Central Nervous System Infections
To reiterate, a brain abscess is a localized infection of the brain parenchyma that arises from direct spread from a contiguous site (otitis media, mastoiditis, sinusitis, odontogenic, etc.) or hematogenous dissemination. Usually, multifocal brain abscesses are seen in hematogenous routes of infection and solitary brain abscesses are from contiguous spread. Brain abscesses can also present as a post-neurosurgical complication. Unfortunately, no clear source of infection is determined in 20-40% of cases!
As you would expect, if the cause of the brain abscess is due to local spread, then the abscess is likely to be polymicrobial (involving oral flora such as anaerobes). The empiric cocktail of ceftriaxone 2g IV q12 and metronidazole 500mg IV/PO q8h is recommended.
If you suspect brain abscess is from a hematogenous source such as endocarditis, then empiric vancomycin is added until blood culture results are known.
If the patient is post-neurosurgery, include coverage for MRSA with vancomycin IV (goal trough 15-20) as well as Pseudomonas with cefepime 2g IV q8h.
Patients typically have headache, and only ~50% of patients present with fever. Other symptoms include AMS, focal neurologic deficits, and seizures (30-60% of patients). Obtain blood cultures and CNS imaging as soon as possible (while MRI is more sensitive, contrast-enhanced CT is often faster.) Remember, LP is contraindicated because of the potential for increased intracranial pressure and possible herniation.
Ideally, cultures are obtained prior to starting antibiotics if the patient is stable; otherwise, start empiric antibiotics immediately. If imaging shows an abscess <2.5 cm and the patient is clinically stable, medical therapy may be successful without surgery if the causative organism(s) are identified. If >2.5 cm, abscesses should be surgically drained. Get Neurosurgery on board as soon as possible! If substantial cerebral edema or AMS is present, some experts recommend steroid administration as well.
Note: In HIV/immunocompromised patients, consider Listeria, fungal organisms (such as Cryptococcus neoformans, Aspergillus, Coccidioides) as well as Toxoplasma and Nocardia. Because the differential is broad, cultures (bacterial, fungal, AFB) are incredibly important. Add ampicillin if you suspect Listeria and add Bactrim if you suspect Nocardia. Get that ID consult!
Antibiotics are typically continued for 4-8 weeks but duration should be guided by follow-up imaging using the same imaging modality that was previously used.