The most common organism recovered from cultures is the bacterium Streptococcus. However, a wide variety of other bacteria (Proteus, Pseudomonas, Pneumococcus, Meningococcus, Haemophilus), fungi and parasites may also cause the disease. Fungi and parasites are especially associated with immunocompromised patients. Organisms that are most frequently-associated with brain abscess in patients with AIDS are Mycobacterium tuberculosis, Toxoplasma gondii and Cryptococcus neoformans, though in infection with the latter organism, symptoms of meningitis generally predominate.
Bacterial abscesses rarely (if ever) arise de novo within the brain, although establishing a causes can be difficult in many cases. There is almost always a primary lesion elsewhere in the body that must be sought assiduously, because failure to treat the primary lesion will result in relapse. In cases of trauma, for example in compound skull fractures where fragments of bone are pushed into the substance of the brain, the cause of the abscess is obvious. Similarly, bullets and other foreign bodies may become sources of infection if left in place. The location of the primary lesion may be suggested by the location of the abscess: infections of the middle ear result in lesions in the middle and posterior cranial fossae; congenital heart disease with right-to-left shunts often result in abscesses in the distribution of the middle cerebral artery; and infection of the frontal and ethmoid sinuses usually results in collection in the subdural sinuses.
Ring enhancement may also be observed in cerebral hemorrhages (bleeding) and some brain tumors. However, in the presence of the rapidly progressive course with fever, focal neurologic findings (hemiparesis, aphasia etc) and signs of increased intracranial pressure, the most likely diagnosis should be the brain abscess.
Surgical drainage of the abscess remains part of the standard management of bacterial brain abscesses. The location and treatment of the primary lesion also crucial, as is the removal of any foreign material (bone, dirt, bullets, and so forth).
There are a few exceptions to this rule: Haemophilus influenzae meningitis is often associated with subdural effusions that are mistaken for subdural empyemas. These effusions resolve with antibiotics and require no surgical treatment. Tuberculosis can produce brain abscesses that look identical to bacterial abscesses on CT imaging and surgical drainage or aspiration is often necessary to make the diagnosis, but once the diagnosis is made no further surgical intervention is necessary.
Once an almost always fatal disease before the CT era. If the abscess is treated before the person goes into a coma then the death rate has been estimated from 5% to 20% although it is greater in cases of multiple abscesses, when raised intracranial pressure is observed and depending on the level of neurological dysfunction on presentation. Early treatment and the patients overall health has an effect on prognosis. Other factors include: antibiotic resistance or the abscess location. An abscess deep within the brain is more difficult to treat than others.