Brain abscess overview
Editor-In-Chief: C. Michael Gibson, M.S., M.D.  Associate Editor(s)-in-Chief: Farwa Haideri 
Brain abscess Microchapters
Brain abscess overview On the Web
American Roentgen Ray Society Images of Brain abscess overview
Risk calculators and risk factors for Brain abscess overview
Brain abscess (or cerebral abscess) is an abscess caused by inflammation and collection of infected material coming from local (ear infection, dental abscess, infection of paranasal sinuses, infection of the mastoid air cells of the temporal bone, epidural abscess) or remote (lung, heart, kidney etc.) infectious sources within the brain tissue. The infection may also be introduced through a skull fracture following a head trauma or surgical procedures. Although underlying pathology (tumor, blood, etc.) can sometimes be a nidus for infection, the majority of cases occur in a previously healthy brain. Brain abscess is usually associated with congenital heart disease in young children. It may occur at any age but is most frequent in the third decade of life. Due to advanced treatments of the ear, sinus, and orofacial infections, brain abscess occurrences are now rare, only present in about 1,500 to 2,500 infections each year in the United States. If left untreated, a brain abscess can be fatal. Common complications include abscess rupture, hydrocephalus, and brain herniation. With treatment, the mortality rate decreases to about 10% of the population.
Prior to the 1800s, brain abscesses were almost uniformly fatal, and were rarely diagnosed preceding an autopsy. French surgeon S.F. Morand was the first to complete a successful drainage of a brain abscess during the 16th century. It was not until the late 19th century that methodical developments of surgery were available to treat these abscesses.
Brain abscesses are usually polymicrobial in nature. Infected brain cells, white blood cells, live and dead bacteria, and fungi collect in an area of the brain. Although underlying pathology (tumor, bloodetc.) can sometimes be a nidus for infection. Bacterial abscesses rarely arise de novo within the brain. The most common organism recovered from cultures is the bacterium Streptococcus. A wide variety of other bacteria may cause brain abscess; these include Proteus, Pseudomonas, Pneumococcus, Meningococcus, and Haemophilus.
The majority of cases of brain abscess are due to infections with either bacteria or fungi. Common causes of brain abscess include Cryptococcus neoformans, Staphylococcus aureus, Toxoplasma gondii, and Viridans streptococci. The germs that cause a brain abscess usually reach the brain through the blood. Germs may also travel from a nearby infected area (such as an ear infection), can enter the body during an injury (from a gun or knife wound), or result from surgery.
Differentiating brain abscess from other Conditions
Brain abscess must be differentiated from metastatic tumors, necrotic tumors, and lymphomas. The true diagnosis for a brain abscess is sometimes not determined until biopsy.
Epidemiology and Demographics
There has been a remarkable shift in the epidemiology of brain abscesses over the past several years. Traditionally, and still the case in underdeveloped countries, most brain abscesses are due to under-treated otitis and sinusitis. More recently, especially in the U.S., more cases are being seen in immunocompromised patients. The prevalence of brain abscess accounts for approximately 1 in 10,000 hospital admissions.
Common risk factors in the development of brain abscesses are immunodeficiency, chronic diseases, and congenital heart diseases. In adults, drugs that suppress the immune system and a weakened immune system are prevalent risk factors. In children, sinusitis and otitis are seen with greatest frequency.
Natural History, Complications, and Prognosis
Common complications include abscess rupture, hydrocephalus, and brain herniation. With treatment, the mortality rate decreases to about 10% of the population. Early detection and treatment is preferential. After surgery, some patients can experience long-term neurological problems. While the mortality rate was 40% in 1960, it has dropped down to 15% within the past decade alone. About 70% of patients with brain abscesses have a good outcome, rarely with minimal neurological sequelae.
History and Symptoms
A complete history will help determine the correct therapy and helps in determining the prognosis. Specific areas of focus when obtaining a history from the patient include a history of a clinical triad of fever, headaches, and focal neurological deficits and immunosuppression. The symptoms and findings depend largely on the specific location of the abscess in the brain. Most symptoms are caused by a combination of increased intracranial pressure from space-occupying lesions (headache, vomiting, confusion, coma) or infection (fever, fatigue etc.)
Patients with brain abscess generally appear healthy. Most findings are neuromuscular in nature. Significant findings on physical examination include significant derangements in vital signs, including high-grade fever and dysarthria. Aphasia ataxia are two common neuromuscular examinations.
Most laboratory tests are not diagnostic for brain abscess. In 25% of findings, the cerebrospinal fluid CSF is normal or shows unspecific changes. An increase of the erythrocyte sedimentation rate (ESR) has also been associated with brain abscess. Some patients have been seen to herniate after CSF procedure. It is difficult to isolate the pathogens from the CSF.
The diagnosis of brain abscess is established by a computed tomography (CT) (with contrast) examination. The CT scan can detect edema, hydrocephalus, midline shifts, or ventricular ruptures.
Magnetic resonance imaging (MRI) is the diagnostic procedure of choice to diagnose a brain abscess. MRI scans appear more sensitive than CT for detecting cerebral edema and early changes correlated to a brain abscess because they can stage the abscess and gauge the response to therapy with more accuracy.
An electrocardiogram (EEG) can also be used to diagnose a brain abscess by providing detailed information in regards to localization of the abscess. In most cases, the result is abnormal in patients as the brain abscess lateralizes to the side of the lesion.
The treatment of brain abscess includes prompt administration of antimicrobial therapy and occasionally drainage to reduce the mass effect. Neurosurgery should always be consulted upon diagnosis. The decision of whether to surgically drain, aspirate, or simply administer antimicrobial therapy depends on the number of abscesses, their size, and their location.
If the size of the brain abscess exceed 2.5 cm in diameter, surgery is the mainstay treatment. Otherwise, the decision to drain the tumor surgically vs. aspiration is again based on the number of abscesses in respect to their size and location.
Primary preventive strategies for brain abscess include antibiotics, treatment for sinusitis, and highly active antiretroviral therapy for patients with HIV. Secondary prevention for brain abscess include continued treatment of predisposing causes in appropriate patients.