive necrosis; surrounded by inflammatory cells and zone of fibrous tissue, with proliferation of small blood vessels (granulation tissue). Marked vasogenic edema is associated with the neovascularization. Fibroblasts are recruited from the vascular system. Outside the fibrous capsule is a zone of reactive gliosis.