Brain Abscess (Infectious)

A brain abscess is a focal suppurative infection within the brain parenchyma, evolving through four pathologic stages: early cerebritis (days 1-3), late cerebritis (days 4-9), early capsule formation (days 10-13), and late capsule formation (>14 days). Etiology varies by source: contiguous spread from sinusitis, otitis, mastoiditis, or odontogenic infection; hematogenous seeding from endocarditis, pulmonary infection, or bacteremia; direct inoculation through trauma or neurosurgery; and cryptogenic (15-30%).

Microbiology depends on source: streptococci (especially Streptococcus anginosus group, S. milleri) are most common, followed by staphylococci (post-traumatic, post-surgical), anaerobes (Bacteroides, Fusobacterium from sinus/dental), gram-negative bacilli (chronic otitis), and polymicrobial infections. Immunocompromised hosts have additional pathogens: Toxoplasma gondii, Nocardia, Aspergillus, Candida, Cryptococcus.

Clinical presentation varies but classic triad of fever, headache, and focal neurologic deficit is present in <50%. Diagnosis relies on contrast-enhanced MRI (gold standard) showing ring-enhancing lesion with restricted diffusion (highly specific). CT-guided or stereotactic aspiration provides microbiologic diagnosis and therapeutic drainage. Empiric antimicrobial therapy depends on suspected source: ceftriaxone plus metronidazole (sinus/dental), vancomycin plus ceftriaxone plus metronidazole (post-traumatic/post-surgical), modified for immunocompromised patients. Duration is typically 6-8 weeks IV with imaging-guided endpoint.