CT and MRI are the most important diagnostic tools. In the CT scan, the capsule of the abscess enhances and the center of the abscess and surrounding edematous white matter are hypodense. With MRI, in T1-weighted images, the capsule enhances and the interior of the abscess is hypointense and shows restricted diffusion; in T2-weighted images, the surrounding edema is apparent and the capsule is hypointense and there is variable diffusion restriction within the lesion (see Fig. 31-2, right). The abscess capsule tends to be thinner on the side directed to the lateral ventricle. Cerebritis appears as dot-sized areas of decreased density that enhance with gadolinium. Practically all abscesses larger than 1 cm produce positive scans. There is almost no likelihood of cerebral abscess if enhanced CT and MRI studies are negative. Blood cultures, sedimentation rate, and chest radiography are indispensable in the complete diagnosis of brain abscess, although it must be acknowledged that blood cultures are likely to be unrevealing except in cases of acute endocarditis.
If there is no apparent source of infection and there are only signs and symptoms of a mass lesion, the differential diagnosis includes tuberculous or fungal abscess, glioma, metastatic carcinoma, toxoplasmosis, subdural hematoma, subacute infarction of the basal ganglia or thalamus, and resolving cerebral hemorrhage or infarction. Sometimes only surgical exploration will settle the issue, but one must be cautious in interpreting especially stereotactic biopsy if only inflammatory and gliotic tissue is obtained, as these changes may appear in the neighborhood of either abscess or tumor.
During the stage of cerebritis and early abscess formation, which is essentially an acute focal encephalitis, intracranial operation accomplishes little and probably adds only further injury and swelling of brain tissue and possibly dissemination of the infection. Some cases can be cured at this stage by the adequate administration of high-dose antibiotics. Even before bacteriologic examination of the intracerebral mass, certain antibiotics can be given, with the choice based on the predisposing condition (vancomycin, a secondor third-generation cephalosporin such as ceftriaxone, and either meropenem or metronidazole). If a penicillinor oxacillin-sensitive organism is suspected or isolated, those agents are superior to vancomycin. These drugs are given intravenously in divided daily doses. Metronidazole is so well absorbed from the gastrointestinal tract that it can be administered orally, 500 mg q6h.
This choice of antimicrobial agents is based on the fact that anaerobic streptococci and Bacteroides are often among the causative organisms and that most abscesses are polymicrobial. Evidence of staphylococcal infection can be presumed if there has been recent neurosurgery or head trauma or a demonstrable bacterial endocarditis with this organism. Abscesses caused by bacteria of oral origin must be considered to have a high frequency of gram- negative organisms; penicillin and metronidazole are usually adequate but a thirdor fourth-generation cephalosporin, such as cefotaxime or cefepime intravenously, is often used. In abscess of odontogenic origin, coverage of the gram negative organisms with a cephalosporin may not be necessary. In all cases, several weeks of treatment are advised.