Brain abscess is a free or encapsulated collection of pus that usually occurs in the temporal lobe, cerebellum, or frontal lobes. It can vary in size and may present singly or multilocularly. Brain abscess has a relatively low occurrence. Although it can occur at any age, it’s most common in people ages 10 to 35 and is rare in older adults.
An untreated brain abscess is usually fatal; with treatment, the prognosis is only fair. About 30% of patients develop focal seizures. Multiple metastatic abscesses secondary to systemic or other infections have the poorest prognosis.
A brain abscess usually occurs secondary to some other infection, especially otitis media, sinusitis, dental abscess, and mastoiditis. Other causes include subdural empyema; bacterial endocarditis; human immunodeficiency virus infection; bacteremia; pulmonary or pleural infection; pelvic, abdominal, and skin infections; and cranial trauma, such as a penetrating head wound or compound skull fracture.
This condition also occurs in about 2% of children with congenital heart disease, possibly because the hypoxic brain is a good culture medium for bacteria. Common infecting organisms are pyogenic bacteria, such as Staphylococcus aureus and Streptococcus viridans. Penetrating head trauma or bacteremia usually leads to staphylococcal infection; pulmonary disease, to streptococcal infection. In up to 25% of patients, an original source isn’t discovered.
A brain abscess usually begins with localized inflammatory necrosis and edema, septic thrombosis of vessels, and suppurative encephalitis. This is followed by thick encapsulation of accumulated pus, and adjacent meningeal infiltration by neutrophils, lymphocytes, and plasma cells. Increasing pressure in the brain results in more damage.